Additional protection: High sensitivity RCDs
An additional measure of protection against the hazards of contact with live parts is provided by the use of residual current operating device, which operates at 30 mA or less, and are referred to as High Sensitivity RCDs
All the preceding protective measures are preventive, but experience has shown that for various reasons they cannot be regarded as being infallible. Among these reasons may be cited:
- Lack of proper maintenance
- Imprudence, carelessness
- Normal (or abnormal) wear and tear of insulation; for instance flexure and abrasion of connecting leads
- Accidental contact
- Immersion in water, etc. A situation in which insulation is no longer effective.
In order to protect users in such circumstances, highly sensitive fast tripping devices, based on the detection of residual currents to earth (which may or may not be through a human being or animal) are used to disconnect the power supply automatically, and with sufficient rapidity to prevent injury to, or death by electrocution, of a normally healthy human being (see Fig. F8).
These devices operate on the principle of residual current measurement, in which any difference between the current entering a circuit and that leaving it (on a system supplied from an earthed source) is flowing to earth, either through faulty insulation or through contact of an earthed part, such as a person, with a live conductor.
Standardized residual current devices, referred to as RCDs, sufficiently sensitive for protection against contact with live parts are rated at 30 mA of residual current. The response time as defined in IEC 61008 and 61009 is fast enough to provide protection. See Fig. F9.
According to IEC 60364-4-41, additional protection by means of high sensitivity RCDs (IΔn ≤ 30 mA) must be provided for:
- circuits supplying general use socket-outlets with a rated current ≤ 32 A liable to be used by ordinary persons,
- for circuits supplying mobile equipment with a rated current ≤ 32 A for use outdoors,
- AC final circuits supplying luminaires, in dwelling.
It is also recommended to limit the number of socket-outlets protected by a high sensitivity RCD (e.g. 10 socket-outlets for one RCD).
The page Requirements applicable to special installations and locations itemizes various common locations in which RCDs of high sensitivity are mandatory (in some countries), but in any case, are highly recommended as an effective protection against both contact with live parts and contact with conductive parts in fault conditions.
High sensitivity RCDs are suitable for all types of earthing systems. See Fig. F10 (a) for TT or TN systems, and (b) for IT system.