The RCD is a circuit breaker which
continuously compares the current in the phase with that in
the neutral. The difference between the two (the residual
current) will he flowing to earth, because it has left the
supply through the phase and has not returned in the neutral
(see {Fig 5.22}). There will always be some residual current
in the insulation resistance and capacitance to earth, but
in a healthy circuit such current will he low, seldom
exceeding 2mA.
Fig 5.22 The meaning of
the term residual current
The purpose of the residual current
device is to monitor the residual current and to switch off
the circuit quickly if it rises to a preset level. The
arrangement of an RCD is shown in simplified form in {Fig
5.23}. The main contacts are closed against the pressure of
a spring, which provides the energy to open them when the
device trips. Phase and neutral currents pass through
identical coils wound in opposing directions on a magnetic
circuit, so that each coil will provide equal but opposing
numbers of ampere turns when there is no residual current.
The opposing ampere turns will cancel, and no magnetic flux
will be set up in the magnetic circuit.
Residual earth current passes to the
circuit through the phase coil but returns through the earth
path, thus avoiding the neutral coil, which will therefore
carry less current. This means that phase ampere turns
exceed neutral ampere turns and an alternating magnetic flux
results in the core. This flux links with the search coil,
which is also wound on the magnetic circuit, inducing an
e.m.f. into it. The value of this e.m.f. depends on the
residual current, so it will drive a current to the tripping
system which depends on the difference between phase and
neutral currents. When the amount of residual current, and
hence of tripping current, reaches a pre-determined level,
the circuit breaker trips, opening the main contacts and
interrupting the circuit.
For circuit breakers operating at low
residual current values, an amplifier may be used in the
trip circuit. Since the sum of the currents in the phases
and neutral of a three-phase supply is always balanced, the
system can be used just as effectively with three-phase
supplies. In high current circuits, it is more usual for the
Fig 5.23 Residual
current circuit breaker
phase and neutral conductors to simply
pass through the magnetic core instead of round coils wound
on it.
Operation depends on a mechanical
system, which could possibly become stiff when old or dirty.
Thus, regular testing is needed, and the RCD is provided
with a test button which provides the rated level of
residual current to ensure that the circuit breaker will
operate. All RCDs are required to display a notice which
draws attention to the need for frequent testing which can
be carried out by the user, who presses a test button,
usually marked T. {Table 5.10} shows the required notice.
This installation, or part of
it, is protected by a device which automatically
switches off the supply if an earth fault develops.
Test quarterly by pressing the button marked 'T' or
'Test'. The device should switch off the supply, and
should then be switched on to restore the supply. If
the device does not switch off the supply when the
button is pressed, seek expert advice |
Table 5.10 - Periodic
test notice for residual current device
The test circuit is shown in {Fig
5.23}, and provides extra current in the phase coil when the
test button is pressed. This extra current is determined by
the value of the resistor R.
There are currently four basic types of
RCD. Class AC devices are used where the residual current is
sinusoidal - this is the normal type which is in the most
wide use. Class A types are used where the residual current
is sinusoidal and/or includes pulsating direct currents -
this type is applied in special situations where electronic
equipment is used. Class B is for specialist operation on
pure direct current or on impulse direct or alternating
current. Class S RCDs have a built-in time delay to provide
discrimination (see below).
It must be understood that the residual
current is the difference between phase and neutral
currents, and that the current breaking ability of the main
contacts is not related to the residual operating current
value, There is a widely held misunderstanding of this
point, many people thinking that the residual current
setting is the current breaking capability of the device. It
is very likely that a device with a breaking capacity of
100A may have a residual operating current of only 30mA.
There are cases where more than one
residual current device is used in an installation; for
example, a complete installation may be protected by an RCD
rated at 100mA whilst a socket intended for equipment
outdoors may be protected by a 30mA device. Discrimination
of the two devices then becomes important. For example, if
an earth fault giving an earth current of 250mA develops on
the equipment fed by the outdoor socket, both RCDs will
carry this fault current, and both will become unbalanced.
Since the fault is higher than the operating current of both
devices, both will have their trip systems activated. It
does not follow that the device with the smaller operating
current will open first, so it is quite likely that the
100mA device will operate, cutting off the supply to the
complete installation even though the fault was on a small
part of it. This is a lack of discrimination between the
residual current devices. To ensure proper discrimination,
the device with the larger operating current has a
deliberate delay built into its operation. It is called a
time delayed RCD.