Introduction

Reliability of power supply in any networkdepends on various factors such as :

• A reliable power source

• A strong distribution network with

fully coordinated distributionsystem

• A well maintained and disciplinedload management system

• Proper selection of ratings of alldevices,viz., transformer,switchgear and loads

Distribution System

Let us look at a typical distributionsystem, shown in Fig.1, fed from agrid having a fault level of say, 50 kA.

The incoming circuit breaker A feedsthe bus section of Panel 1. One of theoutgoing feeders controlled by breakerB feeds Panel 2. The load L1 fed fromthis panel is controlled by breaker C.There are other outgoing feeders forboth the panels 1 and 2. Based onthe discrimination scheme envisagedby the consultant or customer, thesystem may provide completediscrimination or partial discriminationor no discrimination at all.

The term “Discrimination” implies ‘co-ordination of the operatingcharacteristics of two or more over-current protective devices so thaton the incidence of an over-currentwithin stated limits, the device

intended to operate will do so whilethe other(s) does (do) not.

In any good distribution system, it isnormally desired that :

• the fault should be cleared

immediately on its occurrence bya device closest to the locationof fault

• only the faulty section is isolated

• healthy feeders are notunnecessarily isolated due to afault elsewhere

• the system is stressed least and

the extent of outage/powerinterruption is kept to a minimum.

Fully Rated System WithDiscrimination

In a fully co-ordinated system, thecircuit breaker nearest to the fault is

Low Volta ge Distribution SystemSelection Of Circuit Breakers

- Mr. T. P. R. SarmaSwitchgear Design& Product Development

Fig.1

Selection of appropriate switchgearensuring suitable discriminationupstream and downstream plays a veryimportant role in ensuring goodperformance of the distribution system.

Guidelines for selection of switchgear,especially the circuit breakers, mayappear very simple as parameters likerated service voltage, ratedcurrent, and frequency areobvious factors consideredduring selection. However,the short-circuitperformance specificationsare sometimes ignored/missed. The price of acircuit breaker of a giventhermal rating will varywidely depending on theshort-circuit capabilities. Inorder to keep the projectcost under control, therecould be a temptation tocompromise on thebreaking capacity/shorttime rating requirements ofthe circuit breakers,without realising itsimpact on the systemreliability and operationale f f i c iency andconvenience.

designed to sense and clear the fault.Other feeders connected to the systemcontinue to remain energised. In Fig.1for a fault at X, the breaker C isexpected to operate for clearing thefault. In case this breaker fails tooperate for any reason, the upstreambreaker B will operate after a pre-determined time delay. Breaker A thatis upstream of breaker B will operateafter a further pre-set time delay onlyif both the breakers B and C fail tooperate. This scheme will provide totaldiscrimination. Discrimination isachieved by matching the time-currentcharacteristics of the protectivedevices.

In this system, all the circuit breakersshould have breaking capacity equalto or more than the prospective systemfault level. The entire bus systemshould also be designed to withstandthe short-circuit stresses till theupstream breaker A clears the faultafter its preset time delay as decidedby the requirements of co-ordinationand discrimination.

Cascade system

Though the good engineering practicecalls for all breakers used in a networkto have breaking capacities equal toor more than the prospective systemfault level, some customers may usebreakers of lower breaking capacitiesfor certain special applications. Thispractice existed in certain countries,especially in the North America till therecent past (may be due to technologylimitations at that time), but today itis not preferred by most consultantsand customers. Modern handbooks onElectrical Engineering do notrecommend this practice for newinstallations. The National ElectricalCode (of USA), i.e., NEC 1999 aswell as IEC standard on ElectricalInstallations of Building-IEC 60364lay down special requirements for suchapplications. Relevant details areextracted and reproduced below:

(i) Reference:- NEC 1999 NationalElectrical Code-Chapter 2Cl.240.86 Series ratings - Wherea circuit breaker is used on acircuit having an available faultcurrent higher than its marked

interrupting rating by beingconnected to the load side of anacceptable over current protectivedevice having the higher rating, thefollowing shall apply:

(a) Marking - The additional seriescombination interrupting rating hallbe marked on the end-useequipment, such as switchboardsand panel boards

(b) Motor Contribution - Series ratingsshall not be used where

(1) Motors are connected on theloadside of the higher rated over-currentdevice and on the line side of thelower rated over-current device,and,

(2) The sum of the motor full loadcurrents exceed 1% of theinterrupting rating of the lower-ratedcircuit breaker. (This clause, i.e.,Cl. 240.86 was added in the NECas an amendment in 1999)

(ii) Reference:- IEC 364 Part 4 Chapter43. Electrical Installations ofBuildingCl. 434.3.1 The breaking capacity(of the switching device) shall notbe less than the prospective short-circuit current at the place of itsinstallation, except where thefollowing paragraph applies.A lower breaking capacity isadmitted if another protectivedevice having the necessarybreaking capacity is installed inthe supply side. In that case, thecharacteristics of the devices mustbe coordinated so that the energylet through by these two devicesdoes not exceed that which canbe withstood without damage bythe device on the load side andthe conductors protected by thesedevices.

