1. the basic functions of lv switchgear schneider... · the protection of circuits - the switchgear...

the protection of circuits - the switchgear - H2-1

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1. the basic functions of LV switchgear

the role of switchgear is that of:c electrical protection;c safe isolation from live parts;c local or remote switching.

National and international standards definethe manner in which electric circuits of LVinstallations must be realized, and thecapabilities and limitations of the variousswitching devices which are collectivelyreferred to as switchgear.The main functions of switchgear are:c electrical protection;c electrical isolation of sections of aninstallation;c local or remote switching.These functions are summarized below intable H2-1.Electrical protection at low voltage is (apartfrom fuses) normally incorporated in circuit

breakers, in the form of thermal-magneticdevices and/or residual-current-operatedtripping devices (less-commonly, residual-voltage-operated devices - acceptable to, butnot recommended by IEC).In addition to those functions shown in tableH2-1, other functions, namely:c over-voltage protection;c under-voltage protection are provided byspecific devices (lightning and various othertypes of voltage-surge arrester; relaysassociated with: contactors, remotely-controlled circuit breakers, and with combinedcircuit breaker/isolators… and so on).

electrical protection isolation controlagainstoverload currents - isolation clearly indicated - functional switchingshort-circuit currents by an authorized fail-proof - emergency switchinginsulation failure mechanical indicator - emergency stopping

- a gap or interposed insulating - switching off forbarrier between the open mechanical maintenancecontacts, clearly visible.

table H2-1: basic functions of LV switchgear.

1.1 electrical protection

electrical protection assures:c protection of circuit elementsagainst the thermal and mechanicalstresses of short-circuit currents;c protection of persons in the eventof insulation failure;c protection of appliances andapparatus being supplied (e.gmotors, etc.).

The aim is to avoid or to limit the destructiveor dangerous consequences of excessive(short-circuit) currents, or those due tooverloading and insulation failure, and toseparate the defective circuit from the rest ofthe installation.A distinction is made between the protectionof:c the elements of the installation (cables,wires, switchgear…);c persons and animals;c equipment and appliances supplied fromthe installation;c the protection of circuits(see chapter H1):v against overload; a condition of excessivecurrent being drawn from a healthy(unfaulted) installation,v against short-circuit currents due tocomplete failure of insulation betweenconductors of different phases or (in TNsystems) between a phase and neutral (orPE) conductor.Protection in these cases is provided eitherby fuses or circuit breaker, at the distributionboard from which the final circuit (i.e. the

circuit to which the load is connected)originates. Certain derogations to this rule areauthorized in some national standards, asnoted in chapter H1 sub-clause 1.4.c the protection of persons againstinsulation failures (see chapter G).According to the system of earthing for theinstallation (TN, TT or IT) the protection willbe provided by fuses or circuit breakers,residual current devices, and/or permanentmonitoring of the insulation resistance of theinstallation to earth.c the protection of electric motors(see chapter J clause 5) against overheating,due, for example, to long term overloading;stalled rotor; single-phasing, etc. Thermalrelays, specially designed to match theparticular characteristics of motors are used.Such relays may, if required, also protect themotor-circuit cable against overload. Short-circuit protection is provided either by typeaM fuses or by a circuit breaker from whichthe thermal (overload) protective element hasbeen removed, or otherwise madeinoperative.

1.2 isolation

a state of isolation clearly indicatedby an approved "fail-proof" indicator,or the visible separation of contacts,are both deemed to satisfy thenational standards of manycountries.

The aim of isolation is to separate a circuit orapparatus, or an item of plant (such as amotor, etc.) from the remainder of a systemwhich is energized, in order that personnelmay carry out work on the isolated part inperfect safety.In principle, all circuits of an LV installationshall have means to be isolated. In practice,in order to maintain an optimum continuity ofservice, it is preferred to provide a means ofisolation at the origin of each circuit.An isolating device must fulfil the followingrequirements:c all poles of a circuit, including the neutral(except where the neutral is a PENconductor) must be open (1);

c it must be provided with a means of lockingopen with a key (e.g. by means of a padlock)in order to avoid an unauthorized reclosureby inadvertence;c it must conform to a recognized national orinternational standard (e.g. IEC 947-3)concerning clearance between contacts,creepage distances, overvoltage withstandcapability, etc. and also:(1) the concurrent opening of all live conductors, while notalways obligatory, is however, strongly recommended (forreasons of greater safety and facility of operation). Theneutral contact opens after the phase contacts, and closesbefore them (IEC 947-1).

H2-2 - the protection of circuits - the switchgear

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1. the basic functions of LV switchgear (continued)

1.2 isolation (continued)

v verification that the contacts of the isolatingdevice are, in fact, open. The verification maybe:- either visual, where the device is suitablydesigned to allow the contacts to be seen(some national standards impose thiscondition for an isolating device located at theorigin of a LV installation supplied directlyfrom a HV/LV transformer),- or mechanical, by means of an indicatorsolidly welded to the operating shaft of thedevice. In this case the construction of thedevice must be such that, in the eventualitythat the contacts become welded together inthe closed position, the indicator cannotpossibly indicate that it is in the open position.v leakage currents. With the isolating deviceopen, leakage currents between the opencontacts of each phase must not exceed:- 0.5 mA for a new device,- 6.0 mA at the end of its useful life.v voltage-surge withstand capability, acrossopen contacts. The isolating device, whenopen must withstand a 1.2/50 µs impulse,having a peak value of 5, 8 or 10 kVaccording to its service voltage, as shown intable H2-2. The device must satisfy theseconditions for altitudes up to 2,000 metres.Consequently, if tests are carried out at sealevel, the test values must be increased by23% to take into account the effect of altitude.See standard IEC 947 and the Noteimmediately preceding table F-10.

service (nominal) impulse withstandvoltage peak voltage(V) (kV)230/400 5 kV400/690 8 kV1,000 10 kV

table H2-2: peak value of impulse voltageaccording to normal service voltage oftest specimen.

Industrial LV switchgear which affordsisolation when open is marked on the frontface by the symbol .This symbol may be combined with thoseindicating other features where a device alsoperforms other functions as shown in figureH2-4.

fig. H2-3: symbol for a disconnector* alsocommonly referred to as an isolator.

switch-disconnector*, also referred to as a load-break isolating switch

circuit breaker suitable for circuit isolation

fig. H2-4: symbols for circuit isolationcapability incorporated in other switchingdevices.

* IEC 617-7 and 947-3.

Note. In this guide the terms "disconnector"and "isolator" have the same meaning.

1.3 switchgear control

switchgear-control functions allowsystem operating personnel tomodify a loaded system at anymoment, according to requirements,and include:c functional control (routineswitching, etc.);c emergency switching;c maintenance operations on thepower system.

In broad terms "control" signifies any facilityfor safely modifying a load-carrying powersystem at all levels of an installation. The

operation of switchgear is an important part ofpower-system control.

functional controlThis control relates to all switching operationsin normal service conditions for energizing orde-energizing a part of a system orinstallation, or an individual piece ofequipment, item of plant, etc.Switchgear intended for such duty must beinstalled at least:c at the origin of any installation;c at the final load circuit or circuits (oneswitch may control several loads).Marking (of the circuits being controlled) mustbe clear and unambiguous.In order to provide the maximum flexibilityand continuity of operation, particularly wherethe switching device also constitutes theprotection (e.g. a circuit breaker orswitch-fuse) it is preferable to include aswitch at each level of distribution, i.e. on

each outgoing way of all distribution and sub-distribution boards.

The manœuvre may be:c either manual (by means of an operatinglever on the switch) or;c electric, by push-button on the switch or ata remote location (load-shedding and re-connection, for example).These switches operate instantaneously (i.e.with no deliberate delay), and those thatprovide protection are invariably omni-polar*.The main circuit breaker for the entireinstallation, as well as any circuit breakersused for change-over (from one source toanother) must be omni-polar units.* one break in each phase and (where appropriate) onebreak in the neutral (see table H1-65).


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emergency switching -emergency stopAn emergency switching is intended tode-energize a live circuit which is, or couldbecome, dangerous (electric shock or fire).An emergency stop is intended to arrest amovement which has become dangerous. Inthe two cases:c the emergency control device or its meansof operation (local or at remote location(s))such as a large red mushroom-headedemergency-stop pushbutton must berecognizable and readily accessible, inproximity to any position at which dangercould arise or be seen;c a single action must result in a completeswitching-off of all live conductors (1) (2);

c a "break glass" emergency switchinginitiation device is authorized, but inunmanned installations the re-energizing ofthe circuit can only be achieved by means ofa key held by an authorized person.

It should be noted that in certain cases, anemergency system of braking, may requirethat the auxiliary supply to the braking-systemcircuits be maintained until final stoppage ofthe machinery.(1) Taking into account stalled motors.(2) In a TN schema the PEN conductor must never beopened, since it functions as a protective earthing wire aswell as the system neutral conductor.

switching-off for mechanicalmaintenance workThis operation assures the stopping of amachine and its impossibility to beinadvertently restarted while mechanicalmaintenance work is being carried out on thedriven machinery. The shutdown is generallycarried out at the functional switching device,with the use of a suitable safety lock andwarning notice at the switch mechanism.


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2. the switchgear and fusegear

2.1 elementary switching devicesdisconnector (or isolator)This switch is a manually-operated, lockable,two-position device (open/closed) whichprovides safe isolation of a circuit whenlocked in the open position. Its characteristicsare defined in IEC 947-3.A disconnector is not designed to make or tobreak current* and no rated values for thesefunctions are given in standards. It must,however, be capable of withstanding thepassage of short-circuit currents and isassigned a rated short-time withstandcapability; generally for 1 second, unlessotherwise agreed between user andmanufacturer. This capability is normallymore than adequate for longer periods of(lower-valued) operational overcurrents, suchas those of motor-starting.Standardized mechanical-endurance,overvoltage, and leakage-current tests, mustalso be satisfied.* i.e. a LV disconnector is essentially a dead-system switching device to be operated withno voltage on either side of it, particularlywhen closing, because of the possibility of anunsuspected short-circuit on the downstreamside. Interlocking with an upstream switch orcircuit breaker is frequently used.

fig. H2-5: symbol for a disconnector(or isolator).

load-breaking switchThis control switch is generally operatedmanually (but is sometimes provided withelectrical tripping for operator convenience)and is a non-automatic two-position device(open/closed).It is used to close and open loaded circuitsunder normal unfaulted circuit conditions.It does not consequently, provide anyprotection for the circuit it controls.IEC standard 947-3 defines:c the frequency of switch operation(600 close/open cycles per hour maximum);c mechanical and electrical endurance(generally less than that of a contactor);c current making and breaking ratings fornormal and infrequent situations.IEC 947-3 also recognizes 3 categories ofload-breaking switch, each of which issuitable for a different range of load powerfactors, as shown in table H2-7.

fig. H2-6: symbol for a load-breakingswitch.

