1
to 411.3.6, remove earth fault current very quickly, less than 0.8
Construction RCD.2.
The Regulations recognize RCDs ‘additional protection’ the event of
failure the provision for basic protection, fault protection carelessness
by the users the installation (IEE Regulation 415.2.
Consumer units can now supplied which incorporate RCD, split
boards, where half the final circuits are RCD protected that any
equipment supplied the consumer unit socket outlet circuits out
of doors circuits which are outside the zone created the protective equi-
potential bonding, such garage greenhouse, can have the special
protection required IEE Regulations 411.1). Modern
RCDs have tripping sensitivities between and 30mA, and therefore a
faulty circuit can isolated before the lower lethal limit human beings
(about 50mA) reached.
N N
E
Casing
Supply Trip
coil
L L
Test button
Load
FIGURE 12. 12. However, more current flows the line con-
ductor than the neutral conductor result fault between live and
earth, out-of-balance flux will result emf being induced the trip
coil which will open the double pole switch and isolate the load. The load cur-
rent fed through two equal and opposing coils wound common
transformer core.
. This is
in direct contrast the protection provided overcurrent devices, which
require low resistance earth loop impedance path.
The basic circuit for single-phase RCD shown Fig.
RCBOs (residual current circuit breaker with overload protection) com-
bines RCD protection and MCB protection into one unit.1.1.3 and 415.3.1. The phase and neutral currents healthy circuit pro-
duce equal and opposing fluxes the transformer core, which induces no
voltage the tripping coil.Basic Electrical Installation Work
252
Additional protection: RCDs
When required provide the very best protection from electric shock
and fire risk, earth fault protection devices are incorporated the instal-
lation. The object the Regulations concerning these devices, 411.8.3.4s
for all final circuits not exceeding 32A and limit the voltage which might
appear any exposed metal parts under fault conditions not more
than They will continue provide adequate protection throughout
the life the installation even the earthing conditions deteriorate
