Any mobile source for a SELV or PELV system that is supplied at low voltage, such as a portable safety isolating transformer for a handlamp, must be selected and erected to meet the requirements for protection by the use of double or reinforced insulation in accordance with Section 412 (Regulation 414.3 refers). This is intended to give extra protection against the effects of possible damage to the equipment when being moved about. A socket-outlet or luminaire supporting coupler in a SELV or PELV system must not have a protective conductor contact and must be suitable only for use with a plug that is dimensionally incompatible with those used for other systems at the same premises (Regulation 414.4.3 refers).
Fig 2 Example of a PELV circuit supplied by a safety isolating transformer
FELV Unlike in a SELV or PELV system, the extra-low voltage in a FELV system is used for functional purposes only; it is not relied upon to contribute to protection against electric shock. A FELV system (see Fig 3) is part of the protective measure of Automatic Disconnection of Supply (ADS), covered by Section 411 of BS 7671. The source of supply for the FELV system must be either a double-wound transformer that provides at least simple separation, or a source that meets the requirements of Regulation 414.3 for a SELV or PELV system (Regulation 411.7.4 refers). It is not permitted to use an autotransformer as the source (although this was permitted by the 16th Edition and earlier of the Wiring Regulations). Basic protection against electric shock in the FELV system is provided by basic insulation rated to suit the nominal voltage of the primary circuit (generally by the use of mains voltage grade wiring) or by barriers and enclosures (Regulation 411.7.2 refers). Fault protection is provided by connecting the exposed-conductive-parts of the FELV system to the earthed protective conductor of the primary of the source, the circuit supplying the primary being subject to protection by Automatic Disconnection of Supply (Regulation 411.7.3 refers). A live conductor of the FELV system may also be connected to the protective conductor of the primary of the source. Socket-outlets and luminaire supporting couplers in a FELV system must have a protective conductor contact and must use a plug that is not dimensionally compatible with those used in any other system at the same premises (Regulation 411.7.5 refers). An example of where a FELV system might be used is a motor control circuit.
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Fig 3 Example of a FELV circuit supplied by a double-wound transformer
FELV circuit connected to Earth
Double-wound transfer Protective device
Protective device
FELV current-using equipment
LV source
FELV circuit
FELV live part may be connected to cpc of primary source
NICEIC Connections Summer 2011
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