Chapter A General rules of electrical installation design

F20
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F - Protection against electric shock
Fig. F32 : Fire-risk location
Fire-risk
location
5 Implementation of the TT system
In high fire risk locations (see Fig. F32)
RCD protection at the circuit-breaker controlling all supplies to the area at risk is
necessary in some locations, and mandatory in many countries.
The sensitivity of the RCD must be y 500 mA, but a 300 mA sensitivity is
recommended.
Protection when exposed conductive parts are not connected
to earth (see Fig. F33)
(In the case of an existing installation where the location is dry and provision of
an earthing connection is not possible, or in the event that a protective earth wire
becomes broken).
RCDs of high sensitivity (y 30 mA) will afford both protection against indirect-contact
hazards, and the additional protection against the dangers of direct-contact.
Fig. F33 : Unearthed exposed conductive parts (A)
5.2 Coordination of residual current protective
devices
Discriminative-tripping coordination is achieved either by time-delay or by subdivision
of circuits, which are then protected individually or by groups, or by a combination of
both methods.
Such discrimination avoids the tripping of any RCD, other than that immediately
upstream of a fault position:
b With equipment currently available, discrimination is possible at three or four
different levels of distribution :
v At the main general distribution board
v At local general distribution boards
v At sub-distribution boards
v At socket outlets for individual appliance protection
b In general, at distribution boards (and sub-distribution boards, if existing) and on
individual-appliance protection, devices for automatic disconnection in the event of
an indirect-contact hazard occurring are installed together with additional protection
against direct-contact hazards.
Discrimination between RCDs
The general specification for achieving total discrimination between two RCDs is as
follow:
b The ratio between the rated residual operating currents must be > 2
b Time delaying the upstream RCD
Discrimination is achieved by exploiting the several levels of standardized sensitivity:
30 mA, 100 mA, 300 mA and 1 A and the corresponding tripping times, as shown
opposite page in Figure F34.
Schneider Electric - Electrical installation guide 2008