When closing a switch to energize a circuitthere is always the possibility that an(unsuspected) short circuit exists on thecircuit. For this reason, load-break switchesare assigned a fault-current making rating,i.e. successful closure against theelectrodynamic forces of short-circuit currentis assured. Such switches are commonlyreferred to as "fault-make load-break"switches.Upstream protective devices are relied uponto clear the short-circuit fault.


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nature utilization category typical applicationsof frequent infrequentcurrent operation operationalternating AC-20A AC-20B connecting and disconnectingcurrent under no-load conditions

AC-21A AC-21B switching of resistive loadsincluding moderate overloads

AC-22A AC-22B switching of mixed resistiveand inductive loads, includingmoderate overloads

AC-23A AC-23B switching of motor loads orother highly inductive loads

table H2-7: utilization categories of LV a.c. switches according to IEC 947-3.

Category AC-23 includes occasionalswitching of individual motors. The switchingof capacitors or of tungsten filament lampsshall be subject to agreement betweenmanufacturer and user.The utilization categories referred to in tableH2-7 do not apply to an equipment normallyused to start, accelerate and/or stopindividual motors. The utilization categoriesfor such an equipment are dealt with inchapter J, table J5-4.

Example:A 100 A load-break switch of category AC-23(inductive load) must be able:c to make a current of 10 In (= 1,000 A) at apower factor of 0.35 lagging;c to break a current of 8 In (= 800 A) at apower factor of 0.35 lagging;c to withstand short-circuit currents (not lessthan 12 In) passing through it for 1 second,where 12 In equals the r.m.s. value of the a.c.component, while the peak value (expressedin kA) is given by a factor "n" in table XVI ofIEC 947- Part 1, reproduced below for readerconvenience (table H2-8).

test current I power-factor time-constant n(A) (ms)

I i 01 500 0.95 5 1.41 1 500 < I i 3 000 0.9 5 1.42 3 000 < I i 4 500 0.8 5 1.47 4 500 < I i 6 000 0.7 5 1.53 6 000 < I i 10 000 0.5 5 1.710 000 < I i 20 000 0.3 10 2.020 000 < I i 50 000 0.25 15 2.150 000 < I 0.2 15 2.2

table H2-8: factor "n" used for peak-to-rms value (IEC 947-part1).

bistable switch (télérupteur)This device is extensively used in the controlof lighting circuits where the depression of apushbutton (at a remote control position) willopen an already-closed switch or close anopen switch in a bistable sequence.Typical applications are:c two-way switching on stairways of largebuildings;c stage-lighting schemes;c factory illumination, etc.Auxiliary devices are available to provide:c remote indication of its state at any instant;c time-delay functions;c maintained-contact features.

fig. H2-9: symbol for a bistable remotely-operated switch (télérupteur) .


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2. the switchgear and fusegear (continued)

contactorThe contactor is a solenoid-operatedswitching device which is generally heldclosed by (a reduced) current through theclosing solenoid (although variousmechanically-latched types exist for specificduties). Contactors are designed to carry outnumerous close/open cycles and arecommonly controlled remotely by on-offpushbuttons.The large number of repetitive operatingcycles is standardized in table VIII of IEC947-4-1 by:c the operating duration: 8 hours;uninterrupted; intermittent; temporary of 3,10, 30, 60 and 90 minutes;c utilization category: (for definition see tableJ5-4) for example, a contactor of categoryAC3 can be used for the starting andstopping of a cage motor;c the start-stop cycles (1 to 1,200 cyles perhour);c mechanical endurance (number of off-loadmanœuvres);c electrical endurance (number of on-loadmanœuvres);

c a rated current making and breakingperformance according to the category ofutilization concerned.

Example:A 150 A contactor of category AC3 must havea minimum current-breaking capability of 8 In(= 1,200 A) and a minimum current-makingrating of 10 In (= 1,500 A) at a power factor(lagging) of 0.35.

controlcircuit

power circuit

fig. H2-10: symbol for a contactor.

discontactor*A contactor equipped with a thermal-typerelay for protection against overloadingdefines a "discontactor". Discontactors areused extensively for remote push-buttoncontrol of lighting circuits, etc., and may alsobe considered as an essential element in amotor controller, as noted in sub-clause 2.2."combined switchgear elements".The discontactor is not the equivalent of a

circuit breaker, since its short-circuit current-breaking capability is limited to 8 or 10 In. Forshort-circuit protection therefore, it isnecessary to include either fuses or a circuitbreaker in series with, and upstream of, thediscontactor contacts.*This term is not defined in IEC publicationsbut is commonly used in some countries.

fusesFuses exist with and without "fuse-blown"mechanical indicators.Fuses break a circuit by controlled melting ofthe fuse element when a current exceeds agiven value for a corresponding period oftime; the current/time relationship beingpresented in the form of a performance curvefor each type of fuse.Standards define two classes of fuse:c those intended for domestic installations,manufactured in the form of a cartridge forrated currents up to 100 A and designatedtype gG in IEC 269-3;c those for industrial use, with cartridge typesdesignated gG (general use); and gM and aM(for motor-circuits) in IEC 269-1 and 2.The main differences between domestic andindustrial fuses are the nominal voltage andcurrent levels (which require much largerphysical dimensions) and their fault-currentbreaking capabilities.Type gG fuse-links are often used for theprotection of motor circuits, which is possiblewhen their characteristics are capable ofwithstanding the motor-starting currentwithout deterioration.A more recent development has been theadoption by the IEC of a fuse-type gM formotor protection, designed to cover starting,and short-circuit conditions. This type of fuseis more popular in some countries than inothers, but at the present time the aM fuse incombination with a thermal overload relay ismore-widely used.

two classes of LV cartridge fuse arevery widely used:c for domestic and similarinstallations type gGc for industrial installations type gG,gM or aM.

A gM fuse-link, which has a dual rating ischaracterized by two current values. The firstvalue In denotes both the rated current of thefuse-link and the rated current of the fuse-holder; the second value Ich denotes thetime-current characteristic of the fuse-link asdefined by the gates in Tables II, III and VI ofIEC 269-1.These two ratings are separated by a letterwhich defines the applications.For example: In M Ich denotes a fuseintended to be used for protection of motorcircuits and having the characteristic G. Thefirst value In corresponds to the maximumcontinuous current for the whole fuse and thesecond value Ich corresponds to the Gcharacteristic of the fuse link. For furtherdetails see note at the end of sub-clause 2.1.An aM fuse-link is characterized by onecurrent value In and time-currentcharacteristic as shown in figure H2-14.Important: Some national standards use a gI(industrial) type fuse, similar in all mainessentails to type gG fuses.Type gI fuses should never be used,however, in domestic and similar installations.

fig. H2-11: symbol for fuses.

2.1 elementary switching devices (continued)


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fusing zones -conventional currentsThe conditions of fusing (melting) of a fuseare defined by standards, according to theirclass.c class gG fusesThese fuses provide protection againstoverloads and short-circuits.Conventional non-fusing and fusing currentsare standardized, as shown in figure H2-12and in table H2-13.v the conventional non-fusing current Inf isthe value of current that the fusible elementcan carry for a specified time without melting.Example: a 32 A fuse carrying a current of1.25 In (i.e. 40 A) must not melt in less thanone hour (table H2-13)v the conventional fusing current If (=I2 infig.H2-12) is the value of current which willcause melting of the fusible element beforethe expiration of the specified time.Example: a 32 A fuse carrying a current of1.6 In (i.e. 52.1 A) must melt in one hour orless (table H2-13).IEC 269-1 standardized tests require that afuse-operating characteristic lies between thetwo limiting curves (shown in figure H2-12) forthe particular fuse under test. This meansthat two fuses which satisfy the test can havesignificantly different operating times at lowlevels of overloading.v the two examples given above for a 32 Afuse, together with the foregoing notes onstandard test requirements, explain why

these fuses have a poor performance in thelow overload range.v it is therefore necessary to install a cablelarger in ampacity than that normally requiredfor a circuit, in order to avoid theconsequences of possible long termoverloading (60% overload for up to one hourin the worst case).By way of comparison, a circuit breaker ofsimilar current rating:v which passes 1.05 In must not trip in lessthan one hour; andv when passing 1.25 In it must trip in onehour, or less(25% overload for up to one hour in the worstcase).

gM fuses require a separate overloadrelay, as described in the note at theend of sub-clause 2.1.

I

t

Inf

minimumpre-arcingtime curve

fuse-blowncurve

I2

1h.

fig. H2-12: zones of fusing and non-fusingfor gG and gM fuses.

class rated current* conventional non- conventional fusing conventionalIn (A) fusing current current If time h

Inf I2gG In i 4 A 1.5 In 2.1 In 1gM 4 < In < 16 A 1.5 In 1.9 In 1

16 < In i 63 A 1.25 In 1.6 In 163 < In i 160 A 1.25 In 1.6 In 2160 < In i 400 A 1.25 In 1.6 In 3400 < In 1.25 In 1.6 In 4

table H2-13: zones of fusing and non-fusing for LV types gG and gM class fuses (IEC 269-1and 269-2-1).

* Ich for gM fuses

c class aM (motor) fusesThese fuses afford protection against short-circuit currents only and must necessarily beassociated with other switchgear (such asdiscontactors or circuit breakers) in order toensure overload protection < 4 In. They arenot therefore autonomous. Since aM fusesare not intended to protect against low valuesof overload current, no levels of conventionalnon-fusing and fusing currents are fixed. Thecharacteristic curves for testing these fusesare given for values of fault current exceedingapproximately 4 In (see figure H2-14), andfuses tested to IEC 269 must give operatingcurves which fall within the shaded area.Note: the small "arrowheads" in the diagramindicate the current/time "gate" values for thedifferent fuses to be tested (IEC 269).

class aM fuses protect againstshort-circuit currents only, and mustalways be associated with anotherdevice which protects againstoverload.

x In

t

4In

fuse-blowncurve

minimumpre-arcingtime curve

fig. H2-14: standardized zones of fusingfor type aM fuses (all current ratings).


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rated short-circuit breakingcurrentsA characteristic of modern cartridge fuses isthat, owing to the rapidity of fusion in the caseof high short-circuit current levels*, a currentcut-off begins before the occurrence of thefirst major peak, so that the fault currentnever reaches its prospective peak value(fig. H2-15).This limitation of current reduces significantlythe thermal and dynamic stresses whichwould otherwise occur, thereby minimizingdanger and damage at the fault position.The rated short-circuit breaking current of thefuse is therefore based on the r.m.s. value ofthe a.c. component of the prospective faultcurrent.No short-circuit current-making rating isassigned to fuses.*for currents exceeding a certain level, depending on thefuse nominal current rating, as shown below in figureH2-15A.

