electrical installation guide appendix

7/30/2019 Electrical Installation Guide Appendix

1/22low-voltage service connections - D581

D

electrical installation practiceaccording to IECinternational standards

1. rules and regulations EMC1

2. electromagnetic disturbances

2.1 disturbances by conduction EMC22.2. radiation EMC8

3. cabling of equipment and systems3.1 earthing EMC93.2 masses (non-conducting metal parts) EMC113.3 attenuating effects EMC153.4 installation and cabling rules EMC163.5 EMC components and solutions EMC17

4. local network problems EMC20

Appendix EMC


2/22Appendix EMC - 1

EMC

1. rules and regulations

Legislation on EMC throughout the world isbroadly divided into two philosophies. In"liberal" countries, any parasitic interferencewith radio reception is illegal, but noemission-level limit for the source ofinterference is imposed. However, in cases oflitigation the methods of measuring, and thelimits of emission level laid down by theCISPR, serve as reference. For example,Japan is a "liberal country" in which the VCCIstandards (that correspond technically to theinternational publications of CISPR) assumesthat the attitude of civic responsibilityprevailing is adequate at the present time.For countries more rigidly "regulated",emission levels exceeding a standardizedlimit are illegal. For example, in the U.S.A.,Automatic Data Processing (ADP) systemsare protected by obligatory emission-levelstandards defined by the Federal law FCC

part 15. Checking procedures differ,depending on whether the level for class A(procedure for "compliance") or class B isconcerned. In the case for class B (domesticenvironment), certification is then required.European regulations are effectively placedbetween the two foregoing attitudes. Theparasitic-emission level and an excessively-sensitive reception device are both illegal.Compliance with the EMC standards,although only constituting a presumption ofconformity to the essential requirements, is,nevertheless, the preferred means ofchecking. Moreover, the Europeanregulations are applicable to all apparatuses,systems and commercialised installations,without exception.


3/222 - Appendix EMC

EMC

2. electromagnetic disturbances

Problems of EMC (ElectroMagneticCompatibility) often arise when equipment,which is highly sensitive to extraneouselectrical disturbances (commonly referred toas "interference" or "parasites") is located inan environment subject to electromagneticdisturbances.Since sources of electromagnetic disturbanceare numerous and inevitable, anddesensitizing an equipment (generallyelectronic) to counter the effect ofdisturbances is difficult to achieve,consideration of the physical layout of thesensitive equipments and related cabling,relative to the sources of disturbance,becomes necessary.This is the principal means of ensuring asatisfactory degree of immunity for the largemajority of sensitive electronic devices.There are two recognized modes of

electromagnetic interference:c conducted disturbances propagated alongcables, wires, etc.c radiated disturbances by stationaryinduction (magnetic or electrostatic fields*)and/or by electromagnetic (radio) waves

travelling in space.The magnitudes of electromagneticdisturbances are expressed by fourparameters: two for the conduction mode andtwo for the radiation mode.For the conduction mode, measurements aremade in the traditional quantities, viz: volts(U) and amps (I). For the radiatedelectromagnetic waves the electric andmagnetic field strengths are measured involts per metre (E) and in amperes permetre (H), respectively.The frequency is one of the principal featuresthat characterise an electromagnetic wave.In EMC studies the solutions adapted differaccording to whether the disturbance is at lowfrequency (LF) or at high frequency (HF).

2.1 disturbances by conduction2.1.1 Disturbances by conduction: modesof propagationElectrical energy, whether useful signals orpower, or unwanted parasites, propagatealong a 2-wire circuit in one of two modesonly, viz: in differential mode or in commonmode.

Differential modeThe differential mode is the normal way ofconducting current through a 2-wire circuit.This mode is sometimes referred to as seriesmode, normal mode, or symmetrical mode.In the differential mode the current flowing inone conductor is in exact phase opposition tothat in the other conductor, i.e. flowing in theopposite direction at every instant.The voltage is measured between the twoconductors.

Common modeThe common mode is parasitic. It issometimes also referred to as parallel mode,longitudinal mode or asymmetrical mode.Common-mode currents pass through all theconductors of a cable in the same direction.The return path for such currents is via the

earth, earth-bonding connections and theprotective earthing conductors, cablesheaths, etc. Since the earth is no longerused as a conducting medium for telefax,useful signals are no longer transmitted in thecommon mode through the associatedcables. A potential difference in commonmode is measured between the mass (localzero voltage reference terminal) and themean value of potential of all the conductorsof the cable circuit being tested. It may bepresent in the absence of any current flow.

fig. EMC-1: signal or disturbance in thedifferential mode.

fig. EMC-2: disturbance in commonmode.

Differential mode disturbances are the mostsevere at low frequencies. By lowfrequencies (LF) it is understood in EMCstudies to concern all frequencies lower than9 kHz. This convention means that a verylarge number of electrical disturbances areconsidered as being LF phenomena.In electrical power networks disturbances inthe differential mode are numerous. One maycite, for example, interruptions of supply of

short or long duration, voltage fluctuationsand dips, phase instability, lamp flicker,variations of frequency: harmonics andvoltage spikes. The effect of an electromagneticdisturbance depends largely on its duration.Permanent (maintained) disturbancesprincipally affect analogue-type circuits, whiletransient and impulsive disturbances interfereespecially with digital circuits.

* Stationary (but changing in magnitude) magnetic andelectric induction fields generally, are only significant inclose proximity to the sources and are easily countered byplacing sensitive equipment at a suitable distance from

them. A notable exception is the case where some tens ofthousands of amps, i.e. short-circuit fault currents, flow in apower cable.

I

I

U equipment

I

equipment

2

I

2U



EMC

Electromagnetic disturbances couple readilywith cables in the common mode, particularlyat high frequencies (HF), since they act asradio antennae. Several kinds of couplingbetween neighbouring circuits can occur.The problems of common mode recurfrequently in EMC cases. A conductingenvironment is always good for EMC,due to it equipotential quality.Only disturbances in the differential mode canbe filtered locally, cable by cable.As indicated by its name, the common modeis common to all the cables of a givenequipment. Common mode problems at HFare particularly critical in an insulatedenvironment, or where the mass (the zero-voltage refernce for all electronic circuits) is"floating" with respect to earth (i.e. insulatedfrom the earth).A common mode voltage is always

detrimental. If it cannot be reduced, it isimportant, at least, to prevent it fromdeveloping into a differential modedisturbance.

This is the role of insulated and/orsymmetrical connections.A galvanic insulation is only effective at lowfrequencies. A symmetrical connection, alsoreferred to as "balanced", can remaineffective up to high frequencies. Thedissymmetry of a differential connectionoriginates mainly from its end circuits. Animbalance at an end circuit can be caused byan electrical and/or geometric dissymmetry.In any case, a connection by simple coaxialcable to transmit signals at low frequencies isnot recommended.Correcting measures which may produceharmful secondary effects must be combinedwith other precautions in order that thesystem effectively counters the entire rangeof disturbances (LF and HF, of large andsmall amplitudes). The combination of thedifferent corrective measures (galvanic

separation, symmetrical connections andovervoltage protection) is referred to ascoordinated protection.

2.1.2 LF disturbances by conductionLF disturbances include all types of parasiticinterference of which the range of significantfrequencies is lower than 9 kHz. Thefrenquency of 9 kHz is a conventional upperlimit, below which electrical phenomena maybe analysed in simple terms, using customaryequivalent linear circuit techniques, based onresistances, inductances (self- and mutual-)and capacitances. By definition, a LFdisturbance exists for a relatively "long" time

(at least a hundred microseconds). Theenergy level of a conducted LF disturbancecan be considerable and is very easilymeasured.The impedance of a cable at very lowfrequencies is practically equivalent to itsresistance only. At several kiloherz mostcables of small cross-sectional-area (c.s.a.)and even at 50 Hz (for cables of large c.s.a.)the lineal inductance of a conductor is of theorder of 1 H/m, and its impedance increaseslinearly with frequency. For example, large 1-core cables at 50 Hz, installed in trefoil, havea lineal impedance of approximately0.3 per km. This feature is important whenconsidering harmonic frequencies in a

network.Selecting a c.s.a. larger than 35 mm2 for aprotective conductor would effectively reducethe heating of the conductor when carryingfault current (since its resistance would belower) but would have a negligible effect onthe equipotential distribution: the inductanceof a cable being (as noted above) practicallyindependent of its c.s.a.

