Additional protection: High sensitivity RCDs

An additional measure of protection against the hazards of direct contact is provided by the use of residual current operating device, which operate at 30 mA or less, and are referred to as RCDs of high sensitivity

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. F6).

Fig. F6: High sensitivity RCD

These devices operate on the principle of differential current measurement, in which any difference between the current entering a circuit and that leaving it (on a system supplied from an earthed source) be flowing to earth, either through faulty insulation or through contact of an earthed part, such as a person, with a live conductor.
Standardised residual-current devices, referred to as RCDs, sufficiently sensitive for protection against direct contact are rated at 30 mA of differential current.
According to IEC 60364-4-41, additional protection by means of high sensitivity RCDs (I∆n ≤ 30 mA) must be provided for circuits supplying socket-outlets with a rated current ≤ 20 A in all locations, and for circuits supplying mobile equipment with a rated current ≤ 32 A for use outdoors.
This additional protection is required in certain countries for circuits supplying socket-outlets rated up to 32 A, and even higher if the location is wet and/or temporary (such as work sites for instance).
It is also recommended to limit the number of socket-outlets protected by a RCD (e.g. 10 socket-outlets for one RCD).
Chapter P section 3 itemises various common locations in which RCDs of high sensitivity are obligatory (in some countries), but in any case, are highly recommended as an effective protection against both direct and indirect contact hazards.