Is it OK to calculate the Zs?

Is it acceptable to calculate Zs (the earth loop reading at the furthest point on a given circuit), and why - when one does calculate it - it's not always very accurate.

Clearly, when a Zs reading is obtained and it is lower than the external loop impedance Ze + (R1+R2), this is because of parallel earth paths, but sometimes it is also higher which, I'm led to believe is to do with RCDs/MCBs.

If it is OK to calculate the Zs, then how can we be sure that it is correct (if - in different situations - the Zs can end up higher or lower)? Please could you explain this?

Also, which is the correct and proper method, as I'm tired of telling my foreman that carrying out a Zs check isn't necessary

Answer 1: The measurement of Zs of a final circuit can be determined using two methods:

1. By direct measurement.
2. By adding the value of Ze to the R1+R2 of the circuit at the end of that circuit.

It is necessary to carry out a Zs measurement by either of the above methods to record in the relevant column in the test schedule, and also to ensure compliance with table 41.3 of BS7671: 2008.

It may be that you are measuring from the busbar of the consumer unit or distribution board - you would then be reading the resistance of the MCB/RCD, which would give you a higher reading because of the resistance of these devices.

See 'Snags and Solutions, Part 3 - Inspection and Testing Snag No. 32 and No. 40. These provide explanations on how to measure and calculate these variables.

Answer 2: If an R1 = R2 value is obtained for a circuit and a Ze is confirmed at the mains position, then it can be reliably found by adding the Ze + (R1+R2) as a calculated maximum Zs.

If, at that point, you are unsure, or wish to confirm that the Zs is as expected, then a Zs test should be carried out - if it is indeed safe to do so.

Therefore, it is not a formal requirement that you must carry out a Zs test. However, such a test would help to confirm that the calculated Zs is indeed accurate and in compliance.