8.5.4 - Tests for electrical separation of circuits

This section is concerned with tests necessary to ensure the safety of separated extra-low voltage (SELV), protective extra-low voltage (PELV) and functional extra-low voltage (FELV) circuits which are explained in {7.16}. In general, the requirement is a thorough inspection to make sure that the source of low voltage (most usually a safety isolating transformer) complies in all respects with the British Standard concerned, followed by an insulation test between the extra-low voltage and low voltage systems. The test is unusual in that a 500 V dc supply (from an insulation resistance tester) must be applied between the systems for one minute, after which the insulation resistance must not be less than 5 MOhms for SELV or PELV systems, or 0.5 MOhms for FELV systems. A further test at 3750 V dc for one minute is passed if no flashover occurs. This test in particular can be dangerous, and special care should be taken.

For SELV and FELV circuits, additional inspection must ensure that the low voltage requirements (not exceeding 50 V ac or 120 V dc) are met. If the voltage exceeds 25 V ac or 70 V dc (60 V ripple-free), barriers and enclosures must be tested to IP2X (see {Table 2.1})and a 500 V dc insulation test applied for one minute between the live conductors and metal foil wrapped round the insulation should produce a result of at least 0.5 MOhms

When insulation testing on electrically isolated circuits or on equipment which might be damaged by the test voltage, phase and neutral must be connected together and the test applied between them and earth.