5.4Purpose of a residual current device#

The fundamental rule for protection against electric shock is that:

1. 1.hazardous live parts shall not be accessible, and
2. 2.accessible conductive parts shall not be hazardous
3. a.under normal operating conditions, and
4. b.under a single fault condition.

The normal protection measures against hazardous live parts include:

1. 1.protection by the insulation of live parts
2. 2.protection by means of barriers or enclosures
3. 3.protection by means of obstacles or placing out of reach, and
4. 4.protection by use of extra low voltage.

Sometimes these preventative measures fail due to lack of maintenance, imprudence, carelessness, normal wear and tear, age or environmental effects. Where electrical equipment is contained in an earthed metal box, any current leakage from the equipment is likely to flow in the earthed enclosure. This could lead to dangerous potential differences between that enclosure and other earthed metalwork which is also open to touch. The main purpose of the RCD is to limit the severity of electric shock due to indirect contact. This occurs when a person touches an exposed conductive part that is not normally live but has become live accidentally, due to some fault condition. RCDs with a sensitivity of 30 mA are designed to monitor the residual current, operate, and disconnect the electrical supply, automatically clearing the fault with sufficient speed to prevent serious injury, fibrillation of the heart, or death by electrocution of a normal healthy human being (refer Section 7 for the effects of current through the body).