Interruptions (long or transitory)An interruption is a total disappearance of thepower-supply system voltage. In the case ofa fault occurring on the HV network of apower-supply system, a consumer willnormally experience a "voltage dip"sometimes followed by a brief interruption.This interruption will occur only if the HVsystem is an overhead-line (O/H line) system,and the consumer is being supplied from thesection of line on which the fault has

occurred. So-called "fleeting faults" onoverhead lines are very common, and consistof flashovers (of insulators) to earthed metalby momentary overvoltage due to lightning ora short-circuit through a large bird, or again,directly to earth through, for example, a wettree branch, etc.In more than 80% of these incidents such afault will disappear during the brief period ofautomatic interruption, and normal supply willbe restored.The automatic sequence for the elimination offleeting faults on O/H lines is included in theprotection scheme for the line. Theinterruptions in these schemes are normallylimited to less than 0.5 seconds. An

underground cable supply network reducesthe number of interruptions to about 10% onlyof those of O/H line systems, butunderground cable faults are not self-clearing, so that lengthy shutdowns arenecessary to locate the fault and to effectrepairs.



EMC

2. electromagnetic disturbances (continued)

FlickerFlicker describes a condition of small but

frequently recurring voltage dips caused byloads which require relatively heavy currentfor brief and regularly repeated periods.The impedance of a LV network is made upmainly of cable impedance and theimpedance of the HV/LV transformersupplying the network. The greater the kVArating of the transformer, the lower itseffective impedance. In public power-supplysystems the flicker problem is more commonon rural systems, particularly at the end oflong lines. It is a problem on a line which issupplying arc furnaces, arc-welding machinesand, generally, where heavy loads arefrequently switched.Flicker creates an objectionable annoyance

for persons working under incandescentlighting. The effect is purely physiological, nodysfunctioning of electronic equipment willoccur due to flicker.Flicker is objectionable only where heavyoverloads and frequent switching arecombined, or where the impedance of thesystem is high. Standardized parameter limitsand a flicker meter are described in IECpublications 1000-3-3 and 1000-4-15.

2.1 disturbances by conduction(continued)

For industrial installations subject to flicker, amodification to the installation is sometimesnecessary. Among the possible corrective

measures available, the most effectiveinclude: separate cables for heavy loadspreferably with each large load suppliedthrough an individual HV/LV transformer,division of the load, increase the time lags inautomatic control systems, reduction in thework-cycle rate, time-wise staggering andspreading of operations which requireimpulsive power demands, together with theinstallation of a static reactive-powercompensator. Technically, a reduction in thesource impedance is an excellent solution.In final general-distribution circuits at LV, the3-phase symmetrical short-circuit current isusually within the 500-5,000 A range. Inindustry, the short-circuit current at LV mayexceed 10 kA on a circuit of large c.s.a. closeto the source substation. This value never,however, exceeds 100 kA.

Fluctuations and voltage dipsA fluctuation of voltage is a rapid change ofsupply voltage not exceeding + 10% (thegenerally accepted limits at distribution level)during normal operation. A "dip" is a sudden

drop of voltage, caused principally byswitching loads which, at the instant of

energization, require a greater current thanthe normal rated value, e.g. small-motorstarting currents, the switching on of largeresistive heating devices and incandescentlamps, etc. Such dips are transitory only, butare often more severe than those classed asflicker, generally exceeding 10%. The durationof a "dip" lasts from 10 ms to approximately 1 s.Voltage reductions which exceed 10% and1 s, due, for example, to starting large motors,or, as previously described, due to systemfaults, are simply referred to as "voltage drop"and the extent of the drop and its duration arespecified. Voltage fluctuations have littleeffect on electronic circuits generally.Sensitive, precision electronic-control devices,

electronic calculators of early design, andelectronic (HF) fluorescent lighting tubes,however, may be adversely affected.A well-designed electronic device cantolerate, without difficulty, voltage fluctuationsup to + 8%.A voltage dip at a point on a HV system isgenerally due to a short-circuit faultelsewhere on the same network. The closerthe fault to the point in question, the moresevere the dip. The severity of the dip isdefined by two parameters: the magnitude ofthe drop as a percentage of the systemnominal voltage, and its duration inmilliseconds.Voltage dips are generally due to wind-blowndebris (tree branches, etc.), electric storms,

or faults on the lines (broken insulators) oroccur on the installation of a neighbouringconsumer.

Faults on VHV (very high voltage)transmission lines are rare and are usuallydue to lightning, or to exceptionally severecold weather.The consequence of voltage dips (whenfollowed by an interruption) is a complete lossof supply to electronic (and power) devices.Relays will drop out and motors controlled byelectronic speed-variation and regenerative-braking devices will be deprived of brake control.Even if there is no supply interruption, a largeand long (up to 1 second) voltage dip maycause similar malfunctions.Means of countering these problems at theleast cost requires individual analysis of eachcase. To overcome the voltage-dip problem,

many low-power electronic equipments haveindividual power packs with an autonomy ofseveral hundred milliseconds for 100% lossof supply voltage.For heavy-duty power supplies, the period ofautonomy amounts only to approximately20 milliseconds, the limiting factor being thesize of the energy-storing capacitors required.Rotating machines (motor/generators) have

most influenced beetween thethree phase to phase voltages

time

10

depth(% de Un)

400 V

Ueff.

360 V

clearingtime

= 0,3 s

duration= 0,4 s

fig. EMC-4: caracteristics of a voltage dip.

fig. EMC-3: number of variations perminute.

3

2

1

0,5

0,3

.5 .7 1 10 100 1000

U/Un in %



EMC

UnbalanceThe amplitude of an ac voltage is expressedby its rms (root-mean-square) value. Thevoltage between a phase conductor and theneutral is referred to as the phase voltage,while that measured between any two phasesis called the line voltage. The line voltageequalsetimes the phase voltage, on anormally balanced 3-phase system(e= 1.732). A 3-phase system may bedefined simply by the amplitude of 3 voltages,either line or phase values.In order to define a sinusoidal system whichis in an unbalanced state, however, thevalues of current and voltage of each phase

are then, in the general case, the sum of3 rotating vector components. The threecomponents of each phase are known as:c the positive phase-sequence componentc the negative phase-sequence componentc the zero phase-sequence component.A balanced 3-phase system is composed ofpositive phase-sequence components only.An unsymmetrical system is said to beunbalanced; its negative- and zero-phasesequence components are generally bothpresent, together with the positive phase-sequence component.A common cause of imbalance is that ofdifferent levels of loading on the 3 phases.Unbalanced loading results in unbalanced

voltages being applied to 3-phase motors.Increased losses occur in the rotors of themotors, and in cases of excessive imbalance,motors can be destroyed by overheating.Single-phase (line-to-line) loads are notnormally adversely affected by imbalance.Small degrees of imbalance (0.5-1%) areinevitable on LV 3-phase 3-wire networks,and up to 2 or 3% can be tolerated for severalminutes by all loads.When an imbalance of voltage is consideredto be excessive (> 2% for example), it isadvisable to correct the balance of phaseloading. Where it is not possible to improvethe balance, the situation may be eased byincreasing the fault level at the circuitconcerned by changing the supply

transformer.An average HV/LV distribution transformer(> 100 kVA) has a short-circuit voltage of5-6%. Special transformers are available withinterlaced windings which limit the leakagereactance to give a short-circuit voltage ofapproximately 2%.A low short-circuit voltage effectively means alow source impedance (with higher fault-current level) a situation which improves thevoltage balance, and (incidentally) improvesthe form of the voltage wave (if it happens tobe distorted) by reducing the harmoniccontent of the wave. A modern method ofimproving a condition of imbalance, thoughpresently rather costly, is to install a static

compensator. It consists of a system whichstores energy in an inductor or capacitor, andrestores this energy to the system at theappropriate instants.An active filter constitutes one of thepreferred solutions for limiting disturbancesgenerated by arc furnaces during the start-upphase.

