Chapter A General rules of electrical installation design

F - Protection against electric shock
Residual current load break switches are
covered by particular national standards.
RCDs with separate toroidal current
transformers are standardized in IEC 60947-2
appendix M
I1 I2
Fig. F66 : The principle of RCD operation
I3
8 Residual current devices (RCDs)
Residual current circuit-breakers and RCDs with separate
toroidal current transformer (see Fig. F65)
RCDs with separate toroidal CTs can be used in association with circuit-breakers or
contactors.
Fig. F65 : RCDs with separate toroidal current transformers
8.2 Description
Principle
The essential features are shown schematically in Figure F66 below.
A magnetic core encompasses all the current-carrying conductors of an electric
circuit and the magnetic flux generated in the core will depend at every instant on
the arithmetical sum of the currents; the currents passing in one direction being
considered as positive (Ι1), while those passing in the opposite direction will be
negative (Ι2).
In a normally healthy circuit Ι1 + Ι2 = 0 and there will be no flux in the magnetic core,
and zero e.m.f. in its coil.
An earth-fault current Ιd will pass through the core to the fault, but will return to the
source via the earth, or via protective conductors in a TN-earthed system.
The current balance in the conductors passing through the magnetic core therefore
no longer exists, and the difference gives rise to a magnetic flux in the core.
The difference current is known as the “residual” current and the principle is referred
to as the “residual current” principle.
The resultant alternating flux in the core induces an e.m.f. in its coil, so that a current
I3 flows in the tripping-device operating coil. If the residual current exceeds the value
required to operate the tripping device either directly or via an electronic relay, then
the associated circuit-breaker will trip.
8.3 Sensitivity of RDCs to disturbances
In certain cases, aspects of the environment can disturb the correct operation of
RCDs:
b “nuisance” tripping: Break in power supply without the situation being really
hazardous. This type of tripping is often repetitive, causing major inconvenience and
detrimental to the quality of the user's electrical power supply.
b non-tripping, in the event of a hazard. Less perceptible than nuisance tripping,
these malfunctions must still be examined carefully since they undermine user safety.
This is why international standards define 3 categories of RCDs according to their
immunity to this type of disturbance (see below).
Schneider Electric - Electrical installation guide 2008
F37
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