Chapter A General rules of electrical installation design

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F - Protection against electric shock
Fig. F47 : Fire-risk location
2 y Irm y 4In
Great length of cable
Fire-risk
location
PE or PEN
Fig. F48 : Circuit-breaker with low-set instantaneous magnetic
tripping
Phases
Neutral
PE
6.4 Protection in high fire-risk location
According to IEC 60364-422-3.10, circuits in high fire-risk locations must be
protected by RCDs of sensitivity y 500 mA. This excludes the TN-C arrangement and
TN-S must be adopted.
A preferred sensitivity of 300 mA is mandatory in some countries (see Fig. F47).
6.5 When the fault current-loop impedance is
particularly high
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)
b Install a circuit-breaker which has a lower instantaneous magnetic tripping level, for
example:
2In y Irm y 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.
b Schneider Electric solutions
v Type G Compact (2Im y Irm y 4Im)
v Type B Multi 9 circuit-breaker
Suggestion 2 (see Fig. F49)
b 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 (y 30 mA) RCDs; generally
one RCD for a number of socket outlets on a common circuit.
b Schneider Electric solutions
v RCD Multi 9 NG125 : IΔn = 1 or 3 A
v Vigicompact REH or REM: IΔn = 3 to 30 A
v Type B Multi 9 circuit-breaker
Fig. F49 : RCD protection on TN systems with high earth-faultloop
impedance Fig. F50 : Improved equipotential bonding
6 Implementation of the TN system
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.
Schneider Electric - Electrical installation guide 2008