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Differences between extra-low voltage systems Hybrid ring/radial final circuits The Electrical Installation Condition Report RCD testing for a lighting circuit with no circuit protective conductor snags and solutions: A practical guide to everyday electrical problems
Differences 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 differences 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. PELV
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
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. Requirements for SELV and PELV
SELV system
An exposed-conductive-part of a SELV system must not be connected to:
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Earth,
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an exposed-conductive-part of another system,
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a protective conductor of any system.
Summer 2011 NICEIC Connections
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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 specified requirements. Regulation 414.3 gives the requirements for these sources.
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14/6/11 15:19:25
