4.2 Surge limitersNom<strong>in</strong>al voltage Arc<strong>in</strong>g U (V) Example : limiter to beof a limiter -Un- at power on 1.2/50 chosen for a 230/400 V(V) (NF C 63-150) frequency impulse wave network…250 400 < U < 750 < 1,750 … if connected betweenthe earth and the neutral.440 700 < U < 1,100 < 2,500 … if connected betweenthe earth and the phase.660 1100 < U < 1,600 < 3,500Fig. 20 : the nom<strong>in</strong>al voltage of a surge limiter must be adapted to network voltage.<strong>The</strong> previous section clearly expla<strong>in</strong>s why thesurge limiter is an “essential accessory” of the<strong>IT</strong> <strong>earth<strong>in</strong>g</strong> <strong>system</strong> and thus stipulated bystandards. It also protects the PIM aga<strong>in</strong>stovervoltages.Its clipp<strong>in</strong>g thresholds for overvoltages at powerfrequencies and for common mode impulseovervoltages are def<strong>in</strong>ed by standardNF C 63-150 (see fig. 20 ). <strong>The</strong>se thresholds arelower than the specified withstand of equipmentused on <strong>LV</strong> networks (230/400 V).It must be connected as close as possible to theMV/<strong>LV</strong> transformer between neutral and earth,or between a phase and earth if thetransformer’s secondary connection is of thedelta or non-distributed neutral k<strong>in</strong>d.NB:c limiters are not necessary on networksdownstream of a <strong>LV</strong>/<strong>LV</strong> transformer,c standard IEC 60364 does not specify use ofsurge limiters, as it considers that occurrence ofan MV/<strong>LV</strong> fault is rare. However, when this faultdoes occur, its consequences are frequentlyserious.OperationA surge limiter consists of two conductivecomponents separated by an <strong>in</strong>sulat<strong>in</strong>g film(see fig. 21 ).Impulse overvoltages generate arc<strong>in</strong>g betweenthe two conductive components, but do notshort-circuit the limiter.Energetic overvoltages melt the <strong>in</strong>sulat<strong>in</strong>g film,thus allow<strong>in</strong>g the run-off of a high current toearth. <strong>The</strong> cartridge must then be replaced: itsshort-circuit<strong>in</strong>g is reported by the PIM just as an<strong>in</strong>sulation fault. Moreover, it is useful, for livefault track<strong>in</strong>g, to consider its earth connection <strong>in</strong>the same way as a feeder, particularly if thisconnection is normally <strong>in</strong>accessible (such aswhen, for example, the limiter is placed <strong>in</strong> thetransformer cubicle).Important characteristicWhen all the application frames are properly<strong>in</strong>terconnected, the double fault concern<strong>in</strong>g boththe arced surge limiter and an <strong>in</strong>sulation fault on“Insulat<strong>in</strong>g film”disappears dur<strong>in</strong>ghigh powerovervoltagesArc<strong>in</strong>g zone dur<strong>in</strong>glow power overvoltagesInsulat<strong>in</strong>g caseConnection padFig. 21 : surge limiter pr<strong>in</strong>ciple (Merl<strong>in</strong> Ger<strong>in</strong> Cardewtype).a phase, becomes a short-circuit. Limiterwithstand must then be sufficient for the timerequired to elim<strong>in</strong>ate the fault current (Forexample, 40 kA must be withstood for 0.2 sec forMerl<strong>in</strong> Ger<strong>in</strong> Cardew limiters).In the rare case of the second <strong>in</strong>sulation faultoccurr<strong>in</strong>g upstream of the <strong>in</strong>com<strong>in</strong>g circuitbreaker,the double fault is elim<strong>in</strong>ated by theMV protection devices (just as for an upstreamshort-circuit on the ma<strong>in</strong> <strong>LV</strong> switchboard).For this reason, the time delay sett<strong>in</strong>g of thetransformer’s MV protection must take the thermalwithstand [f (I 2 t)] of the surge limiter <strong>in</strong>to account.<strong>The</strong> cross-section of the connection conductorupstream and downstream of the surge limitermust also have the same thermal withstand. Itscross-section is calculated <strong>in</strong> standardNF C 15-100.Cahier Technique Schneider Electric no. 178 / p.20
4.3 Why use an impedance?An impedance can be connected between thenetwork and the earth, normally between thetransformer neutral and the earth. Its value isapproximately 1,700 Ω at 50 Hz.Its purpose is to reduce variations <strong>in</strong> potentialbetween network and earth, caused by MVdisturbances or fluctuations <strong>in</strong> potential of thelocal earth. It is therefore particularlyrecommended for short networks supply<strong>in</strong>gmeasurement <strong>in</strong>struments sensitive to thispotential and for networks placed next tocommunication networks (Bus).A read<strong>in</strong>g of the table <strong>in</strong> figure 5 shows thatwhen the network is very slightly capacitive(case 1), the neutral impedance Z N causes thefault current to <strong>in</strong>crease, which neverthelessrema<strong>in</strong>s very low (≈ 250 mA <strong>in</strong> figure 5). Thiseffect is even slighter when the network is highlycapacitive (cases 2 and 3). In practice, thisimpedance effects only very slightly the contactvoltage U C which rema<strong>in</strong>s less than U L <strong>in</strong> soundnetworks.F<strong>in</strong>ally, presence of a resistance <strong>in</strong> the impedanceenables a reduction of the ferromagneticresonance hazard.Cahier Technique Schneider Electric no. 178 / p.21