Anthony Smith asks about R1+R2 calculations and whether – if a new test instrument calculates the answer - is this sufficient? His question is answered by the NICEIC (Theme – Electrical Testing):
Question: I'm really confused, I've been instructing my engineers for years that R1+R2 values must be measured and never calculated, just because you have a path to earth when obtaining a earth fault loop impedance (Zs) reading does not mean that you have continuity of a circuit protective conductor - the Zs value in a PIR could be the result of many things.
I've just been looking at a new multi-function test instrument, which to my amazement, has a function that calculates R1+R2 values. These are not true values - but are values, none-the-less.
Have I missed something?
Having a function like this will only invite testers to calculate R1+R2 values!
Or have things now changed? Do my engineers now only have to calculate these values?
Answer: No, you haven't missed anything. The tester calculates an R1+R2, but the person carrying out the testing should then confirm this as being true by testing the circuit for its R1+R2 value. This is purely an indicator of the value and should not be relied upon because of parallel paths.
Any test and inspection should ideally involve dead testing with R2 values and, if this is not done for any reason, it should be mentioned as a limitation on the report from the outset.
This would make the integrity of the report more reliable, and would not leave a grey area as to the validation of the test results.
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