5.9.3 - Regulations for residual current devices

The primary purpose of the residual current device is to limit the severity of shock due to indirect contact. In other words, it will detect and clear earth faults which otherwise would could lead to dangerous potential differences between pieces of metalwork which are open to touch. If the sensitivity of the device (its operating residual current) is low enough, it may also be used to limit the shock received from direct contact in the case of the failure of other measures. A problem which may occur here is nuisance tripping, because the operating current may be so low that normal leakage current will cause operation. For example an RCD with a sensitivity of 2 mA will switch off the supply as soon as a shock current of 2 mA flows, virtually preventing a fatal shock. The difficulty is that normal insulation resistance leakage and stray capacitance currents can easily reach this value in a perfectly healthy system, and it may thus be impossible to keep the circuit breaker closed. The sum of the leakage currents in circuits protected by an RCD should never be more than 25% of the operating current of the device. Normal earth leakage current from equipment and appliances will, of course vary with the condition of the device. Maximum permitted leakage currents are listed in Appendix L of the 2nd Edition of Guidance Note 1, and vary from 0.25 mA for Glass II appliances to 3.5mA for information technology equipment (see {7.8.2})

Some RCDs (usually electronic types) will not switch off unless the mains supply is available to provide power for their operation. In such a case, mains failure may prevent tripping whilst danger is still present, (due to, for example, charged capacitors). Such RCDs may only be used where there is another means of protection from indirect contact, or where the only people using the installation are skilled or instructed so that they are aware of the risk.

In some cases RCD are designed so that their operating parameters, such as the rated residual current or the time delay, can be adjusted. If such an RCD can be operated by an ordinary person (rather than by a skilled or instructed person then such adjustments must only be possible by a deliberate act using a key or a tool which results in a visible indication of the setting.

If a residual current circuit breaker is set at a very low sensitivity, it can prevent death from electric shock entirely. However, the problem is that a safe current cannot be determined, because it will vary from person to person, and also with the time for which it is applied. The Regulations require a sensitivity of 30 mA for RCDs intended to provide additional protection from direct contact.

An RCD must not be used in an installation with neutral and earth combined (TNG system using a PEN conductor) because there will be no residual current in the event of a fault to cause the device to operate, since there is no separate path for earth fault currents.

RCD protection is required for socket outlets where:

1. - they are part of a TT system (no earth terminal provided by the Electricity Supply Company),

2. - they are installed in a bedroom which contains a shower cubicle, or

3. - the socket outlet(s) are likely to feed portable equipment used outdoors.

4. - they are installed in zones B or C of a swimming pool and comply with BS EN 60309-2.

Protection by an RCD with a rating of 30 mA is required for fixed electrical equipment installed in a bathroom or in zone C of a swimming pool.

Although residual current devices are current-operated, there are circumstances where the combination of operating current and high earth-fault loop impedance could result in the earthed metalwork rising to a dangerously high potential. The Regulations draw attention to the fact that if the product of operating current (A) and earth-fault loop impedance exceeds 50, the potential of the earthed metalwork will be more than 50 V above earth potential and hence dangerous. This situation must not be allowed to arise

Fig 5.24 - Danger with an RCD when earth-fault loop impedance is high. In this case, p.d. from earth to exposed conductive parts will be 1000 Ohms x 0.09 A = 90 V

RCDs must he tested to ensure correct operation within the required operating times. Such tests will be considered in {8.6.3}.

Special requirements apply to RCDs used to protect equipment having normally high earth leakage currents, such as data processing and other computer-based devices. These installations are considered in {7.8.2}.