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Diverted neutral current / broken PEN?

Jamie Bate

Hello all,

Diverted Neutral Currents (DNC) seem to be a big area for debate especially with the push for EV's and the implications that they can cause.

Are there any official figures regarding contacting the DNO regarding these DNC's or recording of it other than on testing documentation (EIC/EICR etc.) ?

This still seems to be an area that needs more information, as variations including the location of the fault, the time of day, year etc. when the test was completed.

If testing a standard PME 100A single phase domestic supply and getting around 8mA on the earthing conductor during the installation being switched on and then reduces 0.01mA when the installation is isolated will probably be more to do with the instrument and 0.01mA = 0.00001A and would indicate that there is no issue. If this stayed at 8mA (0.008A) this is potentially an issue with a broken/deteriorating PEN conductor, however this could change due to variations across neighbouring installations.

When completing this test prior to any other work on the installation and getting readings when not expecting them this is going to impact on any electrical work being carried out, what is deemed as notifiable to the DNO? 

I have heard of anything 5A and above being reportable to the DNO which seems reasonable, however what if you were to get a reading of 4.5A, due to those variations this could be higher taking it beyond the 5A at other points in the day etc.

Would it be more appropriate to have anything above 1A reportable to the DNO so they can make their checks and note this on the job/ testing documentation, and where does it become a safety issue regarding work in/on the installation with a broken/deteriorating PEN conductor, as a job could be completed and several weeks later an incident could occur due to further loading and changes to an installation?

There is lots of information, about how and why etc, but no physical figures, I am aware that Smart metering maybe able to measure volts/current/power etc however this still seems a little vague.

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  • The real issue is you probably do not know you have a PEN fault in parallel with buildings with good service bonding, as noted, the plumbing or what have you will be comparable or lower resistance than an intact supply cable, so the Zs test may well pass, and unless you know what it was before, a bit of degradation would be hard to spot. 

    The time you find out, and by then the PEN may have been open circuit for years, is when the plumbing or gas pipes or whatever are changed, and instead of the usual  little sparkle, as the current interrupts and the open circuit voltage barely twitches and if at all, it only rises by single figures, you get something far more unpleasant ., where a significant chunk of the full phase voltage is available to do some damage.

    Rare, but also both impressive and dangerous to the unsuspecting plumber.

    Mike

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  • The real issue is you probably do not know you have a PEN fault in parallel with buildings with good service bonding, as noted, the plumbing or what have you will be comparable or lower resistance than an intact supply cable, so the Zs test may well pass, and unless you know what it was before, a bit of degradation would be hard to spot. 

    The time you find out, and by then the PEN may have been open circuit for years, is when the plumbing or gas pipes or whatever are changed, and instead of the usual  little sparkle, as the current interrupts and the open circuit voltage barely twitches and if at all, it only rises by single figures, you get something far more unpleasant ., where a significant chunk of the full phase voltage is available to do some damage.

    Rare, but also both impressive and dangerous to the unsuspecting plumber.

    Mike

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