Q & A of the Day – Why is continuity of R1+R2 bigger than measured earth loop impedance?

Question: During the testing of a small power 16A radial circuit, protected by a Type C MCB, the continuity of protective conductor test result (R1+R2), was in excess of the measured earth loop impedance test result (Zs). Is there a technical reason for this, and would this test result be acceptable?

Answer: The possibilities include problems with continuity or earth fault loop test instrument accuracy. Alternatively, the use of no trip test setting on some earth fault loop impedance testers {not always as accurate} only passes 15mA instead of 20-25A test current.

You do not mention whether any RCD protection of the circuit is in place. The Zs measurement and R1+R2 measurement may also be lower if measurement is carried out with other parallel connections to earth in place during test, i.e. main protective conductors connected/SWA connected and used in addition to the CPC of cable / metal conduit connected during testing of conductors / flying tails to back boxes/ etc…You do not specify the wiring type or method of installation either. Knowing this would help.

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