Frequency variationsThe European network performs, in practice,as an infinite system as far as frequencystability is concerned, in that load changes donot sensibly affect the frequency. On smallerprivate systems, and especially on singlegenerators, where the rotational inertia issmall and the regulating system of the primemover is generally rudimentary, the frequencywill vary (within reasonable limits) each timethe load changes abruptly. Diesel engineprime movers are less stable, in terms offrequency, than turbines. Frequencyvariations do not unduly disturb electronicequipments. Converters based on currentchopping principles are insensitive tofrequency changes. All modern devices andcomponents should be capable of correctperformance during frequency changes of+ 4% throughout a 10 minute period.

Only very large systems with transformersoperating at the limit of saturation may, whenthe system voltage is at its maximum, besubjected to overheating by a long-term low-frequency condition. AC motors (locked to thefrequency) will experience speed variationscorresponding to those of the frequency. Onthe other hand, the inertia of motors tends tosmooth out other sudden disturbancesoccurring on a network.

HarmonicsAny non-linear load (fluorescent lamp, Graetzbridge, arc furnace, etc.) takes a non-sinusoidal current from the network. Such acurrent is composed of a sinusoidalcomponent at the frequency of the system

and is known as the fundamental component,together with other sinusoidal componentswhich are whole-number multiples of thefundamental frequency. These latter arereferred to as harmonic components.Conventionally, harmonics up to the rank of40 only are considered in power systems,i.e. 2 kHz for 50 Hz systems and 2.4 kHz for60 Hz systems. Supplies to electronic circuits,power regulators based on Graetz bridge,and fluorescent lighting equipment, are rich inharmonics.Distortion of a voltage waveform is onerousfor associated equipments; it is expressed asa percentage. It is proportional to theharmonic content of the current and to theimpedance of the source. The effect of

distortion is to increase the heating losses inmotors. In an ADP environment, a distortionof 5% may be considered to be normal. Allelectronic components can tolerate a globalfactor of distortion including possible inter-harmonics of at least 8%. An inter-harmoniccurrent has a frequency which is not a whole-number multiple of the fundamental (i.e.system) frequency. A distinction is madebetween "true" inter-harmonics generated atdiscrete frequencies, and those forming partof a continuous spectrum.Even-numbered harmonics are generatedonly by asymmetrical rectifiers and loadcurrents which contain a dc component. A dccomponent can readily saturate a power-

supply transformer. Most non-linear loads(satured transformers, fluorescent tubes,power supply circuits which use current-chopping techniques, etc.) only generateodd-numbered harmonics.

sufficient autonomy to cope with voltage dips.Finally, uninterruptible power supply units cansuppress voltage dips, and maintain thepower supply during a period of completeinterruption.


7/22



EMC

The installation engineer has practically onlyone way to protect the installation againstovervoltages, and that is to install overvoltagelimiting devices on the supply-circuitconductors. Overvoltages occurring on publicLV distribution networks are lower in energythan those occurring in heavy-currentindustrial networks: the energy on publicnetworks rarely exceeding 100 Joules.The only really dangerous case is that of alightning stroke on a line close to theinstallation.

New overvoltage surge arresters based onthe use of varistors of high energy-dissipationratings allow an effective protection of all LVsystems and equipments downstream of thepoint of arrester installation.Failure of the zinc-oxide varistor will cause athermal fusing element (which is connected inseries with it) to blow, and open the circuit,thereby avoiding a short-circuit to earth via afaulty or damaged arrester. The cable fromthis device must be connected by theshortest possible route to the mass of thedistribution board, i.e. the common earthingbar, and not to the earthing electrode, whichis generally too far away (see Sub-clauseL 1.4).

2.1.3. HF disturbances by inductionAt HF i.e. conventionally above 1 MHz,interference phenomena becomeconsiderably more complicated. Powerconductors become efficient antennae,electromagnetic fields, even when weak,produce considerable interference, all cablesare affected, and some may resonate, etc.HF phenomena are severe, frequent, difficultto analyse and are cause to reconsider theestablished practices in electronics cableinstallation.The inductance of cables becomes more of aproblem at HF than at low frequencies. Thelineal inductance of any conducting structurefollowing a sensibly straight route isapproximately 1 H/m. Furthermore, aninterconnection of a length exceeding athirtieth (1/30) of a wavelength becomes

practically incapable of ensuringequipotentiality between the twointerconnected masses. Beyond /30, aconductor becomes an effective radiatingantenna but, if radiating, it fails to performcorrectly as an equipotential conductor.The wavelength corresponding to afrequency of 1 MHz is 300 metres. Thedistance between any equipment and themain earth bar being generally greater than10 metres, one may deduce that the natureand the quality of the earthing is of noconsequence at frequencies exceeding1 MHz. A simple dictum: a large conductor isgood, but a short conductor is better.HF disturbances by conduction in commonmode through cables are, in fact, consideredto be the principal problem for EMCspecialists. The reduction of common modedisturbances at HF through cables can beachieved by one or more of the followingthree stratagems:1 - attenuation effects: close interconnection(networking) by equipotential conductors of"masses" and/or screened cables2 - filters between conductors andmechanical mass of each equipment3 - ferrites on "problem" cables.An electronic circuit, e.g. a card supportingchips, etc., should never be allowed to "float"relative to its conducting envelope, acondition which is to be avoided at all costs inthe presence of HF interference. The natural

(so-called "stray") capacitances of the cardcomponents, less than a pico-farad, can besufficient to cause interference with anelectronic circuit. To limit rapidly varyingvoltages between an electronic circuit and itsenvironment, the connecting of the filter 0 V(reference voltage) terminal to an envelopingmetallic housing, connected to earth or not, isan excellent preventive measure.

HF spikesThe range of frequencies which presents thegreatest difficulties, both in radiation and inprotection against the radiated energy, is theVHF band from 30 to 300 MHz, also referredto as the "metric" band. Almost all electricarcs, sparks, electrostatic discharges andstarting contacts (such as dry contacts,starting contact for striking an arc inelectroluminescent tubes, operation of circuitbreakers and other switching devices on HVsystems) generate impulses (spikes) whichare conducted in common mode andradiated. The radiation spectrum covers therange of the above-mentioned VHF band.The amplitude of the HF current spikes canattain a peak of several tens of amperes.Digital circuits are particularly vulnerable tosuch spikes. Respecting the standard for

immunity IEC 1000-4-4 is a highly-recommended means of achievingsatisfactory protection and EMC of aninstallation.

Maintained HF disturbancesFrequency converters, electronic speedcontrollers, Graetz bridges and electric-motorcommutating brushes also generate commonmode HF disturbances. The peak value ofthese disturbances can reach and evenexceed 1 ampere. One solution is to install anefficient filter at the supply-source and/or atthe disturbed equipment. Another solution isthe use of power cables that include ascreen, which is earthed at both ends.For sources of heavy interference, it is

recommended to form a network ofequipotential interconnections of all massesin the neighbourhood of the offending source,in particular all metallic cable ways, ducts,trays, etc.



