We measure conductor to conductor IR and not surface to conductor. (Granted metal containment systems will be earthed.)
In a new installation, we expect hundreds of MΩ. For periodic testing, it is not so much the absolute value, but any change which is of interest. (Assuming that previous reports are available.) If we get a result around 1 MΩ now, it has probably been falling and may well continue to fall. In any test regime, an acceptable value must take account of any expected change before the next test. It follows that the pass value will be above the minimum safe value. IMHO, a margin of a factor of 10 would not be unreasonable.
Or Zoomup may be correct - it was simply a reasonable engineering judgment.
We measure conductor to conductor IR and not surface to conductor. (Granted metal containment systems will be earthed.)
In a new installation, we expect hundreds of MΩ. For periodic testing, it is not so much the absolute value, but any change which is of interest. (Assuming that previous reports are available.) If we get a result around 1 MΩ now, it has probably been falling and may well continue to fall. In any test regime, an acceptable value must take account of any expected change before the next test. It follows that the pass value will be above the minimum safe value. IMHO, a margin of a factor of 10 would not be unreasonable.
Or Zoomup may be correct - it was simply a reasonable engineering judgment.