ReminderShort-circuit currents initially contain d.c.components, the magnitude and duration ofwhich depend on the XL/R ratio of the fault-current loop.Close to the source (HV/LV transformer) therelationship I peak / Irms (of a.c. component)immediately following the instant of fault, canbe as high as 2.5 (standardized by IEC, andshown in figure H2-15A).At lower levels of distribution in aninstallation, as previously noted, XL is smallcompared with R and so for final circuits Ipeak / I rms ~ 1.41, a condition whichcorresponds with figure H2-15 above andwith the "n" value corresponding to a powerfactor of 0.95 in table H2-8.

The peak-current-limitation effect occurs onlywhen the prospective r.m.s. a.c. componentof fault current attains a certain level. Forexample, in the above graph the 100 A fusewill begin to cut off the peak at a prospectivefault current (r.m.s.) of 2 kA (a). The samefuse for a condition of 20 kA r.m.s.prospective current will limit the peak currentto 10 kA (b). Without a current-limiting fusethe peak current could attain 50 kA (c) in thisparticular case.As already mentioned, at lower distributionlevels in an installation, R greatlypredominates XL, and fault levels aregenerally low.This means that the level of fault current maynot attain values high enough to cause peak-current limitation. On the other hand, the d.c.transients (in this case) have an insignificanteffect on the magnitude of the current peak,as previously mentioned.

Note on gM fuse ratings.A gM type fuse is essentially a gG fuse, thefusible element of which corresponds to thecurrent value Ich (ch = characteristic) whichmay be, for example, 63 A.This is the IEC testing value, so that its time/current characteristic is identical to that of a63 A gG fuse.This value (63 A) is selected to withstand thehigh starting currents of a motor, the steady-state operating current (In) of which may bein the 10-20 A range.

t

I

0.005s

0.02s

0.01s

prospectivefault-current peakrms value of the a.c.component of theprospective fault currentcurrent peaklimited by the fuse

Tf TaTtc

Tf: fuse pre-arc fusing timeTa: arcing timeTtc: total fault-clearance time

fig. H2-15: current limitation by a fuse.

1 2 5 10 20 50 1001

2

5

10

20

50

100

(a)

(b)

(c)

peak currentcut-offcharacteristiccurves

maximum possible current peak characteristic i.e. 2.5Ir.m.s. (IEC)

160A

100A

50A

nominalfuseratings

prospective fault current (kA) peak

a.c. component of prospectivefault current (kA) r.m.s.

fig. H2-15A: limited peak current versusprospective r.m.s. values of the a.c.component of fault current for LV fuses.

2.1 elementary switching devices (continued)


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This means that a physically smaller fusebarrel and metallic parts can be used, sincethe heat dissipation required in normalservice is related to the lower figures(10-20 A).A standard gM fuse, suitable for this situationwould be designated 32M63 (i.e. In M Ich).The first current rating In concerns thesteady-load thermal performance of the fuse-link, while the second current rating (Ich)relates to its (short-time) starting-currentperformance.It is evident that, although suitable for short-circuit protection, overload protection for themotor is not provided by the fuse, and so aseparate thermal-type relay is alwaysnecessary when using gM fuses.The only advantage offered by gM fuses,therefore, when compared with aM fuses, arereduced physical dimensions and slightlylower cost.

2.2 combined switchgear elementsSingle units of switchgear do not, in general,fulfil all the requirements of the three basicfunctions, viz: protection, control andisolation.Where the installation of a circuit breaker isnot appropriate (notably where the switchingrate is high, over extended periods)combinations of units specifically designed forsuch a performance are employed.The most commonly-used combinations aredescribed below:

switch and fuse combinationsTwo cases are distinguished:c the type in which the operation of one(or more) fuse(s) causes the switch to open.This is achieved by the use of fuses fittedwith striker pins, and a system of switchtripping springs and toggle mechanisms.This type of combination is generally used forcurrent levels exceeding 100 A, and iscommonly associated with a thermal-typeovercurrent relay for overload protection (forwhich the fuses alone may not be suitable).If the switch is classified as AC22 or AC23,and associated with a motor-overload type ofthermal relay, the ensemble, i.e. switch,striker-pin fuses and overload relay, issuitable for the control and protection of amotor circuit,and:c the type in which a non-automatic switch isassociated with a set of fuses in a commonenclosure.In some countries, and in IEC 947-3, theterms "switch-fuse" and "fuse-switch" havespecific meanings, viz:v a switch-fuse comprises a switch (generally2 breaks per pole) on the upstream side ofthree fixed fuse-bases, into which the fusecarriers are inserted (figure H2-17(a)),v a fuse-switch consists of three switchblades each constituting a double-break perphase.These blades are not continuous throughouttheir length, but each has a gap in the centrewhich is bridged by the fuse cartridge. Somedesigns have only a single break per phase,as shown in figures H2-17(a) and (b).

fig. H2-16: symbol for an automatic-tripping switch-fuse, with a thermaloverload relay.

fig. H2-17 (a): symbol for a non-automaticswitch-fuse.

fig. H2-17 (b): symbol for a non-automaticfuse-switch.


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The current range for these devices is limitedto 100 A maximum at 400 V 3-phase, whiletheir principal use is in domestic and similarinstallations.To avoid confusion between the first group(i.e. automatic tripping) and the secondgroup, the term "switch-fuse" should bequalified by the adjectives "automatic" or"non-automatic".

fuse - disconnector+ discontactorfuse - switch-disconnector+ discontactorAs previously mentioned, a discontactor doesnot provide protection against short-circuitfaults. It is necessary, therefore, to add fuses(generally of type aM) to perform thisfunction.The combination is used mainly for motor-control circuits, where the disconnector orswitch-disconnector allows safe operationssuch as:c the changing of fuse links (with the circuitisolated);c work on the circuit downstream of thediscontactor (risk of remote closure of thediscontactor).

The fuse-disconnector must be interlockedwith the discontactor such that no opening orclosing manœuvre of the fuse-disconnector ispossible unless the discontactor is open(figure H2-18 (a)), since the fuse-disconnector has no load-switching capability.A fuse-switch-disconnector (evidently)requires no interlocking (figure H2-18 (b)).The switch must be of class AC22 or AC23 ifthe circuit supplies a motor.

fig. H2-18 (a): symbol for a fuse-disconnector + discontactor.

fig. H2-18 (b): symbol for a fuse-switch-disconnector + discontactor.

circuit-breaker + contactorcircuit-breaker + discontactorThese combinations are used in remotely-controlled distribution systems in which therate of switching is high, or for control andprotection of a circuit supplying motors.The protection of induction motors isconsidered in chapter J, clause J5.

2.2 combined switchgear elements (continued)



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3. choice of switchgear

3.1 tabulated functional capabilitiesAfter having studied the basic functions of LVswitchgear (clause 1, table H2-1) and thedifferent components of switchgear(clause 2), table H2-19 summarizes thecapabilities of the various componentsto perform the basic functions.

isolation control electrical protectionswitchgear functional emergency switching emergency stop switching for overload short-circuit differentialitem (mechanical) mechanical

maintenanceisolator (or cdisconnector)(4)switch (5) c c c (1) c (1) (2) cresidual c c c (1) c (1) (2) c cdevice(RCCB) (5)switch- c c c (1) c (1) (2) cdisconnectorcontactor c c (1) c (1) (2) c c (3)bistable-switch c c (1) c(telerupteur)fuse c c ccircuit c c (1) c (1) (2) c c cbreaker (5)circuit breaker c c c (1) c (1) (2) c c cdisconnector(5)residual c c c (1) c (1) (2) c c c candovercurrentcircuit breaker(RCBO) (5)point of origin of all points where, in general at the at the supply at the supply origin origin origin ofinstallation each for operational incoming circuit point to each point to each of each of each circuits where(general circuit reasons it may to every distribution machine machine circuit circuit the earthingprinciple) be necessary board and/or on the system is

to stop the machine appropriateprocess concerned TN-S, IT, TT

table H2-19: functions fulfilled by different items of switchgear.

(1) Where cut-off of all active conductors is provided(2) It may be necessary to maintain supply to a braking system(3) If it is associated with a thermal relay (the combination is commonly referred to as a "discontactor")(4) In certain countries a disconnector with visible contacts is mandatory at the origin of a LV installation supplied directly from a HV/LV transformer(5) Certain items of switchgear are suitable for isolation duties (e.g. RCCBs according to IEC 1008) without being explicitly marked as such.

3.2 switchgear selectionSoftware is being used more and more in thefield of optimal selection of switchgear.Each circuit is considered one at a time, anda list is drawn up of the required protectionfunctions and exploitation of the installation,among those mentioned in table H2-19 andsummarized in table H2-1.A number of switchgear combinations arestudied and compared with each otheragainst relevant criteria, with the aim ofachieving:c satisfactory performance;c compatibility among the individual items;from the rated current In to the fault-levelrating Icu;c compatibility with upstream switchgear ortaking into account its contribution;c conformity with all regulations andspecifications concerning safe and reliablecircuit performance.

In order to determine the number of poles foran item of switchgear, reference is made tochapter H1, clause 7, table H1-65.Multifunction switchgear, initially more costly,reduces installation costs and problems ofinstallation or exploitation. It is often foundthat such switchgear provides the bestsolution.


H2

4. circuit breakers

the circuit breaker/disconnector fulfillsall of the basic switchgear functions,while, by means of accessories,numerous other possibilities exist.

As shown in table H2-19 the circuit breaker/disconnector is the only item of switchgearcapable of simultaneously satisfying all thebasic functions necessary in an electricalinstallation.Moreover, it can, by means of auxiliary units,provide a wide range of other functions, forexample: indication (on-off - tripped on fault);undervoltage tripping; remote control… etc.These features make a circuit-breaker/disconnector the basic unit of switchgear forany electrical installation.

functions possible conditionsisolation ccontrol functional c

emergency switching c (with the possibility of a trippingcoil for remote control)

switching-off for mechanical cmaintenance

protection overload cshort-circuit cinsulation faulty c (with differential-current relay)undervoltage c (with undervoltage-trip coil)

remote control c added or incorporatedindication and measurement c (generally optional with an electronic

tripping device)

table H2.20: functions performed by a circuit-breaker/disconnector.

4.1 standards and descriptions

industrial circuit breakers mustconform with IEC 947-1 and 947-2 orother equivalent standards.Corresponding European standardsare presently being developed.Domestic-type circuit breakersshould conform to IEC standard 898,or an equivalent national standard.

standardsFor industrial LV installations the relevant IECstandards are, or are due to be:c 947-1: general rules;c 947-2: part 2: circuit breakers;c 947-3: part 3: switches, disconnectors,switch-disconnectors and fuse combinationunits;c 947-4: part 4: contactors and motor-starters;c 947-5: part 5: control-circuit devices andswitching elements;c 947-6: part 6: multiple function switchingdevices;c 947-7: part 7: ancillary equipment.Corresponding European and many nationalstandards are presently in the course ofharmonization with the IEC standards, withwhich they will be in very close agreement.For domestic and similar LV installations, theappropriate standard is IEC 898, or anequivalent national standard.


