Meba Electric receives many enquires relating to the inspection and testing of electrical installations and the applicable requirements.The enquires 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.
1. What is an RCD and what does it do?
A residual current device (RCD) is defined 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 line and neutral conductors.
RCDs are used to provide protection against the specific dangers that may arise in electrical installations, including:
●additional protection; and
●protection against fire.
An RCD does not provide protection against overcurrent. Overcurrent protection is provided by a fuse or a circuit-breaker. However, combined RCD and circuit-breakers are available and are designated RCBOs.
2. Characteristics of RCDs
RCDs are defined by a series of three main electrical characteristics:
●The rating of the device in amperes, I.
●The rated residual operating current of the protective device in amperes, IΔn.
●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 in this article we will consider the rated residual operating current of the protective device, IΔn.
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.
Regulation 514.12.2, states the notice that shall be fixed in a prominent position at or near the origin of the installation, where an installation requires a residual current device:
RCDs must be tested; the requirements are stated in the following Regulations:
●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.
●Where an RCD with a rated residual operating current, IΔn, not exceeding 30 mA is used to provide additional 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 line conductor of the protected circuit and the associated cpc. Any load or appliances should be disconnected prior to testing.
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.