Chapter A General rules of electrical installation design

F - Protection against electric shock
A
RCD
Fig. F29 : Distribution circuits
RCD
B
RCD
RA1 RA2
Distant location
5 Implementation of the TT system
5.1 Protective measures
Protection against indirect contact
General case
Protection against indirect contact is assured by RCDs, the sensitivity IΔn of which
50 V
complies with the condition I �n i (1) (1)
RA
The choice of sensitivity of the residual current device is a function of the resistance
RA of the earth electrode for the installation, and is given in Figure F28.
Schneider Electric - Electrical installation guide 2008
IΔn Maximum resistance of the earth electrode
(50 V) (25 V)
3 A 16 Ω 8 Ω
1 A 50 Ω 25 Ω
500 mA 100 Ω 50 Ω
300 mA 166 Ω 83 Ω
30 mA 1666 Ω 833 Ω
Fig. F28 : The upper limit of resistance for an installation earthing electrode which must not be
exceeded, for given sensitivity levels of RCDs at U L voltage limits of 50 V and 25 V
Case of distribution circuits (see Fig. F29)
IEC 60364-4-41 and a number of national standards recognize a maximum tripping
time of 1 second in installation distribution circuits (as opposed to final circuits). This
allows a degree of selective discrimination to be achieved:
b At level A: RCD time-delayed, e.g. “S” type
b At level B: RCD instantaneous
Case where the exposed conductive parts of an appliance, or group of
appliances, are connected to a separate earth electrode (see Fig. F30)
Protection against indirect contact by a RCD at the circuit-breaker level protecting
each group or separately-earthed individual appliance.
In each case, the sensitivity must be compatible with the resistance of the earth
electrode concerned.
High-sensitivity RCDs (see Fig. F31)
Fig. F30 : Separate earth electrode Fig. F31 : Circuit supplying socket-outlets
(1) 25 V for work-site installations, agricultural establishments, etc.
According to IEC 60364-4-41, high sensitivity RCDs (y 30 mA) must be used for
protection of socket outlets with rated current y 20 A in all locations. The use of such
RCDs is also recommended in the following cases:
b Socket-outlet circuits in wet locations at all current ratings
b Socket-outlet circuits in temporary installations
b Circuits supplying laundry rooms and swimming pools
b Supply circuits to work-sites, caravans, pleasure boats, and travelling fairs
See 2.2 and chapter P, section 3
F19
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