How to Test Separation of Circuits 

Protection by SELV or PELV 

BS7671 Regulation 612.4 
The separation of circuits shall be verified in accordance with Regulation 612.4.1 in the case of protection by SELV, Regulation 612.42 in the case of protection by PELV and Regulation 612.4.3 in the case of protection by electrical separation. The resistance values obtained in the tests of Regulations 612.4.1 to 612.4.3 shall be at least of the circuit with the highest voltage present in accordance with Table 61 

Before proceeding with tests an inspection should be carried to verify: 

  • The source of supply complies with Regulation 414.3 e.g. A safety isolating transformer to BS EN 61558-2-6 
  • The output voltage may have to be measured to confirm it does not exceed 50 V a.c. or 120 V d.c. If the output voltage exceeds 25 V a.c. or 60 V d.c., there is a need to check out the means provided for basic protection is in compliance with Regulations 416.1 and 416.2 i.e. the use of insulation barriers and enclosures.

All live parts of the SELV circuits must be tested to ensure that the electrical separation requirements of the Regulations are satisfied. This is done by testing between the live conductors of each SELV circuit strapped together and the live conductors of any higher circuits strapped together. The test voltage is 500 V d.c. and the minimum insulation resistance is 1 MΩ. (These values are quoted in Section 2.7.8 and Table 2.2 of the IEE Guidance Notes 3).


Testing separation of circuits


An additional resistance test is required if the cables of an SELV circuit are grouped with circuits of different voltages. This test is between the live conductors of the SELV circuit and the protective conductor of the highest voltage circuit present. Again, the test should be carried out at 500 V, with a minimum resistance of 1 MΩ.


Testing Electrical Separation 

The source of supply must provide at least simple separation as set out in Regulation 413.3.2 and if in any doubt, the output voltage should be measured and should not exceed 500 VTesting is required to confirm that the live parts of the separated circuit are electrically separate form other conductors in the same enclosure including earth. A further test is carried out between the separated circuit and other adjacent circuits. The test is done at 500 V d.c. by testing between the live conductors of the separated circuit connected together, and the live conductors of any other adjacent circuit strapped together or to earth. The resulting minimum insulation resistance value should be not less than 1 MΩ If the separated circuit supplies more than one item of equipment there are a number of additional requirements to be met. 

They include the following: 

(i) A continuity test is to be carried out between all exposed parts of the separated circuit to ensure they are all equipotential bonded together. This is to be followed by an insulation resistance test between the exposed conductive parts of the separated circuit and the protective conductor, or exposed conductive parts of other circuits or extraneous-conductive-parts. The insulation resistance should not be less than 1 MΩ. 

(ii) A check made to ensure the protective conductor contact in any socket is connected to the equipotential bonding conductor.

(iii) A check made to ensure all flexible cables to electrical equipment have been provided with a protective conductor connecting to the equipotential bonding except for class II equipment

(iv) Verification of the fault loop impedances between live conductors to ensure safe disconnection times are met during fault conditions. Tables used for earth loop impedance values (41.2, 41.3 and 41.4) can be used to determine maximum impedance values for L-N Loop Impedance values.