c <strong>The</strong> correspond<strong>in</strong>g contact voltage isU C =R pe I d , i.e.:v if one of the faults is on the neutralmU =0,8U ,c 0 or21 ( + m)v if the double fault concerns two phasemconductors U c=0,8eU021 ( + m ).NB: this method is not applicable for <strong>in</strong>stallationssupplied by generator set, as, due to highgenerator impedance compared with theimpedance of the supplied network, voltage atthe orig<strong>in</strong> of the network <strong>in</strong> question is low whena fault occurs (> 400 0.02 0.08Fig. 16 : maximum break<strong>in</strong>g time specified for the <strong>IT</strong><strong>earth<strong>in</strong>g</strong> <strong>system</strong> by <strong>in</strong>stallation standards (* for s<strong>in</strong>glephasenetworks).c Protection of the neutral conductor when it isdistributed.Figure 3b shows that when a double fault occurs,the two SCPDs detect the fault current but eachone on a s<strong>in</strong>gle phase or on the neutral.This situation calls for particular monitor<strong>in</strong>g ofSCPD characteristics: this is because if thecables of the two feeders have similar crosssections,the two SCPDs play an equal role <strong>in</strong>break<strong>in</strong>g, but if the cross-sections are different,there is a risk of only one SCPD, the one withCahier Technique Schneider Electric no. 178 / p.16
the lowest rat<strong>in</strong>g, perform<strong>in</strong>g break<strong>in</strong>g. It is thusnecessary to verify that its break<strong>in</strong>g capacity onone phase, thus under e U 0 , is greater than I d .For this reason, circuit-breaker manufacturersspecify the s<strong>in</strong>gle-phase break<strong>in</strong>g capacities oftheir devices accord<strong>in</strong>g to each nom<strong>in</strong>al voltage,and standard IEC 60947-2 specifies a testsequence for circuit-breakers designed forprotection of <strong>IT</strong> networks. Devices fail<strong>in</strong>g tosatisfy the requirements of these tests must bemarked: <strong>IT</strong>Protection must also be confirmed for the neutralconductor when its cross-section is less thanthat of the phase conductors. Note that four-polecircuit-breakers (the fourth pole has a half rat<strong>in</strong>g)can be used to protect cables with neutral crosssectionhalf of phase cross-section.It should be stressed that four-pole SCPDs arebecom<strong>in</strong>g <strong>in</strong>creas<strong>in</strong>gly necessary, irrespective ofthe <strong>in</strong>stallation <strong>earth<strong>in</strong>g</strong> <strong>system</strong> used (TN, TT or<strong>IT</strong>), due to the proliferation of harmonics <strong>in</strong>networks, and thus that the neutral can beoverloaded by harmonic currents of rank 3 andmultiples.c Fuse protection<strong>The</strong> fuse blow<strong>in</strong>g zone is located between twoenvelope curves.Us<strong>in</strong>g the expression of current I d , def<strong>in</strong>ed <strong>in</strong> theprevious chapter, and the condition I fu < I d , it ispossible to determ<strong>in</strong>e the maximum length of theprotected circuit.v If the neutral conductor is distributed:L = 0,8 U 0 S 1max.2ρ ( 1+ m) Ifuv If the neutral conductor is not distributed:L = 0,8 e U 0 S phmax.1+ m2ρ ( ) I fuI fu corresponds to the fuse blow<strong>in</strong>g current with<strong>in</strong>a maximum time stipulated by the standards.It should be checked that this time is compatiblewith protection of persons <strong>in</strong> event of a doublefault.Note that use of fuses <strong>in</strong> the <strong>IT</strong> <strong>earth<strong>in</strong>g</strong> <strong>system</strong>often clashes with the need for multi-polebreak<strong>in</strong>g, <strong>in</strong>clud<strong>in</strong>g that of the neutral conductorwhen distributed.c Circuit-breaker protectionProtection of persons is guaranteed when thefault current is greater than the circuit-breaker’sshort time delay protection sett<strong>in</strong>g.Just as with fuses, the maximum length of theprotected circuit can be determ<strong>in</strong>ed accord<strong>in</strong>g tothe expression of current I d , def<strong>in</strong>ed <strong>in</strong> theprevious chapter and the condition I m < I d .<strong>The</strong> maximum length of the circuit protected by acircuit-breaker is:v with the distributed neutral conductor:SL =0,8U amax 02ρ ( 1+m) I.mv with the non-distributed neutral conductor:SL max =0,8eUa02 ρ ( 1+m ) I.mNote that whether protection is provided by fuse orcircuit-breaker, the fact of distribut<strong>in</strong>g the neutral<strong>in</strong> <strong>IT</strong> divides by e the maximum length protected.c Improvement of tripp<strong>in</strong>g conditions.When tripp<strong>in</strong>g conditions are not satisfied(lengths greater than maximum lengthsprotected), the follow<strong>in</strong>g measures can be taken:v reduce the value of I m of the circuit-breakers:however current discrim<strong>in</strong>ation between circuitbreakersmay be reduced as a result;v <strong>in</strong>crease PE conductor cross-section. <strong>The</strong>impedance of the return circuit of the double faultcurrent is thus reduced and enables an <strong>in</strong>crease<strong>in</strong> maximum length for protection of persons.However, although contact voltage will bereduced, the electrodynamic stresses on thecables will <strong>in</strong>crease.v Increase live conductor cross-section. This isthe most expensive solution and also results <strong>in</strong>an <strong>in</strong>crease <strong>in</strong> three-phase short-circuit currents.v F<strong>in</strong>ally, there is a simple solution that requiresno calculation: use of low sensitivity RCDs onvery long feeders. This solution is also possible<strong>in</strong> <strong>IT</strong>, as the PE conductor is separate from theneutral conductor which is not the case <strong>in</strong> TN-C.Case of application frames with separateearth connectionsWhen an <strong>in</strong>stallation supplies a number ofseparate build<strong>in</strong>gs at a distance from oneanother, their application frames are oftenconnected to separate earth connections. <strong>The</strong>impedance of the path of fault current I d is then<strong>in</strong>creased by the resistance of the two earthconnections <strong>in</strong> question, and the conditionnecessary for protection of persons (respect ofmaximum break<strong>in</strong>g times) can no longer beguaranteed by the short-circuit protection devices.<strong>The</strong> simplest study and <strong>in</strong>stallation solution is touse RCDs. <strong>The</strong>ir sett<strong>in</strong>gs follow the same rulesas <strong>in</strong> TT.To derive maximum benefit from the cont<strong>in</strong>uity ofsupply offered by the <strong>IT</strong> <strong>system</strong>, the RCDs mustbe prevented from tripp<strong>in</strong>g on the first fault bynot sett<strong>in</strong>g their threshold IDn at too low a level,particularly for circuits with a high leakagecapacity, while at the same time respect<strong>in</strong>g the<strong>in</strong>equation: I∆n
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