(iii) Reference:- Switchgear and ControlHandbook edited by Robert. W.Smeaton and published byMcGraw-Hill Book Company.....The cascade system, onceconsidered acceptable is no longerrecommended by thestandards.......... (Text authored byMAX. E. EVERETT, SeniorEngineer Allis-ChalmersCorporation)

...... Note that small moulded-casecircuit breakers or fuses and circuitbreakers should not be cascaded

to achieve adequate short-circuitinterrupting ability unless thecombination is specifically approvedfor that purpose. Small moulded-case breakers tend to open morequickly than larger frame sizes ofmoulded-case breakers andtherefore attempt to perform the jobof interrupting fault currents prior tothe operation of the larger breakerswith which they might be cascaded........ (Text authored by W. B.PERKINS & D. E. MAXWELL,Distribution Equipment Division,Square D. Company)

A careful analysis of theserecommendations will highlight thepoints listed below regarding thecascading of devices:

• In a cascaded system, the customer

is compromising on discrimination

requirements for a cost. For a fault

downstream of the breaker with the

lower breaking capacity, the upstream

breaker will trip along with the down-

stream breaker cutting off power

supply to other healthy feeders as

well. This will reduce the useful life of

the upstream device.

• The specific combination of the

upstream and downstream breakers

should be tested for the required

fault level before selection. NEC

recommendation implies that

thecombination may not have

abreaking capacity equal to that of

the upstream or downstream device.

• If more inductive (motor) loads

areadded to the system on a future

date, a combination,

consideredadequate till then, may

suddenly turn out to be non-

acceptable as per the NEC Code

due to the increased motor

contribution considerations.

• While faults upstream and also

downstream of the cascaded

system are taken care of, the

protection for an internal fault at a

location between the two devices is

not verified and guaranteed.

Fully rated system Cascade system with discrimination

Total discrimination possible Only partial discrimination canbe expected

Only the device closest to Larger outage in case of faultfault needs to operate

As only one device needs to In case of any fault tripping, all devicesoperate for a given fault, life of have to be thoroughly inspected andupstream devices not affected serviced before re-commissioning.

Minimum disruption in case of Short-circuit rating of the combinationany fault has to be assessed by testing prior

to use

All devices are fully rated and Will appear economical (on initial costhence no safety hazard in considerations only), but will work outservice more expensive if total cost of outages

and the cost of lost opportunity areproperly computed.

Could be marginally more Future system upgradation/equipmentexpensive, though extra service replacement needs extreme cautionreliability/less outage will be morethan off-set the extra cost

High reliability Less reliable

Comparison between distributionsystemsA balance sheet analysis of the prosand cons of fully rated system withdiscrimination versus cascadesystem will look as given in the chart.

ConclusionIt is up to the consultant who inconsultation with the end user shouldmake the choice of systems anddevices after assessing the real costof the cheaper-looking option and itsimpact on the long-term serviceperformance, operational convenienceand efficiency of his installation. In mostcases, the final decision will be obvious.

In a cascade system, both theupstream and downstream devicesare expected to operatesimultaneously so that the faultenergy is shared by the breakingdevices. Unless the combination istested for the required fault level,the performance of the combinationcannot be guaranteed in the field.After a major fault is cleared boththe devices of the combination needto be thoroughly examined andreplaced if necessary to ensuresafe operation during any futurefault in the system.

• Any system upgradation or

replacement of any of the protectivedevices, on a future date, can causehavoc unless extreme care isexercised. E.g. replacing one 100AMCCB by another 100A MCCB aspart of maintenance can lead to asafety hazard unless the replacedbreaker is identical to the oldbreaker in all respects.

Referring to Fig.1, if the breaker Cdoes not have the breaking capacity equalto the system fault level, for a fault at X,breakers B and C may have to operatein cascade to clear the fault. This willresult in an undesired power outagefor the other healthy feedersconnected to Panel 2 as thesystem is not, in this case,designed for discrimination betweenbreakers B and C.

There are some similaritiesbetween SCPD of contactors/motor-starters and the cascadesystem of circuit breakers in adistribution feeder. It is well-knownthat the contactors/motor-startershave making and breakingcapacities of only 6-8 times theoperational current rating whichmay be far lower than the systemfault level. In these cases a SCPDis selected so that in case acurrent above the breakingcapacity of the starter is to bebroken, it will be the SCPD andnot the starter that will operate.This is ensured by proper co-ordination of the upstream SCPDand the downstream starter. Thefeeder to be isolated in this caseis only a single motor feeder unlikein a distribution network wheremultiple loads fed by the upstreamfeeder will also get isolatedcausing large and undesired poweroutages.