The IEE’s Mark Coles has written an excellent guide to the inspection and testing of electrical installations – residual current devices (RCDs), which is – indeed – its title. VoltiBULLETIN, with the kind permission of the IEE, presents an abbreviated version of it, the first of a series of articles, here:
The IEE receives many enquiries relating to the inspection and testing of electrical installations and the applicable requirements of BS 7671: 2001 (2004). The queries vary greatly and cover all aspects of inspection and testing, from the initial verification process of domestic installations to the periodic inspection of major industrial installations. In this article, RCDs are examined.
What is an RCD and what does it do?
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 specific dangers which 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; this is provided by a fuse or a miniature circuit-breaker (MCB). However combined RCD and MCBs are available (RCBOs).
Types of RCDs
There are a number of RCD types available. They include RCCBs, RCBOs (used in consumer units and distribution boards), CBRs (used in distribution boards in larger installations), SRCDs (often installed to provide supplementary protection against direct contact for portable equipment used out of doors), PRCDs (plugged into an existing socket-outlet - not part of the fixed installation), and SRCBOs (often installed to provide supplementary protection against direct contact for portable equipment used out of doors).
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. Today, only the current-operated type is recognised.
RCDs are manufactured to harmonised standards and can be identified by with their BS EN numbers. An RCD found in an older installation may not provide protection in accordance with current standards. These are: BS 4293 : 1983 (1993); BS 7071 : 1992 (1998); BS 7288 : 1990 (1998); BS EN 61008-1 : 1995 (2001); and BS EN 61009-1 : 2004.
RCD characteristics
RCDs are defined by three main characteristics: the rating in amperes; the rated residual operating current of the protective device in amperes, IΔn; and whether the device operates instantaneously or incorporates an intentional time delay to permit discrimination. Such devices are called ‘S’ or Selective. IΔn for commercially available devices generally fall into a range between 10 mA and 2.
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. For example, an RCD with an IΔn of 10mA is very sensitive and can be used to protect laboratory benches in schools. AQ 30mA RCD is used to protect portable equipment used outdoors, and certain equipment in showers and bathrooms, as well as to protect socket-outlets in workshops, school laboratories used by performers and entertainers. Swimming pools, caravans, under-floor heating and building sites are all examples of installations requiring use of 30mA RCDs. For an installation forming part of a TT system, a 100 mA RCD is generally installed at the origin. In TN and TT systems, in locations with risks of fire due to the nature of processed or stored materials, wiring systems, except for MICC and busbar trunking systems must be protected against insulation faults to earth by a 300 mA device. In agricultural and horticultural premises, a 500 mA device must be installed to protect equipment against fire and harmful thermal effects (not including effects on animals). Finally, an adjustable ≤2000 mA RCD may be used in specific industrial and distribution applications.
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 which may be expected to occur during normal operation of the connected
load(s) will be unlikely to cause unnecessary tripping of the device. Such
tripping can occur on heating elements, cooking appliances etc. However,
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.
Discrimination
Where two, or more RCDs are connected in series, discrimination must be provided, if necessary, to prevent danger. 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, with their built-in time delay, are used in conjunction with downstream general type RCDs.
Testing
RCDs must be tested: a) by a test simulating an appropriate fault condition and independent of any test facility, or test button, incorporated in the device, or b) where an RCD of 30 mA provides supplementary protection the operating time must not exceed 40 ms at a residual current of 5 IΔn.
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.
There is a range of tests, which are not a specific requirement of BS 7671, but it is recommended that they are carried out.
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’.
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. Instruments used to measure earth fault loop impedance should conform to the requirements of BS EN 61557-3.
Important notice: This IEE document is not reproduced in full. To obtain the full document, which includes much more detail and extra information, as well as extensive tabular information, please contact the IEE at www.iee.org.uk.