way the RCD operates is as follows
The phase and neutral cables from the
supply to the load are passed through a magnetic ring called
a toroid. On the toroid is wound a detector winding which
is taken to a tripping mechanism. It is the make-up of this
tripping mechanism which determines whether the RCD is electromechanical
Whilst the current flowing down the phase
conductor is balanced by that in the neutral the RCD takes
no action but if part of the current flows down to earth through
some metalwork or through a person's body an unbalance occurs
between phase and neutral.
This causes a magnetic field in the toroid
which is picked up by the detector winding, fed to the tripping
mechanism and the RCD opens. It is this speed of opening,
usually between 30 and 50 milliseconds, which gives the protection.
The operation of the electromechanical
tripping mechanism. The mechanism consists of a permanent
magnet which holds a tripping arm closed thus holding the
RCD in the closed position.A spring attempts to pull the arm
away from the magnet.
The Electrical Mechanical Device is called
the RCCB, Residual Current Circuit Breaker.
A winding on the magnet is connected to
the detector winding on the toroid and demagnetises the permanent
magnet in the event of a signal on the detector winding due
to a fault to earth.The arm is pulled away by the spring and
the RCD trips.
In the case of the electronic RCD the
signal from the toroid is fed into an electronic circuit which
accepts the signal, amplifies it and instructs a relay to
open the RCD.
In order to operate the electronic circuit
a mains feed is required into the circuit to feed the amplifier
etc and it is a requirement of the British Standard for electronic
RCDs that if the supply to the electronic circuit fails and
prevents its operation in the event of an earth fault the
RCD must open. This is particularly important in the case
of a loss of neutral which could cause the RCD not to work
even though the phase supply is still connected to the load.
This problem does not arise in the case
of the electromechanical RCCB in which the tripping is powered
by the signal induced into the detector winding on the toroid.
A test circiut is connected from neutral
to phase across the toroid and when the test button is pressed
creates an unbalance of about 2.5 times the normal tripping
current across the RCD which trips it. The use of the test
button which should be operated at least quarterly verifies
the operation of the RCD.
Standard electromechanical RCCBs
are designed to operate on normal supply waveforms and cannot
be guaranteed to operate where none standard waveforms are
generated by loads. The most common is the half wave rectified
waveform sometimes called pulsating dc generated by speed
control devices, semi conductors, computers and even dimmers.
Specially modified RCCBs are available
which will operate on normal ac and pulsating dc.
In addition to the RCCBs which are for
use on single phase and neutral supplies three phase RCDs
are also available and work on the same principle of balance
across the three phases or three phases and neutral.
Most three phase RCDs have the test circuit
connected across one phase and neutral and a problem can arise
if the RCCB is to be used to supply a three phase supply such
as a motor.