H2

descriptionFigure H2-21 shows schematically theprincipal parts of a LV circuit breaker and itsfour essential functions:1 - the circuit-breaking components,comprising the fixed and moving contactsand the arc-dividing chamber.2 - the latching mechanism whichbecomes unlatched by the tripping deviceon detection of abnormal currentconditions.This mechanism is also linked to theoperation handle of the breaker.

3 - a trip-mechanism actuating device:c either: a thermal-magnetic device, in whicha thermally-operated bi-metal strip detects anoverload condition, while an electromagneticstriker pin operates at current levels reachedin short-circuit conditions, or:c an electronic relay operated from currenttransformers, one of which is installed oneach phase.4 - a space allocated to the several types ofterminal currently used for the main power-circuit conductors.

power circuit terminals

trip mechanism and protective devices

latching mechanism

contacts and arc-dividing chamber

fool-proof mechanical indicator

fig. H2-21: principal parts of a circuit breaker.

domestic circuit breakers conformingto IEC 898 and similar nationalstandards perform the basicfunctions of:c isolationc protection against overcurrent.

fig. H2-22: domestic-type circuit breakerproviding overcurrent protection andcircuit isolation features.

some models can be adapted toprovide sensitive detection (30 mA)of earth-leakage current with CBtripping, by the addition of a modularblock, as shown in figure H2-23,while other models (complying withIEC 1009) have this residual-currentfeature incorporated, viz. RCBOs.,and, more recently, IEC 947-2(appendix B) CBRs.

fig. H2-23: domestic-type circuit breakeras above (H2-22) plus protection againstelectric shocks by the addition of amodular block.


H2

4. circuit breakers (continued)

4.1 standards and descriptions (continued)

apart from the above-mentionedfunctions further features can beassociated with the basic circuitbreaker by means of additionalmodules, as shown in figure H2-24;notably remote control and indication(on-off-fault).

fig. H2-24: "Multi 9" system* of LV modular switchgear components.

moulded-case type industrial circuitbreakers conforming to IEC 947-2are now available, which, by meansof associated adaptable blocksprovide a similar range of auxiliaryfunctions to those described above(figure H2-25).

fig. H2-25: example of a modular (Compact NS*) industrial type of circuit breaker capableof numerous auxiliary functions.* Merlin Gerin product.

O-OFF

O-OFF

O-OFFO-OFF

O-OFF

54

32

1

SDE

OF1SD

OF2

OF2SDE

SD

OF1


H2

heavy-duty industrial circuit breakersof large current ratings, conforming toIEC 947-2, have numerous built-incommunication and electronicfunctions (figure H2-26).

fig. H2-26: examples of heavy-duty industrial circuit breakers. The "Masterpact"* providesmany automation features in its tripping module.

These circuit breakers are provided withmeans to adjust protective-device settingsover a wide range, and also with:c a 20 mA output loop;c remote indication contacts;c load indication at the CB.

4.2 fundamental characteristics of a circuit breaker

the fundamental characteristics of acircuit breaker are:c its rated voltage Uec its rated current Inc its tripping-current-level adjustmentranges for overload protection (Ir** orIrth**) and for short-circuit protection(Im)**c its short-circuit current breakingrating (Icu for industrial CBs; Icn fordomestic-type CBs).

rated operational voltage (Ue)This is the voltage at which the circuit breakerhas been designed to operate, in normal(undisturbed) conditions.Other values of voltage are also assigned tothe circuit breaker, corresponding to disturbedconditions, as noted in sub-clause 4.3.

rated current (In)This is the maximum value of current that acircuit breaker, fitted with a specified over-current tripping relay, can carry indefinitely atan ambient temperature stated by themanufacturer, without exceeding thespecified temperature limits of the current-carrying parts.

Example:A circuit breaker rated at In = 125 A for anambient temperature of 40 °C will beequipped with a suitably calibratedovercurrent tripping relay (set at 125 A).The same circuit breaker can be used athigher values of ambient temperaturehowever, if suitably "derated".Thus, the circuit breaker in an ambienttemperature of 50 °C could carry only 117 Aindefinitely, or again, only 109 A at 60 °C,while complying with the specifiedtemperature limit.Derating a circuit breaker is achievedtherefore, by reducing the trip-current settingof its overload relay, and marking the CBaccordingly. The use of an electronic-type oftripping unit, designed to withstand hightemperatures, allows circuit breakers (deratedas described) to operate at 60 °C (or even at70 °C) ambient.

Note: In for circuit breakers (in IEC 947-2) isequal to Iu for switchgear generally, Iu beingrated uninterrupted current.

frame-size ratingA circuit breaker which can be fitted withovercurrent tripping units of different current-level-setting ranges, is assigned a ratingwhich corresponds with that of the highestcurrent-level-setting tripping unit that can befitted.

* Merlin Gerin products.** Current-level setting values which refer to the current-operated thermal and "instantaneous" magnetic tripping devices forover-load and short-circuit protection.


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4.2 fundamental characteristics of a circuit breaker (continued)

overload relay trip-currentsetting (Irth or Ir)Apart from small circuit breakers which arevery easily replaced, industrial circuitbreakers are equipped with removable, i.e.exchangeable, overcurrent-trip relays.Moreover, in order to adapt a circuit breakerto the requirements of the circuit it controls,and to avoid the need to install over-sizedcables, the trip relays are generallyadjustable.The trip-current setting Ir or Irth (bothdesignations are in common use) is thecurrent above which the circuit breaker willtrip. It also represents the maximum currentthat the circuit breaker can carry withouttripping.

That value must be greater than themaximum load current IB, but less than themaximum current permitted in the circuit Iz(see chapter H1, sub-clause 1.3).The thermal-trip relays are generallyadjustable from 0.7 to 1.0 times In, but whenelectronic devices are used for this duty, theadjustment range is greater; typically 0.4 to1 times In.Example (figure H2-27): a circuit breakerequipped with a 320 A overcurrent trip relay,set at 0.9, will have a trip-current setting:Ir = 320 x 0.9 = 288 ANote: for circuit breakers equipped with non-adjustableovercurrent-trip relays, Ir = In.

fig. H2-27: example of a 400 A circuit breaker equipped with a 320 A overload trip unitadjusted to 0.9, to give Ir = 288 A.

short-circuit relay trip-currentsetting (Im)Short-circuit tripping relays (instantaneous orslightly time-delayed) are intended to trip thecircuit breaker rapidly on the occurrence ofhigh values of fault current.Their tripping threshold Im is:c either fixed by standards for domestic typeCBs, e.g. IEC 898, or,c indicated by the manufacturer for industrial-type CBs according to related standards,notably IEC 947-2.

For the latter circuit breakers there exists awide variety of tripping devices which allow auser to adapt the protective performance ofthe circuit breaker to the particularrequirements of a load.

type of pro- overload short-circuittective relay protection protection

domestic thermal- Ir = In low setting standard setting high setting circuitbreakers magnetic type B type C type DIEC 898 3 In i Im < 5 In 5 In i Im < 10 In 10 In i Im < 20 In (1)modular thermal- Ir = In low setting standard setting high settingindustrial (2) magnetic fixed type B or Z type C type D or Kcircuit breakers 3.2 In < fixed < 4.8 In 7 In < fixed < 10 In 10 In < fixed < 14 Inindustrial (2) thermal- Ir = In fixed fixed: Im ≈ 7 to 10 Incircuit breakers magnetic adjustable: adjustable:IEC 947-2 0.7 In i Ir < In - low setting : 2 to 5 In

- standard setting: 5 to 10 Inelectronic long delay short-delay, adjustable

0.4 In i Ir < In 1.5 Ir i Im < 10Irinstantaneous (I) fixedI ≈ 12 to 15 In

table H2.28: tripping-current ranges of overload and short-circuit protective devices for LVcircuit breakers.(1) 50 In in IEC 898, which is considered to be unrealistically high by most European manufacturers (M-G = 10 to 14 In).(2) For industrial use, IEC standards do not specify values. The above values are given only as being those in common use.

overload trip current settingto suit the circuit

Ir

adjustment range

rated current of the tripping unitto suit the circumstances

In0.7 In

circuit-breakerframe-size rating

224 A 288 A 320 A 400 A I


H2

fig. H2-29: performance curve of a circuitbreaker thermal-magnetic protectivescheme.

fig. H2-30 : performance curve of a circuitbreaker electronic protective scheme.

Ir: overload (thermal or short-delay) relay trip-current setting.Im: short-circuit (magnetic or long-delay) relay trip-current setting.I: short-circuit instantaneous relay trip-current setting.PdC: breaking capacity.

isolating featureA circuit breaker is suitable for isolating acircuit if it fulfills all the conditions prescribedfor a disconnector (at its rated voltage) in therelevant standard (see sub-clause 1.2).In such a case it is referred to as a circuitbreaker-disconnector and marked on its frontface with the symbol

All Multi 9, Compact NS and Masterpact LVswitchgear of Merlin Gerin manufacture is inthis category.

rated short-circuit breakingcapacity (Icu or Icn)The short-circuit current-breaking rating of aCB is the highest (prospective) value ofcurrent that the CB is capable of breakingwithout being damaged.The value of current quoted in the standardsis the r.m.s. value of the a.c. component ofthe fault current, i.e. the d.c. transientcomponent (which is always present in theworst possible case of short-circuit) isassumed to be zero for calculating thestandardized value. This rated value (Icu) forindustrial CBs and (Icn) for domestic-typeCBs is normally given in kA r.m.s.Icu (rated ultimate s.c. breaking capacity) andIcs (rated service s.c. breaking capacity) aredefined in IEC 947-2 together with a tablerelating Ics with Icu for different categories ofutilization A (instantaneous tripping) and B(time-delayed tripping) as discussed in sub-clause 4.3.Tests for proving the rated s.c. breakingcapacities of CBs are governed by standards,and include:c operating sequences, comprising asuccession of manœuvres, i.e. closing andopening on short-circuit;c current and voltage phase displacement.When the current is in phase with the supplyvoltage (cos ϕ for the circuit = 1), interruptionof the current is easier than that at any otherpower factor.Breaking a current at low lagging* values ofcos ϕ is considerably more difficult toachieve; a zero power-factor circuit being(theoretically) the most onerous case.

the short-circuit current-breakingperformance of a LV circuit breaker isrelated (approximately) to the cos ϕof the fault-current loop. Standardvalues for this relationship have beenestablished in some standards.

In practice, all power-system short-circuit faultcurrents are (more-or-less) at lagging powerfactors, and standards are based on valuescommonly considered to be representative ofthe majority of power systems.In general, the greater the level of faultcurrent (at a given voltage), the lower thepower factor of the fault-current loop, forexample, close to generators or largetransformers.

Table H2-31 below extracted from IEC 947-2relates standardized values of cos ϕ toindustrial circuit breakers according to theirrated Icu.

