That's obviously advisable when testing in-situ wiring in a situation like this. However, he has tested just a length of unused cable on a drum, with no accessories or anything else connected - and still got a 'low' IR reading (although he gets the expected >999 MΩ when he tests another in-situ circuit which uses different cable, and when he leaves the meter probes not connected to anything)
If the resistance were a 'fixed quantity', then the meter reading would be exactly the same regardless of the test voltage - but that isn't necessarily the case. The whole point of testing at different voltages (or, at least, at a 'high enough voltage') is that resistance may vary according to voltage - otherwise one could do all one's IR testing with, say, 12V, or less.
Certainly very unusual - but everything that he has experienced and reported to us seems to exonerate his meter, and leave a finger pointing firmly at the cable (even though cable that is 'faulty' in this manner is incredibly rare). The results which we hope to see soon 'with a different meter' should settle this, one way or another, for once and all!
Since the consequences of installing large amounts of faulty cable can be horrendous, I've seen it suggested that one should always do that. However, presumably because it's such an incredibly rare occurrence, I don't recall having heard of anyone actually doing that with 'brand new' cable of reputable origin.
Yes, I think we're all very surprised, and most of us probably still find it 'hard (or 'impossible'?) to believe'.
If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.
Select the supplier or trade you require, enter your location to begin your search.
Are you a trade or supplier? You can create your listing free at DIYnot Local
