Diff erences between extra-low voltage systems

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Diff erences between extra-low voltage systems

BS 7671 recognises three types of extra-low voltage (ELV) system: SELV (Separated Extra-Low Voltage), PELV (Protective Extra-Low Voltage) and FELV (Functional Extra-Low Voltage). This article looks at the diff erences between SELV, PELV and FELV and at the main requirements of BS 7671 where each of them is used. Extra-low voltage is defined in Part 2 of BS 7671 as a nominal voltage not exceeding 50 V a.c. rms or 120 V ripple-free d.c., whether between conductors or to Earth.

SELV

A SELV system is an extra-low voltage system electrically separated from Earth and from other systems. To this end the live parts of a SELV system must have: • basic insulation between live parts and other SELV or PELV circuits, • basic insulation between live parts and Earth, and • protective separation from live parts of any other systems by means of double or reinforced insulation, or basic insulation and protective screening for the highest voltage present. (Regulation 414.4.1 refers).

The conductors of each SELV circuit must be separated from the insulated conductors of other circuits by one of four arrangements listed in Regulation 414.4.2, such as physical separation or the enclosure of SELV circuit insulated conductors in a non-metallic sheath or non-metallic enclosure.

It will be seen from Fig 1 that no exposed-conductive-part of the SELV system is connected to Earth or to an exposed-conductive-part of another system or to a protective conductor of any system. (Regulation 414.4.4 refers).

Also, as required by Regulation 414.3, the step-down transformer in Fig 1 is of the safety isolating type, which gives protective separation (equivalent to double or reinforced insulation) between the primary and secondary windings.

Fig 1 Example of a SELV circuit supplied by

a safety isolating transformer

Exposed-conductive-part of a SELV system not to be connected to any other conductive parts

fety isolating ansformer Overcurrent protective device Exposedconductive-part PELV

A PELV system meets all the requirements for a SELV system except those relating to electrical separation from Earth.

In the example of a PELV system in Fig 2, both a live conductor of the PELV circuit and an exposed-conductive-part (the metal case) of the ELV current-using equipment are earthed by being connected to the earthed protective conductor of the circuit supplying the transformer primary, as permitted by Regulation 414.4.1.

SELV system

An exposed-conductive-part of a SELV system must not be connected to: • Earth, • an exposed-conductive-part of another system, • a protective conductor of any system. Requirements for SELV and PELV

SELV and PELV are recognised by BS 7671 as protective measures giving both basic protection and fault protection against electric shock.

A SELV or PELV system must be supplied from a safety isolating transformer or a source providing equivalent safety (such as a motor generator set with suitable isolation from the higher voltage circuit), or a battery or other source independent of the higher voltage circuit, or an electronic device meeting specifi ed requirements. Regulation 414.3 gives the requirements for these sources.

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 eff ects 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).

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. Fig 2 Example of a PELV circuit supplied

by a safety isolating transformer

Fig 3 Example of a FELV circuit supplied

by a double-wound transformer

FELV circuit connected to Earth

Protective device Double-wound transfer

Protective device

L V source

FELV circuit FELV current-using equipment

FELV live part may be connected to cpc of primary source