Icu cos ϕ6 kA < Icu i 10 kA 0.510 kA < Icu i 20 kA 0.320 kA < Icu i 50 kA 0.2550 kA i Icu 0.2

table H2-31: Icu related to power factor(cos ϕ) of fault-current circuit. (IEC 947-2).

c following an open - time delay - close/opensequence to test the Icu capacity of a CB,further tests are made to ensure thatv the dielectric withstand capability;v the disconnection (isolation) performanceandv the correct operation of the overloadprotection,have not been impaired by the test.

I(A)

Im

t (s)

Ir PdC

I(A)

Im

t (s)

Ir PdCI


H2


4.3 other characteristics of a circuit breakerFamiliarity with the following less-importantcharacteristics of LV circuit breakers is,however, often necessary when making afinal choice.

rated insulation voltage (Ui)This is the value of voltage to which thedielectric tests voltage (generally greaterthan 2 Ui) and creepage distances arereferred.The maximum value of rated operationalvoltage must never exceed that of the ratedinsulation voltage, i.e. Ue i Ui.

rated impulse-withstand voltage(Uimp)This characteristic expresses, in kV peak (ofa prescribed form and polarity) the value ofvoltage which the equipment is capable ofwithstanding without failure, under testconditions.For further details see chapter F, clause 2.

category (A or B) and ratedshort-time withstand current(Icw)As already briefly mentioned (sub-clause 4.2)there are two categories of LV industrialswitchgear, A and B, according to IEC 947-2:c those of category A, for which there is nodeliberate delay in the operation of the"instantaneous" short-circuit magnetic-tripping device (figure H2-32), are generallymoulded-case type circuit breakers, and,c those of category B for which, in order todiscriminate with other circuit breakers on atime basis, it is possible to delay the trippingof the CB, where the fault-current level islower than that of the short-time withstandcurrent rating (Icw) of the CB (figure H2-23).This is generally applied to large open-typecircuit breakers and to certain heavy-dutymoulded-case types. Icw is the maximumcurrent that the B category CB can withstand,thermally and electrodynamically, withoutsustaining damage, for a period of time givenby the manufacturer.

fig. H2-32: category A circuit breaker.

fig. H2-33: category B circuit breaker.

I(A)

Im

t (s)

I(A)

Im

t (s)

I PdCIcw


H2

rated making capacity (Icm)Icm is the highest instantaneous value ofcurrent that the circuit breaker can establishat rated voltage in specified conditions. In a.c.systems this instantaneous peak value isrelated to Icu (i.e. to the rated breakingcurrent) by the factor k, which depends on thepower factor (cos ϕ) of the short-circuitcurrent loop (as shown in table H2-34).

Example: a LV circuit breaker has a ratedbreaking capacity Icu of 100 kA r.m.s.Its rated making capacity Icm will be100 x 2.2 = 220 kA peak.

Icu cos ϕ Icm = kIcu6 kA < Icu i 10 kA 0.5 1.7 x Icu10 kA < Icu i 20 kA 0.3 2 x Icu20 kA < Icu i 50 kA 0.25 2.1 x Icu50 kA i Icu 0.2 2.2 x Icu

table H2.34: relation between ratedbreaking capacity Icu and rated makingcapacity Icm at different power-factorvalues of short-circuit current, asstandardized in IEC 947-2.

rated service short-circuitbreaking capacity (Ics)The rated breaking capacity (Icu) or (Icn) isthe maximum fault-current a circuit breakercan successfully interrupt without beingdamaged. The probability of such a currentoccurring is extremely low, and in normalcircumstances the fault-currents areconsiderably less than the rated breakingcapacity (Icu) of the CB. On the other hand itis important that high currents (of lowprobability) be interrupted under goodconditions, so that the CB is immediatelyavailable for reclosure, after the faulty circuithas been repaired.It is for these reasons that a new

characteristic (Ics) has been created,expressed as a percentage of Icu, viz: 25, 50,75, 100% for industrial circuit breakers.The standard test sequence is as follows:c O - CO - CO* (at Ics);c tests carried out following this sequenceare intended to verify that the CB is in a goodstate and available for normal service.For domestic CBs, Ics = k Icn. The factor kvalues are given in IEC 898 table XIV.In Europe it is the industrial practice to use ak factor of 100% so that Ics = Icu.Note: O represents an opening operation.CO represents a closing operation followed by an openingoperation.

fault-current limitationThe fault-current limitation capacity of a CBconcerns its ability, more or less effective, inpreventing the passage of the maximumprospective fault-current, permitting only alimited amount of current to flow, as shown infigure H2-35.The current-limitation performance is given bythe CB manufacturer in the form of curves(figure H2-36 diagrams (a) and (b)).c diagram (a) shows the limited peak value ofcurrent plotted against the r.m.s. value of thea.c. component of the prospective faultcurrent ("prospective" fault-current refers tothe fault-current which would flow if the CBhad no current-limiting capability);c limitation of the current greatly reduces thethermal stresses (proportional I2t) and this isshown by the curve of diagram (b) of figureH2-36, again, versus the r.m.s. value of thea.c. component of the prospective faultcurrent.LV circuit breakers for domestic and similarinstallations are classified in certainstandards (notably European StandardEN 60 898). CBs belonging to a class (of

current limiters) have standardized limiting I2tlet-through characteristics defined by thatclass.In these cases, manufacturers do notnormally provide characteristic performancecurves.

in a correctly designed installation,a circuit breaker is never required tooperate at its maximum breakingcurrent Icu.For this reason a new characteristicIcs has been introduced.It is expressed in IEC 947-2 as apercentage of Icu (25, 50, 75, 100%).

many designs of LV circuit breakersfeature a short-circuit currentlimitation capability, whereby thecurrent is reduced and preventedfrom reaching its (otherwise)maximum peak value (figure H2-35).The current-limitation performance ofthese CBs is presented in the form ofgraphs, typified by that shown infigure H2-36, diagram (a).

fig. H2-35: prospective and actualcurrents.

limitedpeakcurrent(kA)

non-

limite

d cu

rrent

char

acte

ristic

s

150

22

prospective a.c.component (r.m.s.)

150 kA

limited peak current (A2 x s)

2.105

4,5.105

prospective a.c.component (r.m.s.)

fig. H2-36: performance curves of a typical LV current-limiting circuit breaker.

(a) (b)

Icc

t

limited current peak

limitedcurrent

tc

prospecticefault-current

prospecticefault-current peak


H2


current limitation reduces boththermal and electrodynamic stresseson all circuit elements through whichthe current passes, therebyprolonging the useful life of theseelements. Furthermore, the limitationfeature allows "cascading"techniques to be used (see 4.5)thereby significantly reducing designand installation costs.

the advantagesof current limitationThe use of current-limiting CBs affordsnumerous advantages:c better conservation of installation networks:current-limiting CBs strongly attenuate allharmful effects associated with short-circuitcurrents;c reduction of thermal effects:conductors (and therefore insulation) heatingis significantly reduced, so that the life ofcables is correspondingly increased;c reduction of mechanical effects:forces due to electromagnetic repulsion arelower, with less risk of deformation andpossible rupture, excessive burning ofcontacts, etc.;c reduction of electromagnetic-interferenceeffects:less influence on measuring instruments andassociated circuits, telecommunicationsystems, etc.These circuit breakers therefore contributetowards an improved exploitation of:c cables and wiring;c prefabricated cable-trunking systems;c switchgear, thereby reducing the ageing ofthe installation.

Example:On a system having a prospective short-circuit current of 150 kA r.m.s., a circuitbreaker limits the peak current to less than10% of the calculated prospective peakvalue, and the thermal effects to less than 1%of those calculated.Cascading of the several levels of distributionin an installation, downstream of a limitingCB, will also result in important economies.The technique of cascading, described insub-clause 4.5 allows, in fact, substantialsavings on switchgear (lower performancepermissible downstream of the limiting CB(s))enclosures, and design studies, of up to 20%(overall).Discriminative protection schemes andcascading are compatible, in the rangeCompact NS*, up to the full short-circuitbreaking capacity of the switchgear.* A Merlin Gerin product.

4.4 selection of a circuit breaker

the choice of a range of circuitbreakers is determined by:the electrical characteristics of theinstallation, the environment, theloads and a need for remote control,together with the type oftelecommunications systemenvisaged.

choice of a circuit breakerThe choice of a CB is made in terms of:c electrical characteristics of the installationfor which the CB is destined;c its eventual environment: ambienttemperature, in a kiosk or switchboardenclosure, climatic conditions, etc.;c short-circuit current breaking and makingrequirements;c operational specifications: discriminativetripping, requirements (or not) for remotecontrol and indication and related auxiliarycontacts, auxiliary tripping coils, connectioninto a local network (communication orcontrol and indication) etc.,

c installation regulations; in particular:protection of persons;c load characteristics, such as motors,fluorescent lighting, LV/LV transformers, etc.Problems concerning specific loads areexamined in chapter J.The following notes relate to the choice of aLV circuit breaker for use in distributionsystems.

choice of rated current in termsof ambient temperatureThe rated current of a circuit breaker isdefined for operation at a given ambienttemperature, in general:c 30 °C for domestic-type CBs;c 40 °C for industrial-type CBs.Performance of these CBs in a differentambient temperature depends principally onthe technology of their tripping units.

ambienttemperature

ambienttemperature

temperature of airsurrounding the circuit breakers

single CBin free air

circuit breakers installedin an enclosure

fig. H2-37: ambient temperature.

4.3 other characteristics of a circuit breaker (continued)


H2

C60a. C60H: curve C. C60N: curves B and C (reference temperature: 30 °C)rating (A) 20 °C 25 °C 30 °C 35 °C 40 °C 45 °C 50 °C 55 °C 60 °C1 1.05 1.02 1.00 0.98 0.95 0.93 0.90 0.88 0.852 2.08 2.04 2.00 1.96 1.92 1.88 1.84 1.80 1.743 3.18 3.09 3.00 2.91 2.82 2.70 2.61 2.49 2.374 4.24 4.12 4.00 3.88 3.76 3.64 3.52 3.36 3.246 6.24 6.12 6.00 5.88 5.76 5.64 5.52 5.40 5.3010 10.6 10.3 10.0 9.70 9.30 9.00 8.60 8.20 7.8016 16.8 16.5 16.0 15.5 15.2 14.7 14.2 13.8 13.520 21.0 20.6 20.0 19.4 19.0 18.4 17.8 17.4 16.825 26.2 25.7 25.0 24.2 23.7 23.0 22.2 21.5 20.732 33.5 32.9 32.0 31.4 30.4 29.8 28.4 28.2 27.540 42.0 41.2 40.0 38.8 38.0 36.8 35.6 34.4 33.250 52.5 51.5 50.0 48.5 47.4 45.5 44.0 42.5 40.563 66.2 64.9 63.0 61.1 58.0 56.7 54.2 51.7 49.2

circuit breakers with uncompensatedthermal tripping units have a trip-current level that depends on thesurrounding temperature.

uncompensated thermal-magnetic tripping unitsCircuit breakers with uncompensated thermaltripping elements have a tripping-current levelthat depends on the surroundingtemperature. If the CB is installed in anenclosure, or in a hot location (boiler room,etc.), the current required to trip the CB onoverload will be sensibly reduced. When thetemperature in which the CB is locatedexceeds its reference temperature, it willtherefore be "derated". For this reason, CBmanufacturers provide tables which indicatefactors to apply at temperatures different to

the CB reference temperature. It may benoted from typical examples of such tables(tables H2-38) that a lower temperature thanthe reference value produces an up-rating ofthe CB.Moreover, small modular-type CBs mountedin juxtaposition, as shown typically in figureH2-24, are usually mounted in a small closedmetal case. In this situation, mutual heating,when passing normal load currents, generallyrequires them to be derated by a factor of 0.8.

