TN system - When the fault current-loop impedance is particularly high
From Electrical Installation Guide
When the earth-fault current is limited 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. F48)
Fig. F48: Circuit-breaker with low-set instantaneous magnetic tripping
- Install a circuit-breaker which has a lower instantaneous magnetic tripping level, for example: 2In ≤ Irm ≤ 4In
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
- Type G Compact (2Im ≤ Irm ≤ 4Im)
- Type B Multi 9 circuit-breaker
Suggestion 2 (see Fig. F49)
Fig. F49: RCD protection on TN systems with high earth-fault-loop impedance
- 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: IΔn = 1 or 3 A
- Vigicompact REH or REM: IΔn = 3 to 30 A
- Type B Multi 9 circuit-breaker
Suggestion 3
Increase the size of the PE or PEN conductors and/or the phase conductors, to reduce the loop impedance.
Suggestion 4
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.
For TN-C installations, bonding as shown in Figure F50 is not allowed, and suggestion 3 should be adopted.
Fig. F50: Improved equipotential bonding
