IT system - When the fault current-loop impedance is particularly high
From Electrical Installation Guide
When the earth-fault current is restricted due to an inevitably high fault-loop impedance, so that the overcurrent protection cannot be relied upon to trip the circuit within the prescribed time, the following possibilities should be considered:
Suggestion 1 (see Fig. F60)
- Install a circuit-breaker which has an instantaneous magnetic tripping element with an operation level which is lower than the usual setting, for example:
This affords protection for persons on circuits which are abnormally long. It must be checked, however, that high transient currents such as the starting currents of motors will not cause nuisance trip-outs.
- Schneider Electric solutions
- Compact NSX with G trip unit or Micrologic trip unit (2Im ≤ Irm ≤ 4Im)
- Type B Multi 9 circuit-breaker
Fig. F60: A circuit-breaker with low-set instantaneous magnetic trip
Suggestion 2 (see Fig. F61)
Install a RCD on the circuit. The device does not need to be highly-sensitive (HS) (several amps to a few tens of amps). Where socket-outlets are involved, the particular circuits must, in any case, be protected by HS (≤ 30 mA) RCDs; generally one RCD for a number of socket outlets on a common circuit.
- Schneider Electric solutions
- RCD Multi 9 NG125 : ΙΔn = 1 or 3 A
- Vigicompact MH or ME: ΙΔn = 3 to 30 A
Fig. F61: RCD protection
Suggestion 3
Increase the size of the PE conductors and/or the phase conductors, to reduce the loop impedance.
Suggestion 4 (see Fig. F62)
Add supplementary equipotential conductors. This will have a similar effect to that of suggestion 3, i.e. a reduction in the earth-fault-loop resistance, while at the same time improving the existing touch-voltage protection measures. The effectiveness of this improvement may be checked by a resistance test between each exposed conductive part and the local main protective conductor.
Fig. F62: Improved equipotential bonding