NS250N/H/L (reference temperature: 40 °C)rating (A) 40 °C 45 °C 50 °C 55 °C 60 °CTM160D 160 156 152 147 144TM200D 200 195 190 185 180TM250D 250 244 238 231 225

tables H2-38: examples of tables for the determination of derating/uprating factors toapply to CBs with uncompensated thermal tripping units, according to temperature.

ExampleWhat rating (In) should be selected for a CBc protecting a circuit, the maximum loadcurrent of which is estimated to be 34 A;c installed side-by-side with other CBs in aclosed distribution box;c in an ambient temperature of 50 °C.A circuit breaker rated at 40 A would bederated to 35.6 A in ambient air at 50 °C (see

table H2-38). To allow for mutual heating inthe enclosed space, however, the 0.8 factornoted above must be employed, so that,35.6 x 0.8 = 28.5 A, which is not suitable forthe 34 A load.A 50 A circuit breaker would therefore beselected, giving a (derated) current rating of44 x 0.8 = 35.2 A.

compensated thermal-magnetictripping unitsThese tripping units include a bi-metalcompensating strip which allows the overloadtrip-current setting (Ir or Irth) to be adjusted,within a specified range, irrespective of theambient temperature.For example:c in certain countries, the TT system isstandard on LV distribution systems, anddomestic (and similar) installations areprotected at the service position by a circuitbreaker provided by the supply authority.

This CB, besides affording protection againstindirect-contact hazard, will trip on overload;in this case, if the consumer exceeds thecurrent level stated in his supply contract withthe power authority.The circuit breaker (i 60 A) is compensatedfor a temperature range of - 5 °C to + 40 °C.c LV circuit breakers at ratings i 630 A arecommonly equipped with compensatedtripping units for this range (- 5 °C to + 40 °C).


H2


4.4 selection of a circuit breaker (continued)

general note concerning deratingof circuit breakersIt is evident that a CB rated to carry a currentIn at its reference ambient temperature(30 °C) would overheat when carrying thesame current at (say) 50 °C.Since LV CBs are provided with overcurrentprotective devices which (if not compensated)will operate for lower levels of current inhigher ambient temperatures, the CB isautomatically derated by the overload trippingdevice, as shown in the tables H2-38.Where the thermal tripping units aretemperature-compensated, the trippingcurrent level may be set at any valuebetween 0.7 to 1 x In in the ambienttemperature range of - 5 °C to + 40 °C.The reference ambient temperature in thiscase is 40 °C (i.e. on which the rating In isbased).For these compensated units, manufacturers'catalogues generally also give derated valuesof In for ambient temperatures above thecompensated range, e.g. at + 50 °C and+ 60 °C; typically, 95 A at + 50 °C and 90 A at+ 60 °C, for a 100 A circuit breaker.


H2

electronic tripping units are highlystable in changing temperaturelevels.

electronic tripping unitsAn important advantage with electronictripping units is their stable performance inchanging temperature conditions.However, the switchgear itself often imposesoperational limits in elevated temperatures,

as mentioned in the general note above, sothat manufacturers generally provide anoperating chart relating the maximum valuesof permissible trip-current levels to theambient temperature (figure H2-39).

M25N/H/L iiiii 40 °C 45 °C 50 °C 55 °C 60 °Ccircuit breaker A In (A) 2500 2500 2500 2450 2400

maximum adjustment Ir 1 1 1 0.98 0.96circuit breaker B In (A) 2500 2500 2500 2350 2200

maximum adjustement Ir 1 1 1 0.94 0.88

In (A)coeff.

25001

circuit breaker A24000.96

23500.94

22000.88

20 50 55 6035 40 4525 30

θ °C

circuit breaker B

fig. H2-39: derating of two circuit breakers having different characteristics, according tothe temperature.

selection of an instantaneous, orshort-time-delay, trippingthresholdPrincipal charasteristics of magnetic or short-time-delay tripping units. Type classificationaccording to IEC 898. See also table H2-28.

type tripping unit applicationslow setting c sources producing low-short-circuit-current levelstype B (standby generators)

c long lengths of line or cable

standard setting c protection of circuits: general casetype C

high setting c protection of circuits having high initial transienttype D or K current levels (e.g. motors, transformers, resistive

loads)

12 In c protection of motors in association withtype MA discontactors (contactors with overload protection)

I

t

I

t

I

t

I

t

table H2-40: different tripping units, instantaneous or short-time-delayed .


H2



the installation of a LV circuit breakerrequires that its short-circuit breakingcapacity (or that of the CB togetherwith an associated device) be equalto or exceeds the calculatedprospective short-circuit current at itspoint of installation.

selection of a circuit breakeraccording to the short-circuitbreaking capacity requirementsThe installation of a circuit breaker in a LVinstallation must fulfil one of the two followingconditions:c either have a rated short-circuit breakingcapacity Icu (or Icn) which is equal to orexceeds the prospective short-circuit currentcalculated for its point of installation, orc if this is not the case, be associated withanother device which is located upstream,and which has the required short-circuitbreaking capacity.

In the second case, the characteristics of thetwo devices must be co-ordinated such thatthe energy permitted to pass through theupstream device must not exceed that whichthe downstream device and all associatedcables, wires and other components canwithstand, without being damaged in anyway.This technique is profitably employed in:c associations of fuses and circuit breakers;c associations of current-limiting circuitbreakers and standard circuit breakers.The technique is known as "cascading" (seesub-clause 4.5 of this chapter).

The selection of main and principal circuitbreakersc a single transformerTable C-13 (in chapter C) gives the short-circuit current level on the downstream sideof a commonly-used type of HV/LVdistribution transformer. If the transformer islocated in a consumer's substation, certainnational standards require a LV circuitbreaker in which the open contacts areclearly visible*.

Example (figure H2-41):What type of circuit breaker is suitable for themain circuit breaker of an installation suppliedthrough a 250 kVA HV/LV (400 V) 3-phasetransformer in a consumer's substation?In transformer = 360 AIsc (3-phase) = 8.9 kA.A 400 A CB with an adjustable tripping-unitrange of 250 A-400 A and a short-circuitbreaking capacity (Icu) of 35 kA* would be asuitable choice for this duty.* A type Visucompact NS400N of Merlin Gerin manufactureis recommended for the case investigated.c several transformers in parallel(figure H2-42)v the circuit breakers CBP outgoing from theLV distribution board must each be capable ofbreaking the total fault current from alltransformers connected to the busbars, viz:Isc1 + Isc2 + Isc3,v the circuit breakers CBM, each controllingthe output of a transformer, must be capableof dealing with a maximum short-circuitcurrent of (for example) Isc2 + Isc3 only, for ashort-circuit located on the upstream side ofCBM1.From these considerations, it will be seen thatthe circuit breaker of the smallest transformerwill be subjected to the highest level of faultcurrent in these circumstances, while thecircuit breaker of the largest transformer willpass the lowest level of short-circuit current.v the ratings of CBMs must be chosenaccording to the kVA ratings of the associatedtransformers.

Note: the essential conditions for thesuccessful operation of 3-phase transformersin parallel may be summarized as follows:1. the phase shift of the voltages, primary tosecondary, must be the same in all units to beparalleled.2. the open-circuit voltage ratios, primary tosecondary, must be the same in all units.3. the short-circuit impedance voltage (Zsc%)must be the same for all units. For example, a750 kVA transformer with a Zsc = 6% will

share the load correctly with a 1,000 kVAtransformer having a Zsc of 6%, i.e. thetransformers will be loaded automatically inproportion to their kVA ratings.For transformers having a ratio of kVA ratingsexceeding 2, parallel operation is notrecommended, since the resistance/reactance ratios of each transformer willgenerally be different to the extent that theresulting circulating currrent may overload thesmaller transformer.

the circuit breaker at the output of thesmallest transformer must have ashort-circuit capacity adequate for afault current which is higher than thatthrough any of the other transformerLV circuit breakers (fig. H2-42).

250 kVA20 kV/400 V

VisucompactNS400N

fig. H2-41: example of a transformer in aconsumer's substation.

HV

Tr1

LV

CBMA1

B1

CBP

HV

Tr2

LV

CBMA2

B2

CBP

HV

Tr3

LV

CBMA3

B3

E

fig. H2-42: transformers in parallel.


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Table H2-43 indicates, for the most usualarrangement (2 or 3 transformers of equalkVA ratings) the maximum short-circuitcurrents to which main and principal CBs(CBM and CBP respectively, in figure H2-42)are subjected. The table is based on thefollowing hypotheses:c the short-circuit 3-phase power on the HVside of the transformer is 500 MVA;c the transformers are standard 20/0.4 kVdistribution-type units rated as listed;c the cables from each transformer to its LVcircuit breaker comprise 5 metres of single-core conductors;c between each incoming-circuit CBM andeach outgoing-circuit CBP there is 1 metre ofbusbar;c the switchgear is installed in a floor-mounted enclosed switchboard, in anambient-air temperature of 30 °C.Moreover, this table shows selected circuitbreakers of M-G manufacture recommendedfor main and principal circuit breakers in eachcase.

number and minimum S.C. main circuit breakers minimum S.C. rated currentkVA ratings of breaking (CBM) total discrimination breaking cap. In of principal20/0.4 kV capacity of main with out going-circuit of principal circuit breakertransformers CBs (Icu)* kA breakers (CBP) CBs (Icu)* kA (CPB) 250 A2 x 400 14 M08 N1/C 801 N ST 27 NS 250 N3 x 400 27 M08 N1/C 801 N ST 40 NS 250 H2 x 630 22 M10N1/CM1250/C 1001 N 42 NS 250 H3 x 630 43 M10H1/CM1250/C 1001 N 64 NS 250 H2 x 800 24 M12N1/CM1250/C 1251 N 48 NS 250 H3 x 800 48 M12H1/CM1250/C 1251 N 71 NS 250 L2 x 1000 27 M16N1/CM1600 54 NS 250 H3 x 1000 54 M16H2/CM1600 80 NS 250 L2 x 1250 31 M20N1/CM2000 60 NS 250 H3 x 1250 62 M20H1/CM2000 91 NS 250 L2 x 1600 36 M25N1/CM2500 70 NS 250 H3 x 1600 72 M20H2/CM2500H 105 NS 250 L2 x 2000 39 M32H1/CM3200 75 NS 250 L3 x 2000 77 M32H2/CM3200H 112 NS 250 L

table H2-43: maximum values of short-circuit current to be interrupted by main andprincipal circuit breakers (CBM and CBP respectively), for several transformers in parallel.* or Ics in countries where this alternative is practised.