EMC

2.2.1 LF magnetic fieldsAt low frequencies, only the magnetic fieldmay cause problems. Whether it is impulsive(short-circuit, lightning, electronic flash...) ormaintained, the field H is generally producedin close proximity to the affected equipment.Measurement of the field strength requires anoscilloscope and a loop probe only. Themagnetic field at low frequency does notpropagate, but remains in the close proximityof its origin (a transformer or an inductionmotor, for example) and its field strengthinitially decreases rapidly with distance fromthe source according to 1/D3.At greater distances, the rate of decrease isslower and approaches 1/D2. This latter valueis often used when considering the fieldsurrounding busbars or an overhead line. Themagnetic-field strength of a rectilinear currentwith a return path at infinity (such as that dueto lightning) decreases according to 1/D.Severe sources of magnetic fields are thezero-phase sequence currents in supplycables of a TN-C scheme. Loops formed

between the phase conductors and currentsdiverted from the neutral conductor (throughequipotential bonds) are sometimes very

2. electromagnetic disturbances (continued)

2.2 radiationElectrical energy propagation is not onlyconfined to conductors. It can also propagate

in space without material support. Suchpropagation is referred to as electromagneticfields or waves, or Hertzian waves. Such awave is made up of an electric component Ein volts per metre, and a magneticcomponent H in amperes per metre.These radiated fields, when encountering a

conductor (which acts as a receivingantenna), give rise to minute emfs and

currents in the conducting material, i.e. in theform of disturbance by conduction. For cablecircuits, these disturbances are in commonmode. It is therefore possible to protectagainst these radiated fields by means of aFaraday cage arrangement or by (very often)low-pass filters.

large; such currents can amount to severalamperes. The undesirability of the TN-Cscheme (in buildings) for this reason is shownin fig. F14 Chapter F, of the main text. Duringa short-circuit fault, the disturbance isevidently greater to a degree that depends onthe fault-current magnitude.The most common consequence of a LFmagnetic field is a distortion of the image of acathode ray tube (jumps and wave-likemovements of the image, and even changesof colour). A magnetically unscreened CRT(cathode-ray tube), an electron miscroscope,a mass spectrometer or a magnetic readinghead, barely tolerates 1A/m at LF. Moreover,the "stray" loops formed through equipotentialconnections to masses are associated(naturally) with corresponding voltages.Magnetic screening from/of a magnetic fieldis very difficult at frequencies less than10 kHz. The easiest solution is simply toplace the sensitive equipment out of range ofthe offending field. Screening the sensitiveequipment with a thick magnetic shield can

reduce the field strength by a factor of theorder 10.

2.2.2 HF electromagnetic fieldsAt high frequencies, the E and H fields uniteto form indivisible electromagnetic waves inspace. At more than a sixth of a wavelengthfrom a point source, the ratio E/H tends to avalue of 120 = 377 ohms. It is sufficient,therefore, to give the value of one componentin order to deduce the field strength.Numerous industrial, scientific or medicalapparatuses use radio frequencies, mostoften in the 1 MHz to 3 GHz range. Radiotransmitters have power-radiation capabilities

ranging from several milli-watts for radio-control devices, to several mega-watts peakfor radar systems.Walky-talkies, which can be used fortransmission very close to electronicequipment, are sources of disturbance,particularly for low-power analog circuits.An effective way to reduce radio-transmitterfield strength, as "seen" by sensitiveelectronic equipments, is to use antennae asremote from the equipments as possible, andlocated at the greatest height attainable.Since this principle cannot be applied toportable transmitters, their use must berestricted to areas sufficiently remote fromsensitive equipments to ensure trouble-freeoperation of the latter.

Electronic equipments are rarely affected bya field-strength of less than 1 volt per metre.Field strengths exceeding 10 volts per metre,however, very often cannot be tolerated. Therange of frequencies giving the most severeeffects is, again, in the VHF band.At HF a common-mode current in a cablealways produces a radiated wave. Thereciprocal case is also true, i.e. the arrival of aHF wave will produce a common-modecurrent in a cable. The methods of protectionagainst HF fields are the same as those

adopted against disturbances by conductionat the same frequencies.The antenna effect of cables carrying HFcurrent by coupling in common modeconstitutes the principal problem in EMC.



EMC

3. cabling of equipment and systems

In order to cable an electronic systemcorrectly or to correct an unsatisfactoryinstallation, it is often sufficient to apply somesimple elementary rules. From experience,the most important factor is a clearunderstanding of the phenomena and therecognition of their limits. The strictobservance of traditional rules for correctinstallation and cabling has becomenecessary. This is the price to pay for the

achievement of EMC in modern electronicsystems. Many practices which aresatisfactory at LF have proved to be poor oreven catastrophic at HF. Certain cablingoptions can be chosen with confidence.The interconnection of all the non-functionalearths of a single site is one example.Factors which are always favourable shouldbecome standard practice.

3.1 earthingThe expressions "earth", "earth electrode","earth plate", "earth rod" all refer to aconductor which is buried, and in intimatecontact with the soil. The word mass refersto metal parts of equipment (electrical or not -for example, water pipes) which, undernormal conditions, are not intended to carry

current. Bonding conductors, which are usedto interconnect masses are also referred toas "mass". Although all masses in normal LFinstallation practice are connected to earth,the two words, "earth" and the above-notedequivalents should not be confused with"mass". "Mass" is commonly called "ground"in some countries.

3.1.1 the role of earthingThe basic role of an earth electrode is tomaintain all masses in an installation at avoltage close to zero, whether the powersource is earthed or not. This is achieved in aproperly designed installation, regardless ofwhether a faulty condition (which would

otherwise raise the voltage of the installationmasses) occurs in the installation circuits oron the power-supply network, or othersources external to the installation.The role of earthing, therefore, is that of theprotection of persons against the dangers ofelectrocution. The severity of an electricshock is a function of the current whichpasses through the body, and equallyimportant is the path of the current flowthrough the body. Established IEC rules ofprotection against electric shock set safelimits of voltage (referred to as conventionalvoltage limits) above which masses areconsidered to be unacceptably dangerous.For normal 50 Hz or 60 Hz power systems,these values are 50 Vrms for dry locations

and 25 Vrms for wet locations, for examplebathrooms and laundries (see Chapter L ofthe main text for more details).It is recognized that a low contact resistanceof an earth electrode with the mass of theearth cannot always be obtained.Furthermore, its value is rarely constant,depending largely on soil humidity (and soprone to seasonal changes). An essentialfactor in maintaining safety to personnel inthe event of a high earthing resistance, is thatof the equipotential concept. If, for example,all masses are at a common (even normallydangerous) potential, and the earth below thebuilding is at a similar potential, a person cantouch any or several masses at the same

time without danger. This is why electricalappliances with long leads (hedge trimmers,lawn mowers, etc.), which allow the user toleave the equipotential environment of thehouse, must be of Class II insulation level(i.e. doubly insulated).So-called normal leakage currents (noinsulation is perfect) also include the minutecapacitive currents of the wiring to earth.

These currents, and short-circuit-to-earth faultcurrents, flow mainly through the protectiveearthing PE conductors (coloured with yellowand green stripes) and finally back to thesource substation, via the earth (TT system)or via the earth path and (mainly) through theneutral conductor in parallel (TN system).

Since, in the TN case, practically all of thefault (and leakage) currents return to thesource via the neutral conductor, theresistance of the installation earth electrode isnot of primary importance (unless lightningarresters are to be connected to it).For the protection of electronic equipments, itis strongly recommended that common-modecurrents entering the building from externalcables be diverted to earth at the point of entry.A simple galvanic isolation is ofteninsufficient: the overvoltage withstandcapability of a galvanic isolation transformeris typically less that 10 kV. This value isinsufficient on days of intense electricalstorms.

The installation of non-linear voltage-limitingdevices then becomes a necessity.It is important that all incoming metallic pipes,ducts, trunking, etc., be connected to earth attheir entry into the building. This policy canavoid the circulation of currents (from outsidethe building) in the conductors bonding themasses.The installation of overvoltage-protectiondevices must be carried out with theminimum possible common impedancebetween the external circuit and the circuit tobe protected. The length of conductor inseries with the voltage limiter must,consequently, be the shortest possible.The residual voltage "seen" by the protectedequipment is then independent of the

impedance of the earth.Even with a "bad" earth, it is possible toprotect an equipment effectively againstexternal overvoltages: it is necessary andsufficient to connect the voltage limiter to themass of the equipment using the shortestpractical length of cable.

fig. EMC-5: a voltage limiter must beconnected to the mass and not to earth.

protectedelectronicequipment

electric-power line

earthing conductor

voltagelimiter

yes

no

protected

electronicequipment



EMC

3. cabling of equipment and systems (continued)

3.1 earthing(continued)When a voltage limiter is correctly connectedto the mass, the impedance of the earth

electrode is immaterial.A direct lightning stroke on the supply lineclose to the installation requires thedissipation of (typically) 10 kA to 100 kA ofstroke current, most of which passes to earththrough lightning arresters on the lineexternal to the installation. Overvoltageswithin an installation where external arrestersare provided rarely exceed 6 kV due toatmospheric causes.

fig. EMC-6: the entire equipotential"cage" will be at a high absolute potentialduring the brief flow of stroke current.

