Neutral Insulation level

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PP26

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One of the C-phase cables on a Switchboard was below the NETA standard of 100 Mega ohms (the cable read 20 Mega ohms. We were planning to replace 3 sets of 4 cables but the concern came up that one of the cable bundles is stuck due to heaved conduits.

So my thought is to swap the Phase-C and neutral cables at both ends in one conduit. This way Phase-C will have all three good cables and the neutral will end up with one problematic cable. The neutral leg has hardly any current (only the imbalanced current from lighting load).

I would like to hear your thoughts if this would be fine or if it would compromise safety.
 
PP26 said:
Thank you for responding, by any chance, do you have a link for that article ?
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I read it in a post here a short while ago, but then Pauleng came on later and said it was wrong. I'm not qualified to say anymore than that, you might want to seek out Pauleng for further guidance.

You did an internet search?
 
No, it's not gonna be a bare neutral. I tried to search some articles online which relates to this topic but I couldn't find one.

I will probably try again later.
 
Since you mentioned that only the C-phase had a lower resistance I'm assuming that this particular "megger" test was done from the cable to ground?
Were any of the megger tests done from cable to cable? If not then I think they should be done, especially for the conduit that had the lower resistance value to make sure there isn't another issue.

Swapping the C-phase with the neutral should be an improvement over what you have. But I'm not really in a position to say it will be all be OK.
In this situation where the insulation is possibly degraded, the very low potential to ground on the neutral conductor is more important than the neutral having low current levels. This low potential (probably at most several volts) will put a low stress on weak insulation, minimizing the chance of any further degradation. Also the consequences if there's a complete failure in the insulation of the neutral conductor is minimal, if anything, within non-metallic conduits.
 
synchro said:
Since you mentioned that only the C-phase had a lower resistance I'm assuming that this particular "megger" test was done from the cable to ground?
Were any of the megger tests done from cable to cable? If not then I think they should be done, especially for the conduit that had the lower resistance value to make sure there isn't another issue.

Swapping the C-phase with the neutral should be an improvement over what you have. But I'm not really in a position to say it will be all be OK.
In this situation where the insulation is possibly degraded, the very low potential to ground on the neutral conductor is more important than the neutral having low current levels. This low potential (probably at most several volts) will put a low stress on weak insulation, minimizing the chance of any further degradation. Also the consequences if there's a complete failure in the insulation of the neutral conductor is minimal, if anything, within non-metallic conduits.
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Thank you for your response. We also performed cable-cable test, only cable-ground for one of the Phase-C conductor has a 20 Mega Ohm value.
That's what I thought too, having a low value of current flowing into this cable, I would not have that much of thermal stress that could cause more damage on the cable insulation.

However, I would search deeper just to make sure that we are not compromising safety on the solution that we came up.
 
PP26 said:
No, it's not gonna be a bare neutral. I tried to search some articles online which relates to this topic but I couldn't find one.

I will probably try again later.
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The answer is in the NEC. Any on line article would be based on the NEC.
 
You make no mention of operating voltage, test voltage, nor temperature.

You also say C phase and 3 sets of 4 cables in non-metallic conduit. That would imply to me that there is no ground wire in any conduit.

Therefore if you are measuring a low megger reading to ground on one C phase wire where is this partial connection to ground coming from? To me it might suggest the conduit is full of water. That isn't unusual but it could also indicate that conduit has suffered some damage. Is it also the conduit that has "heaved" (whatever that means)?
 
Russs57 said:
Is it also the conduit that has "heaved" (whatever that means)?
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Lots of upward movement ("swelling") of the soil/ground due to the soil/ground freezing. Heaving can damage almost anything.

Probably better to say "conduit damage due to heaving" vs "the conduit heaved", etc.
 
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