Example: (figure H2-44)c circuit breaker selection for CBM duty:In for an 800 kVA transformer = 1.126 A (at410 V, i.e. no-load voltage) Icu (minimum) =48 kA (from table H2-43), the CBM indicatedin the table is a Compact C1251 N(Icu = 50 kA) (by Merlin Gerin) or itsequivalent;c circuit breaker selection for CBP duty:The s.c. breaking capacity (Icu) required forthese circuit breakers is given in the table(H2-43) as 71 kA.A recommended choice for the three outgoingcircuits 1, 2 and 3 would be current-limitingcircuit breakers types NS 400 L, NS 100 Land NS 250 L respectively (by MG) or theirequivalents. The Icu rating in each case =150 kA.These circuit breakers provide theadvantages of:v absolute discrimination with the upstream(CBM) breakers,v exploitation of the "cascading" technique,with its attendant economy for alldownstream components.

fig H2-44: transformers in parallel.

CBP1

3 Tr800 kVA20 kV/400V

CBM

400 A

CBP2

100 A

CBP3

200 A


H2

short-circuit fault-current levels at anypoint in an installation may beobtained from tables.

Choice of outgoing-circuit CBsand final-circuit CBsccccc use of table H1-40From this table, the value of 3-phase short-circuit current can be determined rapidly forany point in the installation, knowing:v the value of short-circuit current at a pointupstream of that intended for the CBconcerned;v the length, c.s.a., and the composition ofthe conductors between the two points.A circuit breaker rated for a short-circuitbreaking capacity exceeding the tabulatedvalue may then be selected.ccccc detailed calculation of the short-circuitcurrent levelIn order to calculate more precisely the short-circuit current, notably, when the short-circuitcurrent-breaking capacity of a CB is slightlyless than that derived from the table, it isnecessary to use the method indicated inchapter H1 clause 4.ccccc two-pole circuit breakers (for phase andneutral) with one protected pole onlyThese CBs are generally provided with anovercurrent protective device on the phasepole only, and may be used in TT, TN-S andIT schemes. In an IT scheme, however, thefollowing conditions must be respected:v condition (c) of table H1-65 for theprotection of the neutral conductor againstovercurrent in the case of a double fault;v short-circuit current-breaking rating:A 2-pole phase-neutral CB must, byconvention, be capable of breaking on onepole (at the phase-to-phase voltage) thecurrent of a double fault equal to 15% of the3-phase short-circuit current at the point of itsinstallation, if that current is i 10 kA; or 25%of the 3-phase short-circuit current if itexceeds 10 kA;v protection against indirect contact: thisprotection is provided according to the rulesfor IT schemes, as described in chapter Gsub-clause 6.2.ccccc insufficient short-circuit current-breaking ratingIn low-voltage distribution systems itsometimes happens, especially in heavy-dutynetworks, that the Isc calculated exceeds theIcu rating of the CBs available for installation,or system changes upstream result in lower-level CB ratings being exceeded.v solution 1: check whether or notappropriate CBs upstream of the CBsaffected are of the current-limiting type,allowing the principle of cascading (describedin sub-clause 4.5) to be applied;

v solution 2: install a range of CBs having ahigher rating. This solution is economicallyinteresting only where one or two CBs areaffected;v solution 3: associate current-limiting fuses(gG or aM) with the CBs concerned, on theupstream side. This arrangement must,however, respect the following rules:- the fuse rating must be appropriate- no fuse in the neutral conductor, except incertain IT installations where a double faultproduces a current in the neutral whichexceeds the short-circuit breaking rating ofthe CB. In this case, the blowing of theneutral fuse must cause the CB to trip on allphases.

cascading: a particular solution toproblems of CBs insufficiently ratedfor S.C. breaking duty.

associating fuses with CBs avoidsthe need for a fuse in the neutral,except in particular circumstances onsome IT systems.




H2

4.5 coordination between circuit breakersPreliminary note on the essential functionof current limiting circuit breakersLow-voltage current-limiting CBs exploit theresistance of the short-circuit current arc inthe CB to limit the value of current.An improved method of achieving current-level limitation is to associate a separatecurrent-limiting module (in series) with astandard CB.A contact bar (per phase) in the modulebridges two (specially-designed heavy-duty)contacts, the contact pressure of which isaccurately maintained by springs. Otherrigidly-fixed conductors are arranged in serieswith, and close to the contact bar, such thatwhen current is passed through theensemble, the electromagnetic force tends tomove the contact bar to open its contacts.This occurs at relatively low values of short-circuit current, which then passes through thearcs formed at each contact. The resistanceof the arcs is comparable with systemimpedances at low voltage, so that thecurrent is correspondingly restricted.

Furthermore, the higher the current, the morethe repulsive force on the bar and the greaterthe arc resistance as its path lengthens, i.e.the current magnitude is (to some extent)self-regulating.The circuit breaker is easily able to break theresulting low value of current, particularlysince the power factor of the fault-currentloop is increased by the resistive impedanceof the arcs.When used in a cascading scheme asdescribed below, the tripping of the limitingCB main contacts is briefly delayed, to allowdownstream high-speed circuit breakers toclear the (limited) current, i.e. the current-limiter CB remains closed.The contact bar in the limiter module resetsunder the influence of its pressure springswhen the flow of short-circuit current ceases.Failure of downstream CBs to trip will result inthe tripping of the current-limiting CB, after itsbrief time delay.

cascadingDefinition of the cascading techniqueBy limiting the peak value of short-circuitcurrent passing through it, a current-limitingCB permits the use, in all circuits downstreamof its location, of switchgear and circuitcomponents having much lower short-circuitbreaking capacities, and thermal andelectromechanical withstand capabilities thanwould otherwise be the case.Reduced physical size and lowerperformance requirements lead to substantialeconomies and to the simplification ofinstallation work.It may be noted that, while a current-limitingcircuit breaker has the effect on downstreamcircuits of (apparently) increasing the sourceimpedance during short-circuit conditions, ithas no such effect at any other time; forexample, during the starting of a large motor(where a low source impedance is highlydesirable).A new range of Compact* current-limitingcircuit breakers with powerful limitingperformances (namely: NS 100, NS 160,NS 250 and NS 400) is particularlyinteresting.

Conditions of exploitationMost national standards permit use of thecascading technique, on condition that theamount of energy "let through" by the limitingCB is less than that which all downstreamCBs and components are able to withstandwithout damage.In practice this can only be verified for CBs bytests performed in a laboratory. Such testsare carried out by manufacturers who providethe information in the form of tables, so thatusers can confidently design a cascadingscheme based on the combination of circuitbreaker types recommended.By way of an example, table H2-45 indicatesthe possibilities of cascading circuit breakertypes* C 60 and NC 100 when installeddownstream of current-limiting CBsNS 250 N, H or L for a 230/400 V or240/415 V 3-phase installation.

* Merlin Gerin products

the technique of "cascading" usesthe properties of current-limitingcircuit breakers to permit theinstallation of all downstreamswitchgear, cables and other circuitcomponents of significantly lowerperformance than would otherwisebe necessary, thereby simplifyingand reducing the cost of aninstallation.

in general, laboratory tests arenecessary to ensure that theconditions of exploitation required bynational standards are met andcompatible switchgear combinationsmust be provided by themanufacturer.


H2


4.5 coordination between circuit breakers (continued)

Advantages of cascadingThe limitation of current benefits alldownstream circuits that are controlled by thecurrent-limiting CB concerned.The principle is not restrictive, i.e. current-limiting CBs can be installed at any point inan installation where the downstream circuitswould otherwise be inadequately rated.The result is:c simplified short-circuit current calculations;c simplification, i.e. a wider choice ofdownstream switchgear and appliances;c the use of lighter-duty switchgear andappliances, with consequently lower cost;c economy of space requirements, sincelight-duty equipment is generally lessvoluminous.

Short-circuit breaking capacity of theupstream (limiter) CBskA r.m.s.150 NS250L10070 NS250H36 NS250N2522

Short-circuit breaking capacity of thedownstream CBs (benefiting from thecascading technique)kA r.m.s.150 NC100LH

NC100LMA100 NC100LS70 NC100LS NC100L

NC100LHNC100LMA

50 NC100L40 C60L i 40 C60L i 4030 C60H C60N C60N

C60L C60H C60HC60L C60L(50 to 63) (50 to 63)NC100H NC100H

25 C60NNC100H

20 C60a C60a15 C60a

tables H2-45: example of cascadingpossibilities on a 230/400 V or 240/415 V3-phase installation.

discrimination may be absolute orpartial, and based on the principles ofcurrent levels, or time-delays, or acombination of both. A more recentdevelopment is based on theprinciples of logic.A (patented) system by Merlin Gerinexploits the advantages of bothcurrent-limitation and discrimination.

discriminative tripping(selectivity)Discrimination is achieved by automaticprotective devices if a fault condition,occurring at any point in the installation, iscleared by the protective device locatedimmediately upstream of the fault, while allother protective devices remain unaffected(figure H2-46).Discrimination between circuit breakers A andB is absolute if the maximum value of short-circuit-current on circuit B does not exceedthe short-circuit trip setting of circuit breakerA. For this condition, B only will trip (figureH2-47).Discrimination is partial if the maximumpossible short-circuit current on circuit Bexceeds the short-circuit trip-current setting ofcircuit breaker A. For this maximum condition,both A and B will trip (figure H2-48).

IscA

IscB

A

B

Iccabsolute discrimination

IccBIrB

Iccpartial discrimination

IccBIrB Ic

B only open A and B opens

fig. H2-46: absolute and partialdiscrimination.


H2

I

t

Irm AIr AIr B

B A

Icc B

Isc downstream of B

I

t

Irm AIr AIr B

B A

A and Bopen

IscA

B only opens

Isc B

fig. H2-48: partial discriminationbetween CBs A and B.

fig. H2-47: absolute discriminationbetween CBs A and B.

1. discrimination based on current levels.This method is realized by setting successiverelay tripping thresholds at stepped levels,from downstream relays (lower settings)towards the source (higher settings).

Discrimination is absolute or partial,according to the particular conditions, asnoted in the above examples.

I

t

Irm AIrm B Isc B

B A

I

t

Isc B

∆t

A

B

A

B

2. discrimination based on stepped timedelays.This method is implemented by adjusting thetime-delayed tripping units, such thatdownstream relays have the shortestoperating times, with progressively longerdelays towards the source.