For transmission and distribution lines at highvoltage, the fault current for one phase toearth returns to the source through the earthand (where provided) through the shieldingconductors above the phase conductors ofthe lines.The provision of an equipotential condition onthe surface of the ground at the base oftransmission towers and, more importantly, atsubstations (the source of the fault current) isa primary concern of design engineers. Theprinciple of equipotential bonding is identicalto that required for a low-voltage installationin a building.A functional earth means an earth electrodewhich is designed to pass load currentthrough the earth, i.e. the earth path acts asone of the circuit conductors. There areseveral installations around the world thatutilise this method. In some countries, d.c. isused in this (economic) way to operate aservice of facsimile transmission.It may be of interest to note that when

telephone cables (which use paper insulationon the conductors) have a high degree ofleakage current and consequently doubtfulsymmetry, a low earth resistance allows thequality of the transmitted signals to bepreserved. Although the magnitude oftelephone signals is low (millivolts to lessthan 1 volt), the quality of modern cablesovercomes the constraints of a "good" earth.To summarise, the protection of persons doesnot depend directly on a low value of earthresistance*; it is rather the establishment of acondition of equipotential between massesthat is of prime importance. Thus, an aircraftin an electric storm presents no danger to thepassengers, who are in a metallic envelopewhich is (within a few volts) perfectlyequipotential. For persons or animals, thedanger is not the magnitude of the absolutepotential, it is the difference of potentialbetween metal parts which can be touchedsimultaneously that is dangerous.

An electronic equipment is not affected by thevalue of earth resistance. At worst, there is a

risk of exposure to overvoltages from anexternal cable, if its protection is insufficient,or is badly cabled. So that the role of themasses is essential, and more important thanthat of the earthing. The only requirement fora satisfactory performance of the electronicequipments is a high degree ofequipotentiality.It is evident that two earths are always lessequipotential than one. Any separate earth,even if said to be interference free isalways detrimental to equipotentiality, and soto the safety of persons and to thesatisfactory functioning of interconnectedequipments. Two non-functional earth-electrode systems on a common site should

always be interconnected.In practice, care must be taken that there isno touch-voltage existing when working on anelectronic equipment interconnectionbetween two buildings (video, access control,local network, information technology (ADP)devices...) if the earthing systems of the twobuildings are not solidly interconnected. It is,as already noted, not possible to be sure ofthe equipotentiality of two separate earths.

* This statement is not true in certain circumstances, notablyin rural areas, where a small transformer supplies anisolated community, for example. The neutral earthelectrode must necessarily have the lowest possibleresistance in such cases. If not, potential gradients on thesurface of the ground can be dangerously high in the vicinityof the electrode during an earth fault.Animals are frequently killed due to this cause.

direct-stroke current

lightning protectionconductors

internal equipments mustbe maintained at

equipotential



EMC

3.2 massesThe majority of malfunctions inelectromagnetic devices, sometimes wrongly

imputed to software problems or human error,are found to be due to an insufficient level ofequipotentiality between interconnected units(probes, cards, actuators).There are two differences between a buriedconductor and a mass conductor. A buriedconductor will dissipate its common modecurrents, but it is always too far from theequipments to be effective at HF.A mass conductor above ground levelpresents two essential virtues to the goodperformance of electronic systems, viz: it isphysically close to the circuits, and it isaccessible.The equipotentiality of equipments and theirmasses is a functional objective.

As long as interference signals circulate inthe masses and not in the electronic circuits,they are harmless. On the other hand, if themasses are not all equipotential and areconnected to earth in star*, for example, HFinterference currents will circulate throughany available path, i.e. via signal cables.Some circuits will therefore be subjected tointerference and even be destroyed.Networking the mass conductors to form aclosely-connected low-impedance bondingsystem is the only economical way to ensurea satisfactory level of equipotentiality to installall sensitive equipments in a "Faraday cage"arrangement (a room enclosed in a mesh ofconductors) would be technically ideal, but isgenerally not economically justified.

By definition, a "mass" is any conductingmaterial which can be touched by any part ofa human body, that is not normally alive, butwhich may become alive as the result of afault. Two masses which are accessible andwithin human reach must present a potentialdifference, under any conceivable faultcondition, that does not exceed the IEC-recommended conventional voltage (UL)

safety limit (of 50 Vrms in dry locations and25 Vrms in wet conditions for ac systems).

These values are the maximum allowed thatcan exist indefinitely in specified conditions ofexternal influences.A dangerous touch voltage can arise during afault if the resistance of the equipotentialconductors is not sufficiently low. In somecases, it is necessary to install supplementaryequipotential conductors in parallel withexisting conductors to satisfy the UL criterion.It should be noted that access to two (ormore) masses is illegal, even if they areassociated with different installations, if theyare connected to different earthing systemswhich are not interconnected.The respect of safety rules is obligatory butnot sufficient, in themselves, to ensure

satisfactory EMC of an installation. In fact, therisk of electrocution only exists by a voltageof a high value and relatively long durationappearing between adjacent masses.An electronic equipment is sensitive to anextensive range of frequencies, or to verybrief impulses. An electrostatic discharge, forexample, is generally of no consequence forits source, but it could be catastrophic for anelectronic device. Normal earths, commonlyconnected in star (i.e. radial) configurationfor example, guarantee the safety of personswhen the relevant standards are respected,but not the satisfactory operation of aninstallation which includes sensitive electroniccomponents.It is certain that more and more electronic

equipments are, or will be, connected to otherapparatuses and devices for the exchange ofinformation. The best way of ensuring asuccessful and durable installationperformance is to establish a high degree ofequipotentiality throughout the entireinstallation.

* i.e. by one of a number of conductors radially connected tothe main earth bar, the ensemble resembling a "star".

3.2.1. loops of mass and between massesA loop of mass is the area included betweena working cable (metering cable, controlcable, power-supply cable, local-networksystem cable) and the mass conductor(generally the nearest PE conductor). Thereare, therefore, as many loops of mass asthere are cables. This is inevitable, whetherthe conductors are galvanically isolated ornot. A galvanic isolation reduces thecirculation of LF currents without, however,reducing the area of the loop. A loop canoscillate strongly at HF, so that large-arealoops constitute the major problem in EMC.

fig. EMC-7: there is an inevitable massloop per cable.

If a current circulates around a mass loop,such a current in common mode may eithersuperimpose noise (interference) on usefulsignals (in differential mode, by conversionfrom common mode to differential mode) ordisturb the electronic circuits at eachextremity.The risk is equally present for radiation from aloop as for reception of interference by theloop. The output stages of electronic circuitsare as sensitive to interference as the inputstages, and are more difficult to filter.The areas enclosed by mass conductorsmust not be confused with those referred toabove as "mass loops". It is preferable toallow parasitic currents to propagate in themasses rather than in the signal cables.These loops between mass conductors arecalled "loops between masses".

fig. EMC-8.

equipment 1 equipment 2

signalcable

massloop

nearest mass conductor


signalcable

mass conductor

loop between masse

mass conductor



EMC


3.2 masses (continued)If two neighbouring masses are notconnected together, the differential potential

difference between them may be significant.A direct connection from one to the other willalways improve the equipotential condition.At least, the masses of all equipments whichexchange data between them should beinterconnected by mass conductors. An evenmore certain way of improving the state ofequipotentiality is to interconnect allequipment masses, whether they exchangedata or not."Mass loops", also called "ground loops",should never be confused with loopsbetween masses. A mass loop is neverfavourable, and its area must be reduced tothe minimum attainable, to reduce as far aspossible the interference effects of disturbing

fields. On the other hand, it is always goodpractice to increase the number and reducethe areas of loops between masses. fig. EMC-9.

