What is an RCD and what does it do?

What is an RCD and what does it do?
1.       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 pre-set limit, the residual operating current (IDn).
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 overcurrent’s. Overcurrent protection is provided by a fuse or a miniature circuit-breaker (MCB).
However, combined RCD and MCBs are available and are designated RCBOs.
2.     Types of RCDs
RCD is the generic term for a device that operates when the residual current in the circuit reaches a predetermined value.
2.1  Older installations with ELCBs
Historically, two basic types of earth leakage circuit-breaker (ELCB) were recognised by the Regulations; the familiar current-operated type and the earlier voltage-operated type. The voltage-operated type ceased to be recognised by the Regulations in 1981and today, only the current-operated type is recognised. The voltage operated device can be distinguished by its two separate earthing terminals– one for the connection of the
earthing conductor of the installation and the other for a connection to a means of earthing. Such devices were often used on installations forming part of a TT system where the means of earthing was an earth electrode.
The major drawback with the voltage operated earth leakage circuit-breaker is that a parallel earth path can  disable the device.
2.2  Recognised devices
RCDs are manufactured to harmonised standards and can be identified by their BS EN numbers. An RCD found  in an older installation may not provide protection in accordance with current standards. The following list identifies the applicable current standards:
> BS 4293 : 1983 (1993)
Specification for residual current operatedcircuit-breakers. (Replaced by BS EN 61008-
1: 1995, BS EN 61008-2-1: 1995 and BS
IEC 61008-2-2: 1990). This Standard
remains current
> BS 7071 : 1992 (1998)
Specification for portable residual current
> BS 7288 : 1990 (1998)
Specification for socket-outlets incorporating
residual current devices. (SRCDs)
> BS EN 61008-1 : 1995 (2001)
Residual current operated circuit-breakers
without integral overcurrent protection for
household and similar uses (RCCBs)
> BS EN 61009-1 : 2004
Residual current operated circuit-breakers
with integral overcurrent protection for
household and similar uses (RCBOs)
2.3  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, IDn.
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, IDn.
3.     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.
3.1  Unwanted tripping
Unwanted tripping of RCDs can occur when a protective conductor current or leakage current causes unnecessary operation of the RCD.
An RCD must be so selected and the electrical circuits so subdivided that any protective conductor current that may be expected to occur during normal operation of the connected load(s) will be unlikely to cause  unnecessary tripping of the device (Regulation 531-02-04 refers). Such tripping can occur on heating elements, cooking appliances etc., which may have elements that absorb a small amount of moisture through imperfect element end seals when cold. When energised, this moisture provides a conductive path for increased leakage and could operate the RCD. The moisture dries out as the element heats up. Although not precluded in BS 7671, it is not a requirement to use an RCD on such circuits if other satisfactory means of protection are available.
Providing an RCD with a higher rated residual operating current may solve the problem but the requirements of the Regulations would still have to be met.
3.2  Discrimination
Where two, or more, RCDs are connected in series, discrimination must be provided, if necessary, to prevent danger (Regulation 531-02-09 refers). During a fault, discrimination will be achieved when the device electrically nearest to the fault operates and does not affect other upstream devices.
Discrimination will be achieved when ‘S’ (Selective) types are used in conjunction with downstream general
type RCDs. The ‘S’ type has a built-in time delay and provides discrimination by simply ignoring the fault for a set period of time allowing more sensitive downstream devices to operate and  remove the fault.
For example, when two RCDs are connected in series, to provide discrimination, the first RCD should
be an ‘S’ type. RCDs with built in time delays should not be used
4.     Labelling
Regulation 514-12-02, states that:
“Where an installation incorporates a residual current device a notice shall be fixed in a prominent position at or near the origin of the installation. The notice shall be in indelible characters not smaller than those here illustrated and shall read as follows:”

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 of the
supply and should then be switched on to restore
the supply.
If the device does not switch of the supply when
the button is pressed, seek expert advice



           5.     Testing
RCDs must be tested. The requirements are stated in the following Regulations:
a. The effectiveness of the RCD must be verified by a test simulating an appropriate fault condition and independent of any test facility, or test button, incorporated in the device (Regulation 713-13-01)
b. Where an RCD of 30mA provides supplementary protection the operating time must not exceed 40 ms at a residual current of 5 IDn.
(Regulation 412-06-02 refers) Tests are made on the load side of the RCD between the phase conductor of the protected circuit and the associated cpc. Any load or appliances should be disconnected prior to testing. RCD test instruments require a few milliamperes to operate; this is normally obtained from the phase and neutral of  the circuit under test.
When testing a three-phase RCD protecting a three-wire circuit, the instrument’s neutral is required to be connected to earth. This means that the test current will be increased by the instrument supply current and
will cause some devices to operate during the 50% test, possibly indicating an incorrect operating time. Under this circumstance it is necessary to check the operating parameters of the RCD with the manufacturer before failing the RCD.
5.1  Range of tests
5.2  Integral test device
An integral test device is incorporated in each RCD. This device enables the mechanical parts of the RCD to be
Verified by pressing the button marked
‘T’ or ‘Test’.
6.     Test Instrument
The test instrument used to test RCDs should be capable of applying the full range of test current to an in- service accuracy, as given in BS EN 61557-6.
This in-service reading accuracy will include the effects of voltage variations around the nominal voltage of the tester. To check RCD operation and to minimise danger during the test, the test current should be applied for no longer than 2s. Instruments conforming to BS EN 61557-6 will fulfil the above requirements.
Device Instrument test current setting Satisfactory result
General purpose RCDs to BS 4293 and RCD protected socket-outlets to BS 7288
50% of operating current Device should not operate 100% of operating current Device should operate in less than 200ms. Where the RCD incorporates an intentional time delay it should trip within a time range from 50% of the rated time delay plus 200ms’ to 100 % of the rated time delay plus 200ms
General purpose RCCBs to BS EN 61008 or RCBOs to BS EN 61009
50% of operating current Device should not operate 100% of operating current Device should operate in less than 300ms unless it is of ‘Type S’ (or selective) which incorporates an intentional
time delay. In this case, it should trip within a time range from 130ms to 500ms
Supplementary protection against direct contact IDn 30mA
Test current at 5 IDn. The maximum test time must not be longer than 40ms, unless the protective conductor potential does not exceed 50V. (The instrument supplier will advise on compliance).
Device should operate in less than 40ms.
I hope this overview will be of service to others, for further information do not hesitate to contact me directly from our website www.safe-electric.com or by E-Mail : – [email protected]