What do the abbreviations mean?

R.C.D.: Residual Current Device is the generic term covering the range of devices incorporating sensing of residual current and includes within the scope R.C.C.B. and R.C.B.O. type products.

R.C.C.B.: Residual Current Circuit Breaker is an RCD which will cause disconnection of electrical supply should a residual current passing through the device exceed a specified level.

R.C.B.O.: Residual Current Circuit Breaker with Overload protection is an RCD which will cause disconnection of electrical supply due to residual current exceeding specified limits together with integral overload; overcurrent and short circuit protection associated with a miniature circuit breaker.

Definitions:

Residual Current: is the vector sum of the currents of all the phases and associated neutral passing through the core balance transformer of an RCD.

Equipotential Zone: the zone within which all conductive parts are maintained at substantially the same potential by bonding to Earth.

When must an RCD be used?

	BS 7671 (16th Edition IEE Regs)
(i) Socket outlets on a TT supply	(471-08-06)
(ii) Sockets to supply portable equipment outside the equipotential zone.	(471-16-01)
(iii) Supply to caravan	(608-13-05)

When is it advisable to install an RCD?

• For protection against risk of fire due to live to earth fault where fault current is insufficient to cause over-current protection device to operate.
• For protection against risk of shock from indirect contact with equipment suffering a live to earth fault.
• For protection against shock in potentially hazardous environment.
• As supplementary protection against shock from directly touching 'Live' parts.

Note: an RCD must not be used as the sole means of protection against touching live parts.

What trip current rating should be selected?

10mA - to gave a high degree of protect on against electric shock in a hazardous environment situation where supplementary protection against shock from accidental direct contact is required. This rating should only be used to supply final circuits where a high risk exists.

30mA - to give a high degree of protection against electric shock in a situation where supplementary protection against shock from accidental direct contact is required when it must be able to trip within 40 milliseconds when a fault current of 150mA is detected. This will also satisfy the IEE/BS condition for supplementary protection of sockets feeding portable equipment outside the equipotential zone.

100mA - to give a degree of protection against electric shock due to indirect contact situation.

Generally this rating should be used to protect groups of circuits and provide overall protection against fire risk.
If lower rated RCD devices are employed down stream then a time delayed 100mA RCD should be employed to ensure discrimination between same.

300mA - gives overall protection against risk of fire from electrical faults in wiring etc, only where sufficient current (typically less than 500mA) may cause incandescence of metal parts in suitable circumstances and in consideration that installed over current devices would require far in excess of 300mA to operate.

If lower rated RCD devices are employed down stream then a time delayed 300mA RCD should be employed to ensure discrimination between same.

NOTE: 10mA; 30mA and 100mA also inherently protect against this risk.

Principles of Operation of RCCB

Brief Explaination of Operating Fundamentals

When a Load is connected to the circuit supplied through an RCD current flows from the SUPPLY through the RCD whereby both phase and neutral form the primaries of a CORE BALANCE TRANSFORMER arrangement, the SECONDARY of which is used as a SENSING COIL to detect any out of balance between the current flowing through the live and neutral conductors in the circuit.

A test circuit is also incorporated whereby connection is made from load phase to supply neutral via a TEST COIL and RESISTOR and activated by a TEST BUTTON.

This test circuit is designed to pass a current well in excess of the related tripping current of the RCD in question.

If a fault occurs on the load side of the RCD whereby a fault current (Ifn) flows between Live and Earth. The Load still demands a current return through the neutral of the RCD of I amps whilst the current flow through the Live becomes I+ Ifn and from this imbalance a corresponding current will be induced electro-magnetically in the sensing coil which if of sufficient magnitude and duration will cause the actuator to function and trip the RCD thereby disconnecting the supply.

However it should be noted that other disturbances that may cause imbalance between phase and neutral can emanate from upstream and/or downstream sources to give rise to the effect of unwanted tripping as identified, in 'Trouble-shooting'.