In the two-level arrangement shown,upstream circuit breaker A is delayedsufficiently to ensure absolute discriminationwith B (for example: Masterpact electronic).

I

t

Irm Adelayed

Isc B

Irm Ainstantaneous

AB

3. discrimination based on a combinationof methods 1 and 2.A mechanical time-delay added to a current-level scheme can improve the overalldiscrimination performance.

Discrimination is absolute ifIsc B < Irm A (instantaneous).The upstream CB has two high-speedmagnetic tripping thresholds:- Irm A (delayed) or a SD* electronic timer- Irm A (instantaneous) standard (Compacttype SA)

* short-delay.

t

IscIrm AIrm B

conventional instantaneousmagnetic-trip characteristic

pressure operatedmagnetic-trip characteristic

table H2-49: summary of methods and components used in order to achieve discriminative tripping.

4. discrimination based on arc-energylevels (Merlin Gerin patent)In the range of short-circuit currents, thissystem provides absolute discriminationbetween two circuit breakers passing thesame fault current. This is achieved by usingcurrent-limiting CBs and initiating CB trippingby pressure-sensitive detectors in the arcing

chambers of the CBs. The heated-airpressure level depends on the energy level ofthe arc, as described in the following pages(figures H2-54 and H2-55).


H2


4.5 coordination between circuit breakers (continued)

current-level discrimination isachieved with stepped current-levelsettings of the instantaneousmagnetic-trip elements.

Current-level discriminationCurrent-level discrimination is achieved withcircuits breakers, preferably limiters, andstepped current-level settings of theinstantaneous magnetic-trip elements.c the downstream circuit breaker is not acurrent-limiter.The discrimination may be absolute or partialfor a short-circuit fault downstream of B, aspreviously noted in 1, above.Absolute discrimination in this situation ispractically impossible because Isc A z Isc B,so that both circuit breakers will generally tripin unison.In this case discrimination is partial, andlimited to the Irm of the upstream circuitbreaker.c the downstream circuit breaker is acurrent limiter.Improvement in discriminative tripping can beobtained by using a current limiter in adownstream location, e.g. for circuitbreaker B.For a short-circuit downstream of B, thelimited level of peak current IB would operatethe (suitably adjusted) magnetic trip unit of B,but would be insufficient to cause circuitbreaker A to trip.Note: All LV breakers (considered here) havesome inherent degree of current limitation,even those that are not classified as current-limiters. This accounts for the curvedcharacteristic shown for the standard circuitbreaker A in figure H2-50.Careful calculation and testing is necessary,however, to ensure satisfactory performanceof this arrangement.c the upstream circuit breaker is high-speed with a short-delay (SD) feature.These circuit breakers are fitted with trip unitswhich include a non-adjustable mechanicalshort-time-delay feature. The delay issufficient to ensure absolute discriminationwith any downstream high-speed CB at anyvalue of s.c. current up to Irms (figure H2-51).

I

I peak

Iscprospective (rms)

current limitationcurve forcircuit breaker(see note)

Isc

A

B

faultupstreamof B

faultdownstreamof B

fig. H2-50: downstream limiting circuitbreaker B.

I

t

Irm Adelayed

A (compact S)

Irm Sinstantaneous

B

only B opens A and B open

fig. H2-51: use of a "selective" circuitbreaker upstream.

Example:circuit breaker A: Compact NS250 N fittedwith a trip unit which includes a SD feature.Ir = 250 A, magnetic trip set at 2,000 Acircuit breaker B: Compact NS100NIr = 100 AThe Merlin Gerin distribution catalogueindicates a discrimination limit of 3,000 A(an improvement over the limit of 2,500 Aobtained when using a standard tripping unit).

Time-based discriminationThis technique requires:c the introduction of "timers" into the trippingmechanisms of CBs;c CBs with adequate thermal and mechanicalwithstand capabilities at the elevated currentlevels and time delays envisaged.Two circuit breakers A and B in series (i.e.passing the same current) are discriminativeif the current-breaking period of downstreambreaker B is less than the non-tripping time ofcircuit breaker A.

Discrimination at several levelsAn example of a practical scheme with (MG)circuit breakers Masterpact (electronicprotection devices).These CBs can be equipped with adjustabletimers which allow 4 time-step selections,such as:c the delay corresponding to a given step isgreater than the total current breaking time ofthe next lower step;

c the delay corresponding to the first step isgreater than the total current-breaking time ofa high-speed CB (type Compact for example)or of fuses (figure H2-52).

I

t

Isc B

A

Isc

B

only B open

current-breakingtime for B

non trippingtime of A

Ir B

fig. H2-52: discrimination by time delay.

discrimination based on time-delayedtripping uses CBs referred to as"selective" (in certain countries).Application of these CBs is relativelysimple and consists in delaying theinstant of tripping of the severalseries-connected circuit breakers in astepped time sequence.


H2

discrimination schemes based onlogic techniques are possible, usingCBs equipped with electronic trippingunits designed for the purpose(Compact, Masterpact by MG) andinterconnected with pilot wires.

Discrimination logicThis discrimination system requires CBsequipped with electronic tripping units,designed for this application, together withinterconnecting pilot wires for data exchangebetween the CBs.With 2 levels A and B (figure H2-53), circuitbreaker A is set to trip instantaneously, unlessthe relay of circuit breaker B sends a signal toconfirm that the fault is downstream of B. Thissignal causes the tripping unit of A to bedelayed, thereby ensuring back-up protectionin the event that B fails to clear the fault, andso on…This system (patented by Merlin Gerin) alsoallows rapid localization of the fault.

A pilot wires

B

fig. H2-53: discrimination logic.

Limitation and discrimination byexploitation of arc energyThe technique of "arc-energy discrimination"(Merlin Gerin patent) is applied on circuitshaving a short-circuit current level u 25 In andensures absolute selectivity between twoCBs carrying the same short-circuit current.Discrimination requires that the energyallowed to pass by the downstream CB (B) isless than that which will cause the upstreamCB (A) to trip (fig. H2-54 (a)).

Operation principleBoth CBs are current limiters, so that theelectromagnetic forces due to a short-circuitdownstream of CB (B) will cause the current-limiting arcing contacts of both CBs to opensimultaneously. The fault current will be verystrongly limited by the resistance of the twoseries arcs. The intense heat of the currentarc in each CB causes a rapid expansion ofthe air in the confined space of the arcingchambers, thereby producing acorrespondingly rapid pressure rise.Above a certain level of current, the pressurerise can be reliably detected and used toinitiate instantaneous tripping.

Discrimination principleIf both CBs include a pressure tripping devicesuitably regulated, then absolutediscrimination between two CBs of differentcurrent ratings can be achieved by settingCB (B) to trip at a lower pressure level thanthat of CB (A) (fig. H2-54). If a short-circuitoccurs downstream of CB (A) but upstreamof CB (B), then the arc resistance of CB (A)only will limit the current. The resulting currentwill therefore be significantly greater than thatoccurring for a short-circuit downstream ofCB (B) (where the two arcs in series cause avery strong limitation, as previously mentioned).The larger current through CB (A) willproduce a correspondingly greater pressure,which will be sufficient to operate itspressure-sensitive tripping unit (diagrams (b)and (c) of fig. H2-54).

As can be seen from figure H2-49 (4), the largerthe short-circuit current, the faster the CB willtrip.Discrimination is assured with this particularswitchgear if:c the ratio of rated currents of the twoCBs u 2.5;c the ratio of the two trip-unit current ratingsis > 1.6, as shown (typically) in figure H2-55.

For overcurrent conditions less than those ofshort-circuits i 25 In, the conventionalprotection schemes are employed, aspreviously described in this chapter.

recently-introduced circuit breakerssuch as Merlin Gerin type NS, usethe principle of arc-energy levels toobtain discrimination.

CB (A) Compact NS

CB (B) Compact NS

Isc = 50 kA

t

CB (A) and CB (B) in series

I

CB (A) only

Isc (limited)

Isc (prospective)

t

CB (B) setting

Pressurein arcingchamber

CB (A) setting

fig. H2-54: arc-energy discriminationprinciples.

NS250NTM260D

NS100NTM100D

CB (A)

CB (B)

fig. H2-55: ratio of rated currents of CBsand of tripping units, must comply withlimits stated in the text, to ensurediscrimination.

(a)

(b)

(c)


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4.6 discrimination HV/LV in a consumer's substationIn general the transformer in a consumer'ssubstation is protected by HV fuses, suitablyrated to match the transformer, in accordancewith the principles laid down in IEC 787 andIEC 420, by following the advice of the fusemanufacturer.The basic requirement is that a HV fuse willnot operate for LV faults occurringdownstream of the transformer LV circuitbreaker, so that the tripping characteristiccurve of the latter must be to the left of that ofthe HV fuse pre-arcing curve.This requirement generally fixes themaximum settings for the LV circuit breakerprotection:c maximum short-circuit current-level settingof the magnetic tripping element;c maximum time-delay allowable for theshort-circuit current tripping element.See also Chapter C sub-clause 3.2.7, andAppendix C1, for further details.

63 A

1250 kVA20 kV / 400 V

VisucompactCM 2000set at 1800 A

full-load current1760 A3-phaseshort-circuitcurrent level31.4 kA

fig. H2-56: example.

c short-circuit level at HV terminals oftransformer: 250 MVA;c transformer HL/LV: 1,250 kVA 20/0.4 kV;c HV fuses: 63 A (table C 11);c cabling, transformer - LV circuit breaker:10 metres single-core cables;c LV circuit breaker: Visucompact CM 2000set at 1,800 A (Ir).What is the maximum short-circuit trip currentsetting and its maximum time delayallowable?The curves of figure H2-57 show thatdiscrimination is assured if the short-timedelay tripping unit of the CB is set at:c a level i 6 Ir = 10.8 kA;c a time-delay setting of step O or A.A general policy for HV fuse/LV circuitbreaker discrimination, adopted in somecountries, which is based on standardizedmanufacturing tolerance limits, is mentionedin chapter C sub-clause 3.2.7, and illustratedin figure C-21.Where a transformer is controlled andprotected on the high-voltage side by a circuitbreaker, it is usual to install separate CT- and/or VT- operated relays, which energize ashunt-trip coil of the circuit breaker.Discrimination can be achieved, together withhigh-speed tripping for faults on thetransformer, by using the methods describedin chapter C sub-clause 3.2.

I

t(ms)

step Cstep Bstep A

step 050

1220

10

100200

1000

CM 2000set at1800 A

1800 AIr

Isc maxi31,4 kA

10 kA0,01

minimumpre-arcingcurve for 63 A HVfuses (currentreferred to thesecondary sideof the transformer)

1Ir

4Ir

6Ir

8Ir

fig. H2-57: curves of HV fuses and LVcircuit breaker.


1. the basic functions of lv switchgear schneider... · the protection of circuits - the switchgear...

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