The IT earthing system (unearthed neutral) in LV

The principle consists of measuring andcomparing voltages between the (+) polarity andthe earth, on the one hand, and the (-) polarityand the earth on the other. This principle makesauxiliary power sources unnecessary, since thenetwork supplies the PIM directly viameasurement sensors (resistances).This technique applies to two-phase AC and DCnetworks and does not allow live fault tracking.For AC networksThe most commonly used PIM are those withinsulation measurement by DC current injection.Permanent measurement of insulationresistance required use of active systems inplace of the previously used passive systems.This resistance can be measured accurately inDC (see fig. 11 ), which is why the first PIMs,placed between the network and the earth,injected a low DC current which flowed throughthe fault. This simple, reliable technique is stillextensively used today, but does not allow livefault tracking.Note that when these PIMs are used on mixednetworks (containing rectifiers without galvanicinsulation), they may be disturbed or evenVNI PIM321NPEI PIMR BFig. 11 : principle of PIM with current injection.“blinded” if a fault is present on the DC part ofthe network.These were followed by PIMs with AC currentinjection at low frequency (< 10 Hz), operatingon the same principle. Although these PIMsallow live fault tracking, they can be “misled” bycable capacities that are seen as insulationfaults and disturbed by frequency converters(variable speed controllers).For all AC and DC networksFinally, nowadays, given that networks arefrequently of the mixed AC/DC kind as well asvariable frequency, the new PIMs are able tomonitor insulation on all types of networks.c Some use squared wave pulses at very lowfrequency (≈ 1Hz). They allow PIM not to bedisturbed by earth leakage capacities, as theyare then immediately loaded then unloaded bythe next strobe pulse of opposite sign. They areuniversal in use and easily adapted to modernnetworks, in particular to those supplying powerelectronic devices which often deform the ACpulse. However, their response time, dependingon the network’s earth leakage capacity, may beas much as a few minutes and does not allowthe detection of intermittent faults.c In order to compensate the usage restrictionsof these PIMs for very long networks andnetworks with a large number of capacitivefilters, the low frequency AC current injectiontechnique has been improved by means of“synchronous demodulation” (see fig. 12 ): thistype of PIM applies a low frequency AC voltagebetween the network and the earth, measuresthe current flowing back via network insulationimpedance and calculates the voltage-currentshift.It is then possible to determine the resistive andcapacitive components of this current and thusrelate the threshold to the resistive componentonly. This upgrade, the result of digitaltechnology, combines the advantages of DCcurrent and low frequency AC current injectionwithout their disadvantages.I BFBF~mAVI R-BF= U BFU BFR NetworkI C-BFC NetworkI C-BFI BFZ NetworkI R-BFFig. 12 : the low frequency AC current injection technique has been improved by means of “synchronousdemodulation”, which enables the insulation drop (resistive leakage) to be distinguished from capacitive leakage.Cahier Technique Schneider Electric no. 178 / p.12