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Max Zs BS7671 17th Edition / Guidance Note 3 Inspection and Testing

Former Community Member over 4 years ago

Hello,

There seems to be some slight differences with regards to acceptable Zs values when comparing the two documents BS7671 17th Edition A3 and Guidance Note 3 Inspection and Testing 17th Edition A3.

For example table 41.3 from BS7671 17th Edition A3 states:

0.4sec trip D6A = 1.82 Ω

Adjusted to 80% as per appendix 14 = 1.456 Ω

However page 121 from Guidance Note 3 Inspection and Testing 17th Edition A3 states:

0.4sec trip D6A = 1.46 Ω

These two documents are produced by IET. Guidance Note 3 Inspection and Testing 17th Edition A3 is rounding up Zs values above the maximum allowable values detailed in BS7671 17th Edition A3, why is that ?

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0 mapj1 over 4 years ago

1.4566666667 Ω does not equal 1.46 Ω.

I beg to differ. The difference is less than nothing in that application,or if you prefer language with rather more mathematical rigour, it is less than the combination of instrumentation error and experimental variation and the results are statistically indistinguishable.

I suggest you go and measure Zs on a real system with a real meter early in the morning, then go out for lunch and polish the meter probes and re-test with the same meter at the same point in the afternoon. The two readings will be both as valid, but are unlikely to be the same in all digits. As an aside what meters do you suggest with this accuracy (not precision) ? Note that two half lengths of meter lead is a good few milli-ohms on its own. I have used both Fluke and Meggar machines and neither is this good.

A typical spec is more like this maker's data for the LTW 425 - and that is a dedicated loop tester, not a compromised design to make a multi function tester all fit in one box, they tend to be worse.

Loop Testing Accuracy

±5% ±0.03Ω

@230V a.c

±10% ±0.02Ω

To quote precision that is not there when recording results is actually wrong as it misleads the reader about the accuracy of the measurement.
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0 mapj1 over 4 years ago

1.4566666667 Ω does not equal 1.46 Ω.

I beg to differ. The difference is less than nothing in that application,or if you prefer language with rather more mathematical rigour, it is less than the combination of instrumentation error and experimental variation and the results are statistically indistinguishable.

I suggest you go and measure Zs on a real system with a real meter early in the morning, then go out for lunch and polish the meter probes and re-test with the same meter at the same point in the afternoon. The two readings will be both as valid, but are unlikely to be the same in all digits. As an aside what meters do you suggest with this accuracy (not precision) ? Note that two half lengths of meter lead is a good few milli-ohms on its own. I have used both Fluke and Meggar machines and neither is this good.

A typical spec is more like this maker's data for the LTW 425 - and that is a dedicated loop tester, not a compromised design to make a multi function tester all fit in one box, they tend to be worse.

Loop Testing Accuracy

±5% ±0.03Ω

@230V a.c

±10% ±0.02Ω

To quote precision that is not there when recording results is actually wrong as it misleads the reader about the accuracy of the measurement.
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Max Zs BS7671 17th Edition / Guidance Note 3 Inspection and Testing

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