Chapter A General rules of electrical installation design

F - Protection against electric shock
Fig. F59 : Fire-risk location
2 y I rm y 4I n
Great length of cable
PE
Fig. F60 : A circuit-breaker with low-set instantaneous
magnetic trip
Fire-risk
location
Phases
Neutral
PE
7 Implementation of the IT system
7.4 Protection in high fire-risk locations
Protection by a RCD of sensitivity y 500 mA at the origin of the circuit supplying the
fire-risk locations is mandatory in some countries (see Fig. F59).
A preferred sensitivity of 300 mA may be adopted.
7.5 When the fault current-loop impedance is
particularly high
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)
b Install a circuit-breaker which has an instantaneous magnetic tripping element with
an operation level which is lower than the usual setting, 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. 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 (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 : ΙΔn = 1 or 3 A
v Vigicompact REH or REM: ΙΔn = 3 to 30 A
Fig. F61 : RCD protection Fig. F62 : Improved equipotential bonding
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.
Schneider Electric - Electrical installation guide 2008
F35
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