The IT earthing system (unearthed neutral) in LV

c Vector chart in presence of a full faultIn event of a full fault on phase 1, the potential ofphase 1 is at earth potential (see fig. 8 ).The neutral-to-earth potential is thus equal tophase to neutral voltage V1, and that of phases2 and 3 with respect to earth is equal tophase-to-phase voltage. If the neutral isdistributed, the fault current is arithmeticallyincreased:I C = 4j Cω V1.However, detection, location and correction ofthis fault must be immediate in order to reducethe risk of a second simultaneous fault occurringwhich would result in opening of the faulty circuits.a) b)23V 1NvN2v 3V 3 V 2II C3 d1T I V 1-T I = I V 3-T I = I V 2-T I TI C2V 1-T = 0V 3-T = V 1 + V 3V 2-T = V 1 + V 2I d = I C = I C2 + I C3I C2 = j C ω v 2I C3 = j C ω v 3I C = 3j C ω V 1I I d I = 3 C ω I V 1 IFig. 8 : vector charts of a network in the IT system, without fault [a] and with an earth fault on phase 1 [b].2.2 Permanent insulation monitors, history and principlesThe first LV electrical distribution networks wereoperated using the IT earthing system.Operators rapidly sought to detect the presenceof the first insulation fault in order to prevent thehazards linked to a short-circuit current ofvarying impedance and the de-energisation of afaulty feeder (with the lowest rating protection) orof the two faulty feeders.The first PIMsThese devices used 3 lamps connected betweenthe phases and the earth (see fig. 9 ).On a sound network, the three lamps form abalanced three-phase load, all lit and with thesame brilliance. When an insulation fault occurs,one of the three lamps is short-circuited by thefault impedance. Voltage is reduced at theterminals of this lamp, and lamp brilliancedecreases. However, voltage at the terminals ofthe other two lamps increases until phase-tophasevoltage is reached. Their luminosity alsoincreases.This system is easy to install and use. However,given that its practical operating threshold is low,attempts were quickly made to try to detectimpedant faults in order to anticipate the full fault.For a DC network(supplied by batteries or by DC generator).The technique of the voltmeter balance(see fig. 10 ) was the first to be used, andindeed is still used today.(1)(2)(3)The OFF indicatorlight indicates thefaulty phase:in this case no. 3.(+) (-)RThe needle indicatesthe faulty polarity; in thiscase the (-) polarity.RFig. 9 : principle of the first PIM.Fig. 10 : principle of the PIM with voltmeter balance.Cahier Technique Schneider Electric no. 178 / p.11