3.2.2 unity of the mass networkThe mass must be unique to be equipotential.There are three methods of connectingmasses which preserve this unity.

1 - Earthing connections in "star": eachequipment has its own earthing cable, whichterminates with all other individual earthingcables on a unique earthing bar.

fig. EMC-10.

The justification of such a philosophy is over-simple: when an equipment develops a leakagecurrent to mass, the remaining equipmentsare assumed to remain at earth potential.But "earth" potential has no real meaning in

practical electronics, all potentials are relativeone to another, the concept of absolute zeropotential (i.e. "remote earth") is abstract.It is often assumed that the "star" configurationof earthing overcomes the problem ofcommon impedance. It is, in fact, quite theopposite! Earthing in the "star" configurationincreases the common impedance (that is,forms a point of common coupling) betweeninterconnected equipments.

fig. EMC-11.

Earthing in "star" can create a commonimpedance between two interconnectedequipments.It is sometimes also assumed that the "star"system of earth connections suppresses themass loops. Between two interconnectedequipments, it is evidently not the case; thearea enclosed by the mass loop can, in fact,be considerable. An electromagnetic fielddue, for example to a lightning discharge, willinduce voltage in the mass loop greater thanthat occurring in any other method of

earthing.

fig. EMC-12.

This long-established "star" earthing methodis now only possible for an equipment whichis, and will remain, isolated from any other.

The method can be suitable only forelectronic analog systems (as opposed todigital systems) with floating sensors, and theelectronic circuits completely isolated fromany other. Such cases are becomingincreasingly rare.With the generalisation of information-datatransmissions over great distances, localnetworks, shared peripherals and, in general,the exchange of signals betweenequipments, the "star" method of earthingmust be abandoned. Moreover, even if theearth connection of each equipment by anindividual conductor is not detrimental, it stillremains a costly method that requires largeamounts of copper and many hours of

installation work.The only reasonable application for the "star"arrangement of earthing (in fact, connectionto mass) is the connecting cable between anequipment and power-supply socket-outlet, orthe nearest distribution board. Thus, in anADP environment, it is reasonable to use thegreen-and-yellow PE conductor of the power-supply circuits to connect each equipment to

equipment 1 equipment 2greater immunity

againstradiation fields by

reductionof area of the mass loop

greater immunity from conducted

interference by multiplication and reductionin area, of loops between masses.

earth cablesinevitably long

authorized method, but costly and notgood for EMC, particularly forinterconnected equipments

earth cables radiating from the main earth bar,figuratively similar to a star

disturbanceon cable

high value ofdifferentialpotential

difference(d.p.d.)

high impedance if theconductor is long


I

I

PE PE

Z

signalcable

mc


signalcable

large area

strongd.p.d.

electro-magneticwave

The more this policy is developed, the moreeffective is the resulting state of

equipotentiality, both at LF and at HF.Connecting the masses to a mesh ofinterconnecting mass cables is alwaysbeneficial, regardless of the nature of theequipments concerned.


14/22


15/22



EMC

lower strength than that of the original field.A cable in close proximity to conductive mass

from end to end is therefore less exposed tothe most severe type of interference, viz: thatof common mode.Attenuation effects can be made moreeffective by arranging the mass, wherepossible, to envelop the conductors to beprotected. In this way, a woven metallicscreen, incorporated in signal cables andconnected to mass, protects the enveloppedconductors above a frequency of 1 MHz withan attenuation factor of at least 300.It is difficult and expensive to shield all theinterconnections in a installation, but it isoften easy to select cable routes whichprovide good attenuation. To benefit from theattenuation effect, it is sufficient to fix cables

on conductive mass throughout the entirecable length. Such masses must be carefullybonded together electrically and to all nearbystructural framework. The quality (i.e. the lowimpedance) of interconnecting bonds is ofprimary importance. The most efficient is adirect contact of sheet metal on sheet metal.

3.3 attenuation effectsThe attenuating effect of a conductingstructure (mass) is defined by the amplitude

of the common-mode interference appearingon a cable installed at a location remote fromany masses, with respect to the amplitude ofthe interference on the same cable due to thesame disturbance, but with the cable installedin close proximity (i.e. clamped firmly) tomass, throughout its length.

fig. EMC-17.

Electrical continuity from one end to the other,and the correct connection to mass atextremities, guarantee an effectiveattenuation factor. It is recommended toconnect cable ways to conductive buildingstructures at intervals along the cable route.The attenuation factor is not reduced bythese additional contacts between masses,but the mass mesh is improved. In a singlecable tray, in order to limit "cross-talk", powercables or, for example, cables of speedcontrollers should not be placed beside small-

signal cables.The ideal, in an industrial environment, wouldbe to install three separate cable trays, i.e.one for measurements and similar functions,one for control and indication circuits, andone for power cables.A copper conductor provides an attenuationfactor of the order 5 if it is installed throughoutthe entire length close to the protected signalcable. It is therefore an advantage toassociate signal cables with interconnectingearthing cables in a common cable way (forinstance, between two buildings). This is stilltrue even if the earths are interconnectedelsewhere. It is always possible to add amass cable adjacent to a particularlysensitive signal cable if necessary. The masscable is then referred to as a "cable ofaccompaniment".A buried cable which is passing an a.c.current in common mode creates a magneticfield in the surrounding soil. This (concentric)field gives rise to (Foucault) currents in theearth and the magnetic energy is dissipatedin the form of heat. The common-mode

attenuation effect of a perforated steel sheetmetal, type "dalle marine"direct contact, sheet metal on sheet metal

frequency(MHz)cable

metal flexible-connectiontresses

(same connectionat 2 extremities)

0,1 0,3 1 3 10 30 100

10

20

40

dB

fig. EMC-16: example of attenuatingeffect (in this case equal to 5).

The attenuating effect is one of the keyfactors in EMC, being effective and not toocostly. In order to exchange signals in goodconditions, i.e. in limiting the interferencepicked up by the signal cables, it is importantto reduce common-mode coupling.Any metallic structure, close to, i.e. in contactwith, and longitudinally parallel to a signal

cable, from one end to the other, can providetwo favourable effects:1 - A more effective meshing of the masses.For d.c. currents, the mesh does not act asan attenuator; its role is to reduce theresistance between masses, not to provide ashielding effect. The galvanic effect of themesh is independent of the proximity of thesignal cables with the mass.2 - An attenuation (shielding) effect. Theeffect of proximity adds to that mentioned inabove, if the word "impedance" replaces"resistance". It is achieved by connectingequipments, which are interconnected to themass of conducting structures which areclose to the signal cables. The benefit is anefficient shielding which is practically cost-

free. The attenuation effect being directlyattributed to mutual induction, there is noattenuation of d.c. interference, as noted in 1.It should be borne in mind that any cable ispotentially an excellent wide-band antenna,especially in the metric range. In order toreduce its radiation ability, a simple, efficientand inexpensive method consists in placingthe cable as close as possible to a massstructure throughout its length, i.e. close to amass cable, metal ducting, structural girder,etc. The attenuation effect produced by amass conductor close to a signal cable issimply explained, as follows. On the occurrenceof an electromagnetic-wave disturbance, acurrent is induced in the mass conductor.

This current generates, according to Lenzslaw, a magnetic field which acts in theopposite sense to the field that produced thecurrent. A signal cable close to the massconductor will therefore be affected by thedifference only of the original field and thereactive field of the mass-conductor current.The resulting field affecting the signal cable isknown as the residual field and is evidently of

victim cable

I

mass conductor

10 V

victim cable

I 2 V



EMC


3.4. installation and cabling rulesTo resolve the majority of EMC problems, it issufficient to respect (rigorously) a fewelementary cabling rules. The firstrequirement is to decide to which group eachcable belongs. The following classes of cablegroups cover most practical installations.

Group n 1 - Measuring circuits (low-levelanalog signals) and supplies to analogprobes. This group is sensitive.

Group n 2 - Digital circuits. This group isalso sensitive (especially to impulses andbursts). It can also interfere with the circuitsof Group n 1.

