Q & A of the Day – Is there a safe way to measure Zs on this circuit?

Question: This concerns taking a direct Zs measurement at the most remote end of a loop-in lighting circuit. Based on the wiring diagram, it appears that the final lighting outlet is not the remote end; it would in fact be the switch controlling that light (i.e. the switch cable running from the final ceiling rose to the associated switch).

My view on this is that if the Zs were to be taken at the lighting outlet, then the line + circuit protective conductor (CPC) resistance of the switch cable would be left out, and so what is believed to be a direct Zs measurement is - in fact - not an accurate measurement.

So really, is the 'R1+R2 then added to the measurement Ze' method the only safe way of determining the Zs on such a circuit?

I bring safety into the matter, because if we were to take the Zs at the switch - which could be a metallic-type - and, unscrewed from the mounting box to probe the terminals, then a shock could occur during the test.

Comments on this matter would be much appreciated.

Answer: Electrical testing inherently involves some degree of hazard, it is the inspector's duty to ensure their own safety, and that of others during the performance of the test procedures. The inspector must be competent, safe working procedures should be observed and the test instruments being used should conform to the appropriate standard.

Where appropriate, the recommendations within HSE guidance note GS38 'Electrical test equipment for use by electrician's' should be observed. In particular, the use of GS38 compliant fused test leads is of importance when using the supply voltage for the purposes of the test in earth loop testing.

It should be noted that some test instrument manufacturers advise the use of non-fused leads and probes when the instrument has in-built electrical protection however, it should also be noted that such electrical protection does not extend to the probes and leads.

Direct measurement of Zs should be made at the furthest point in the circuit, usually at the final connections of the remote accessory however, Regulation 612.9 does state that the relevant impeadances can be determined by an alternative method. We can confirm that the method that you have outlined, that of measuring R1 + R2 during continuity testing and then adding to the measurement of Ze would be an acceptable alternative method to determine Zs.

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