Principles of electrical safety:Prevention of indirect contact injuries

Prevention of indirect contact injuries

Some of the techniques used to prevent direct contact injuries will also prevent indirect contact injuries; this includes the use of SELV or PELV supplies and Class II equipment. However, by far the most common method is the earthing of exposed conductive parts, coupled with the installation of fuses or circuit breakers that disconnect the supply in the event of an earth fault, a technique known as Earthed Equipotential Bonding and Automatic Disconnection of Supply (EEBADS).

The EEBADS technique requires the exposed conductive parts of the apparatus and equipment to be earthed through a protective conductor that is connected to the main earthing terminal of the installation. Overcurrent protective devices, such as fuses and cir- cuit breakers are placed in the phase conductors. When an earth fault occurs, connecting a live conductor to exposed metalwork and thereby creating the conditions for indirect contact injuries, the fault current flows back to the earthed point(s) of the supply system. If the impedance of the fault circuit, the earth fault loop impedance, is low enough to ensure that the fault current is high enough to operate the protective circuit breakers or fuses quickly enough, danger from indirect contact will be prevented. The limiting values of earth loop impedance for different types and ratings of protective devices are listed in BS 7671 (see Table 16.2); this specifies that supplies to hand-held equipment must be disconnected within 0.4 s and those to fixed equipment must be disconnected within 5 s of the fault occurring. In circumstances where sufficiently low values of earth loop imped- ance cannot be achieved the disconnection device may need to be an RCD.

Extraneous conductive parts, being metalwork that does not form part of the instal- lation but which can introduce a potential (usually earth) into the installation, must also be bonded together and earthed. This is usually achieved by using suitably sized bonding conductors to connect water, gas and oil pipes (and any other metallic services) to the installation’s main earthing terminal. Connecting together the exposed and extra- neous conductive parts in this way creates an equipotential zone in which the metalwork is normally held at earth potential; it can raise to higher potentials during fault condi- tions but, since the touchable conductive parts are all connected together, the risk of electrical injury is reduced to an acceptable level. In areas of higher risk, such as bath- rooms and swimming pools, supplementary bonding conductors are used to connect together all the exposed metalwork.

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