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I've just tested a sub-main cable running to an old barn. 0.05 Mohms at 250V. Obviously a fail, but the customer says it's fine because it works. As they're clearly not interested in their own safety I wondered if I could make them see sense by the cost. So my question is; How much does it cost them? Is it simply 240/ 50 000 to see the wasted current.

Thanks, Paul
 
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SparkyChick

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Yes (for a purely resistive load) and it comes out (unless I've got my calculations wrong) as about 10-12 units of electricity a year. Not enough to justify any form of expenditure, but it could have serious safety implications which you should perhaps explain to them.

So, why were you testing the sub-main in the first place? The insulation resistance reading was 0.05Mohms... between what? L-N, L-E, N-E? Were all the readings consistent? Why did you not test at 500v?
 

buzzlightyear

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I've just tested a sub-main cable running to an old barn. 0.05 Mohms at 250V. Obviously a fail, but the customer says it's fine because it works. As they're clearly not interested in their own safety I wondered if I could make them see sense by the cost. So my question is; How much does it cost them? Is it simply 240/ 50 000 to see the wasted current.
What cable was youa testing.
 

Lucien Nunes

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About one watt being dissipated if that's from the line conductor so about £1.50 per annum, nothing at all if it's from the neutral. But it's unusually low and points to some kind of damage. How long and what size is the cable? Could there be joints?

I think the way I would tackle it would be to point out that the low insulation resistance, although bad in itself, is a timely warning that there might be more serious trouble brewing that needs to be found before it destroys the cable.
 

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