In IT grounding systems, when the first insulation fault occurs, the calculation proved there was no risk (contact voltage lower than limit safety voltage).
Automatic de-energizing is therefore not compulsory: this is the main advantage of this system.
To retain this advantage, standards recommend (IEC 60364 - paragraph 4126.96.36.199) the use of an Insulation Monitoring Device (IMD) and locating of the first fault. In point of fact, if a second fault occurs, automatic breaking is vital due to the Electric Shock risk: this is then the role of the SCPDs backed up by the RCDs if required.
Locating the first fault for repairs (curative maintenance) is considerably simplified by the use of a Ground Fault Location Device (GFLD). Predictive maintenance, based on the monitoring (recording) of variations in insulation impedance of each circuit, is also possible.
Operating principle of the IMDs:
A fault on a circuit results in a drop in insulation, or more precisely in resistance of the network compared with earth. The purpose of the IMDs is thus to monitor the value of this resistance. They normally work by injecting an AC or DC current between the network and the earth and by measuring the value of this current (see Figure below).
Injection of a DC current ensures continuous knowledge of network insulation resistance. If this resistance drops below a pre-set threshold, then the IMD reports the fault.
Injection of low frequency AC current (F = a few hertz) monitors fault resistance but with a distortion due to the presence of network leakage capacities. This minor drawback compared with injection frequency, is made up for by an advantage in first fault locating (one single injection device).
LF current injection devices are now available which can separately give the network’s insulation resistance and reactance. Moreover, they enable locating of the first fault without circuit opening and without the problems due to highly capacitive feeders.