Group n 3 - Control and indicationcircuits, including all-or-nothing (AON)relays. This group will interfere with Groupsn 1 and n 2.

Group n 4 - Power-supply cables. Theseare power cables from the public distributionnetwork, or from a private generating source(emergency power supply for example).Currents at this level are switched andchopped (by various power-electronicsequipments, rectifiers, inverters, and so on...).In normal operation these functions generateHF current and voltage components, in andon the supply conductors. Such currents andvoltages constitute a highly-pollutedenvironment for Groups n 1, n 2 and n 3.It is recommended that the cables and wiresof each Group have a distinctive and differentcolour to the other Groups.

Rule n 1 - The "go" and "return"conductors of any circuit must always beplaced as closely together as possible.This general rule applies also to power-supply cables. Do not supply in "star" (i.e.radially) two circuits that are not isolated,which exchange signals.It is necessary, even for the signals of AONrelays with one common conductor, to"accompany" the active conductors with at

least one common conductor per cable or permulti-core cable. For analog or digital signals,the use of two-core cables (or pairedconductors) is the basic minimum precaution.

Rule n 2 - All internal-circuitinterconnecting conductors, cables, etc.should be fixed in close contact withequipotential structures constituting theelectrical mass. This measure ensures thebenefit of interference attenuation previouslydescribed, at practically no cost. Ensure thatunused wires, or cables or free cores are not

currents are damped by this effect, which isnot exactly the same as that of the

attenuation described above, but is ratheranalogous to the action of a transformer witha resistive load. This damping action is

particularly effective where the interference isdue to repetitive trains of transient damped

oscillations (i.e. "bursts"). The Foucaultcurrents in the soil increase the degree ofdamping.

3.3 attenuation effects (continued)

printedcircuit

no

_U +

yes

_U +

printedcircuit

fig. EMC-18.

allowed to move unduly in an equipment.

Rule n 3 - It is recommended to usescreened cables for noisy and forsensitive circuits.Screening is an effective protection againstHF noise, provided that it is connected tomass at least at each end of the cable. It isquite possible to juxtapose two cables ofdifferent groups, provided that at least one(but preferably both) cable(s) is (are)

screened and connected by a flexible wovenmetallic tress to mass at each extremity.Screened cables properly installed areimmune to "cross-talk".

Rule n 4 - Only conductors of the sameGroup can be routed together in a cable,or in the same bundle.For flat ribbon-type multi-core cables, theconductors carrying analog signals shouldseparated from those carrying digital data byat least two conductors mass-connected tothe reference voltage of each card. For digitalconductors, connecting one wire out of two,of a flat ribbon-type cable, to the zero voltageat each end, reduces the HF cross-talkbetween lines by a factor of 5-10. Moreover, it

is detrimental to use one multi-coreconnecting cable link for different Groups. Inpractice, spacing the cables by approximately30 cm is generally sufficient, even in anisolated environment, to reduce the cross-talkto an acceptable level. Crossing two cablesfrom different Groups provides the lowestpossible mutual coupling if the two cablescross at 90. This practice should thereforebe carried out routinely.

Rule n 5 - Any free (i.e. unused)conductors of Groups n 2 or n 4 shouldalways be connected to the mass of thechassis at both ends. By this means, theattenuation effect can often reach a factorexceeding 2. These connections to mass

must be easily removed to free any coreswhich may be needed at a later date. ForGroup n 1 (at very low voltage andfrequency) such a connection could be adisadvantage and is not recommended.Noise at the industrial frequency could causeunacceptable interference.

Rule n 6 - The cables of Group n 4 neednot be screened if they are filtered.It is generally necessary to filter power-supplycables at the point of entry of an equipment.On the other hand, it is difficult to filter powercables supplying speed-change controllers,especially when the peak current is high.It then becomes necessary to screen thecables by flexible metallic tresses or by a

continuous metal tube connected to the massat each end.The opposite case is also true: a cable whichis well screened does not need filtering. In acommon plinth, a screened signal cable haspractically no problems of interference fromneighbouring power-supply cables.


18/22



EMC

3.5.2 EMC filtersAn EMC filter is a protection against

interference by conduction and is generallymade up of a combination of capacitors andinductors. Its role is to allow the passage ofenergy or signals within the band of usefulfrequencies and to reject parasiticfrequencies.Filters in the power-supply circuits are all low-pass filters which allow power-frequencycurrents to flow, but which suppress currentsof higher frequencies. For an interconnectingcoaxial cable, a high-pass filter is anti-parasitic: it allows the HF signal to pass, butrejects any LF interference current.The cable can then be connected to mass atboth ends without any difficulty. A filter at theinput to a radio receiver is a band-pass filterwhich rejects signals outside the band of

frequencies required (as well as anyinterferences). Finally, a harmonic filter is anotch filter, which is tuned to act as a short-circuit (generally phase/phase) at a harmonicfrequency, usually two or more filters for thefirst several odd-numbered harmonics abovethe fundamental frequency, since theseinvariably have the greatest magnitude.An EMC filter being a linear circuit as long asthe inductors remain unsaturated) andpassive, is also bilateral. It is equally effectiveat a given frequency from the interior to theexterior, as in the opposite sense. A filterfunctions firstly by reflection, i.e. by sendingthe energy back towards the source, due to amismatch of the filter/line impedances, then

by absorption, i.e. loss of energy in the formof heat, as it passes through the filter.Since inductors are low-loss components atLF, the L-C filters function principally byreflection. The effectiveness of a filter alsodepends on the upstream and downstreamimpedances. If these impedances vary, theefficiency of the filter referred to as "insertionlosses", will vary. Remark: if a filter canmismatch a line, there is the possibility that itmay also match a line. This is a phenemenonwhich may be observed on LF power-supplyline filters: a resonance (even partial) of thefilter results in a deterioration, at LF, of thelevel transmitted, compared to that when nofilter is present.It should be verified that the resonant

frequency of the filter is not likely to be aproblem (it should be below that of thecurrent-chopping frequency, for example).Filters in power-supply circuits usually useinductors in common mode, also called"current-compensation coils" or"compensated inductors".


3.5 EMC components and solutions (continued)

Such filters evidently present differentdegrees of effectiveness in common mode

than in differential mode.If the inductor of a filter is saturated by thecurrent flowing through it, the effectiveness ofthe filter is greatly impaired.In order to respect the EMC standards, a filteris practically obligatory in power-supplycircuits. Where no filter has been installed, itis often necessary to select one having anefficacity in the order of 30 dB in commonmode at 100 MHz. A power-supply filter must,in order to perform efficiently at HF, beinstalled according to three rules:1 - Screw the filter sheet-metal to sheet-metalin order to limit its impedance to the mass.2 - Arrange the supply cable to enter the filterat the opposite face to that of the outputcircuit, in order to limit common-mode

upstream/downstream coupling.3 - Fix the cables firmly (i.e. clamped) againstthe sheet-metal of the unit to limit radiationfrom the upstream conductors affecting thedownstream circuit.Preferred practice is to install all filters of anequipment on the same metal base whichserves as the potential reference. The notionof equipotentiality at HF is local: eachequipment has to provide, by means of itsconducting envelope, its own potentialreference to the input and output filters and toshielded connection cables.The signal filters are often R-C combinations.A simple resistor of the order of 1 k in serieswith a sensitive line can suffice to reinforce its

immunity. Small inductors in common modecan also be used, with 2, 4, or more, wireswound "two wires in hand".It is interesting to note that these componentsreduce common-mode interference, withoutaffecting the useful signals transmitted indifferential mode.

plated. An electromagnetic shield need not

necessarily be earthed to be effective. For amagnetic field simply its presence issufficient. For electric fields, it is enough thatthe screen acts as potential reference for theinput and output circuits. It may be concludedthat a shield prevents the fields frompenetrating the protected space, but also,and especially, prevents parasitic currentsfrom entering. Thus, shields and filters arenot rivals, but are complementary, one withthe other.If a screen is of excellent quality, with noleakage, it is possible to install input and

output connectors at any convenient point. If,

on the contrary, a screen performs badly, withexcessive leakage (display, keyboard,printed-circuit board or disk reader) then, it isan advantage to group all the input andoutput connectors on a common chassis,remote from the leakage, the role of thischassis being that of a reference-potentialpoint. It may be noted that all modernmicrocomputers have their cables grouped atthe rear face, remote from the disk unitswhich are mounted on the front face.



