RESIDUAL CURRENT DEVICE

An RCD is defined, in BS 7671, as:
‘A mechanical switching device or
association of devices intended to cause
the opening of the contacts when the
residual current attains a given value
under specified conditions’.
An RCD is a protective device used
to automatically disconnect the
electrical supply when an imbalance is
detected between live conductors. In
the case of a single-phase circuit, the
device monitors the difference in
currents between the phase and
neutral conductors. In a healthy
circuit, where there is no earth fault
current or protective conductor
current, the sum of the currents in the
phase and neutral conductors is zero.
If a phase to earth fault develops, a
portion of the phase conductor
current will not return through the
neutral conductor. The device
monitors this difference, operates and
disconnects the circuit when the
residual current reaches a preset limit,
the residual operating current (IΔn).
RCDs are used to provide protection
against the specific dangers that may
arise in electrical installations
including:
> protection against indirect contact
> supplementary protection against
direct contact
> protection against fire and thermal
effects
An RCD on its own does not
provide protection against
overcurrents. Overcurrent
protection is provided by a fuse or a
miniature circuit-breaker (MCB).
However, combined RCD and MCBs
are available and are designated
RCBOs.

Characteristics of RCDs
RCDs are defined by a series of
electrical characteristics, three main
characteristics are:
1. The rating of the device in amperes, I.
2. The rated residual operating
current of the protective device in
amperes, IΔn.
3. Whether the device operates
instantaneously or incorporates an
intentional time delay to permit
discrimination. Such devices are
called ‘S’ or Selective.
Devices are manufactured with
different values of rated current and
rated residual operating current but we
will just consider the rated residual
operating current of the protective
device in amperes,
Applications
The correct device must be selected for the particular application. Choosing
the wrong device could have serious
consequences and could result in
electric shock or fire.