other special locations or installations detailed in Part 7 of BS 7671. However, the specific requirements of Part 7 must also be applied, as appropriate. For example, where a SELV circuit is installed in a location containing a swimming pool or basin, basic protection is required irrespective of the nominal voltage (Regulation 702.414.4.5 refers).
The SELV source The source for a SELV system must be of a type listed in Regulation 414.3. A safety isolating transformer complying with BS EN 61558-2-6 or BS EN 61558-2-8 is commonly used. These types of transformer are designed to provide protective separation (equivalent to double or reinforced insulation) between the primary and secondary windings, and have no provision for an earth connection on the secondary ELV side. As shown in Fig 1, SELV is an ELV system that is electrically separate from Earth and from other circuits, such as the low voltage primary circuit, and because of this separation there is no path for an earth fault current to flow. However, to confirm that protection against shock is provided by SELV the relevant requirements of Regulation Group 414 must be satisfied.
Requirements for SELV circuits (Regulation Group 414.4 ) To prevent this unearthed system from being compromised by earth faults from other circuits, Regulation 414.4.4 prohibits any Table 1: Insulation resistance testing for SELV Tests required to be conducted
Insulation resistance test voltage
Minimum acceptable value (from Table 61)
Test between all live conductors of the SELV* circuit, and Earth (Fig 2 (a)), and, where present, between the live conductors of each SELV circuit and the conductors of any other SELV (or PELV) circuit.
250 V DC
0.5 MΩ
Test between all live conductors of the SELV* circuit, and any live parts of a low voltage (or FELV) circuit (Fig 2 (b)). If the nominal circuit voltage is greater than 500 V the insulation resistance test should be carried out at 1000 V DC (Regulation 612.4 refers).
500 V DC or 1000 V DC
exposed-conductive-part of a SELV system to be connected to Earth or to an exposed-conductivepart, or protective conductor, of another system. Similarly, a socket-outlet or luminaire supporting coupler (LSC) that is connected to a SELV system should not have a protective conductor contact and must be designed so that it is not compatible (dimensionally) with a plug that is used for other systems, such as low voltage, at the same premises (Regulation 414.4.3 refers). Furthermore, to minimise the risk of SELV circuits being exposed to (higher) voltages from other circuits, especially as a result of faults occurring on those circuits, they should be separated from the insulated conductors of other circuits by one of the five arrangements listed in Regulation 414.4.2. These five arrangements are: • using physical separation, • insulating SELV conductors for the highest voltage present, • enclosing insulated SELV circuit conductors in a non-metallic sheath or non metallic enclosure, • confirming that other circuits (and wiring systems) comply with Regulation 412.2.4.1, and • SELV/PELV conductors installed in multi-core cables with circuit conductors at voltages higher than Band I are insulated for the highest voltage present. Irrespective of the arrangements employed for separation, it must be confirmed that the live parts of the SELV circuit satisfies the requirements of Regulation 414.4.1 for the provision of: • 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.
Testing 1 MΩ
*Where appropriate the live conductors may be connected together for the purpose of the test.
As required by Regulation 612.4.1, where the protective measure of extra-low voltage is provided by SELV, separation should be verified by insulation resistance testing. The requirements for the insulation resistance testing of SELV circuits is summarised in Table 1 and the tests illustrated in Fig 2. For the purposes of insulation resistance testing, functional extra-low voltage circuits (FELV) should satisfy the requirements for low voltage circuits (Regulation 612.3.4 refers). It should be noted that the minimum insulation resistance test values prescribed by Table 61
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