EMC

3.5.3 overvoltage protectionThe role of an overvoltage limiter, sometimesreferred to as "surge diverter" or "lightningarrester" (depending on its intended location)is to reduce the risk of destruction tocomponents or entire equipments byinterferences which may occur at excessivevoltage levels.An overvoltage limiter is generally a non-linear unilateral device: it limits the peak valueof voltage at a level which is much lower thanthat of the incoming surge. This reduced levelbeing, in principle, lower than the ratedimpulse withstand capability of alldownstream plant and equipment.The limiting of the voltage peak, however,does not reduce the HF radiation fieldstrength. Conversely, a low-pass filter doesnot limit the voltage peak, the duration ofwhich, at half-peak level, considerably

exceeds the response time of the filter.Thus, an efficient filter which suppressesfrequencies above 10 kHz would have a rise-time of about 35 s. This filter cannot limit anovervoltage due to lightning, the tail-time ofwhich to half-peak is standardized at 50 s.The first voltage limiters used in telephonesystems were gas-filled discharge devices.A gas-filled glass envelope contains twoelectrodes separated one from the other by acalibrated space. An overvoltage ionizes thegas which allows a discharge to occurbetween the electrodes, thereby reducing thepotential and allowing the gas to de-ionize.Such a component is robust and has only asmall parasitic effect.Its occasional failure, often by short-circuitingof the electrodes (i.e. following anovervoltage discharge, the ionized gassometimes provokes a short-circuit at normalworking voltage) means that its reliabilitycannot be guaranteed. In order to protect a

public service supply line, the low arc-voltage,several tens of volts, requires the installationof a varistance in series, to extinguish the arcwhen the surge has been dissipated.Analogous components exist at high voltage("horned spark-gaps" for example). At lowvoltage, "silicon spark-gaps" such as "Trisil"of Thomson (a triac controlled by a Zenerdiode in the trigger circuit) are well adapted tothe protection of telecommunication lines andcircuits.The highly non-linear metal-oxide varistancecomponents are well adapted to theprotection of supply-circuits. A disk of zinc-oxide becomes conductive when the voltageapplied to its two faces exceeds a"knee-point" value. That voltage, proportionalto the thickness of the disk, varies from sometens of volts to several kilovolts. The energythat a component can tolerate depends on

the volume of the disk: from tens of Joules tosome tens of thousand Joules. The maindrawback of varistances is their degradationduring periods of conduction.Zener diodes of very low dynamic resistancehave a precise knee-point voltage and a shortresponse time. Their low energy handlingcapacity, of a fraction of a Joule to severalJoules, limits the use of these components tothe protection of signal circuits. Failure ofsuch a diode always occurs as a short-circuit,a condition that guarantees "fail-safe"protection for the circuits.In all cases, an overvoltage device incommon mode should be connected directlyat the mass of the item to be protected, andnot, as is still often the case, by a long cableconnected radially from a distant earth bar.The response time of an overvoltage limitingdevice depends on the length of itsconnections.



EMC

Local networks present at least one particularproblem: the numerous equipments arespread out, relatively distant one fromanother, and are installed for userconvenience rather than for EMCcoordination considerations, are oftensupplied from different lines, andinterconnected by conventional wiringpractices.

4. local network problems

Such an installation invariably creates anumber of mass loops of very large area.The interconnection of equipments createslarge mass loops. One of the most seriousdangers for local networks is the magneticfield in the areas of the mass loops, createdby the current from a lightning stroke. It maybe noted that a surge induced in the interiorof a building, on average once a year, gives

rise to an overvoltage which can attain, orexceed, 100 volts per square metre of looparea.The meshing of masses should be carried outin the three dimensions (laterally andvertically), especially in multistorey buildingswith network equipments on several floors.Two adjacent floors must be meshed togetherby all conducting metal work which passesthrough the intervening floor. Themultiplication of these conductors affords thefollowing advantages:1 - Improvement of the "verticalequipotentiality" of the building by effectivelyreducing the value of loop inductances, andconnecting them in parallel.

2 - Improvement of the "horizontalequipotentiality" of the building and thesymmetrical flow of surge current directly toearth.3 - Reduction of induction from the magneticfield in the interior of the building. At a pointmidway between two parallel conductorspassing equal currents in the same direction,the magnetic field strength H is zero.Experience has shown that if the masses areineffectively meshed, and interconnectingcables are without attenuation effects, someprinted-circuit boards can be expected to bedestroyed by the induction of even a distantlightning stroke. On the other hand, if themasses are reasonably well interconnected,with conducting cable trays screwed firmly to

the equipment metal frames, a lightningstroke (even direct) produces minorinterference, and causes no destruction ofelectronic circuits. In a badly-meshedenvironment, only equipments completelydisconnected or well shielded are out ofdanger from lightning.

signal cable

I

field H

peripheralcontrol unit

supply cable

supplycable

fig. EMC-21: interconnection ofequipment forms loops with earthedconductors.

The current induced in a mass loop by the

magnetic field of a lightning stroke has thesame form as that of the stroke current; it canexceed 100 A in the case of a large loop.The best solution for limiting the risks ofdestruction is to bring together, for example ina common tray, the signal cables and thepower-supply cables. A shielded cable, withits screen properly connected to mass ateach end is free from cross-talk interference.The presence of a mass cable in intimate(e.g. clamped) contact with a cable (signal orpower-supply) reduces typically by a factor of3 or 4 the interference caused by lightning,provided that the mass cable is connected tomass (at least) at each end. A bolted metalcable duct throughout the entire route of acable has an attenuation factor of the order

30.The woven metal screen of a shielded cable,with short, direct connections to mass at bothends, reduces the induced voltage by a factorof about 100.Local networks processing large amounts ofdata require that the characteristic impedanceof interconnecting signal cables be matchedto the input/output impedances of theinterconnected equipments, in order to avoidlosses by reflections due to mismatchedimpedances.If one of the two matching units of a long lineis disconnected, transmission becomesimpossible.A frequent problem on local networks, apart

from software parameters, is the loss ofavailability caused by electromagneticinterference. The software "filters" errors, butthe useful output is sometimes severelyreduced. The user only notices theseproblems in the rare cases of permanentinterference. The simple observance of theserveral EMC installation rules cited in theforegoing text is sufficient to resolve theseproblems.

fig. EMC-22: lightning producesinterference more often by induction thanby a direct-stroke current.

insulationstressed by 15 kV

signal cable

H

t aera 300 m2

I/t = 100 kA/s

400 m




22/22

EMC

list of "cahiers techniques"

NCT English French Spanish

114 Residual current devices X X X

141 Les perturbations lectriques en BT X X

144 Introduction to dependability design X X145 Etude thermique des tableaux lectriques BT X X

147 Initiation aux rseaux de communication X

numrique

148 High availability electrical power distribution X X

149 EMC: Electromagnetic Compatibility X X

150 Development of LV circuit breakers X X X

to standard IEC 947-2

152 Harmonics in industrial networks X X X

154 LV circuit breaker breaking techniques X X X

155 MV public distribution networks X X X

throughout the world

156 Sret de fonctionnement et tableaux lectriques BT X X

158 Calculation of short-circuit currents X X X

159 Inverters and harmonics X X X

(case studies of non-linear loads)

160 Harmonics upstream of rectifiers in UPS X X

161 Automatic transfering of power supplies X Xin HV and LV networks

162 Les efforts lectrodynamiques X X

sur les jeux de barres en BT

163 LV breaking by current limitation X X

166 Enclosures and degrees of protection X X X

167 Energy-based discrimination X X X

for low-voltage protective devices

172 Earthing systems in LV X X X

173 Earthing systems wolrdwide and evolutions X X X

179 Surtensions et parafoudres en BT, X

Coordination de l'isolement en BT

electrical installation guide appendix

Documents