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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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  • Not really, sadly.

    The DNO have no way to measure diverted current, as on anywhere that has TNC distribution, earth and neutral currents are indistinguishable from the substation end of things, coming in on the same wire, and on the load side there is no requirement for installers or users to even measure let alone report currents flowing in the earth bonding  of any level, and even if there was,  there is no organization to report it to that would keep a record.

    When there is no fault to correct what would you like the DNO do on receiving such a report  that there is 1 amp (or even 50) flowing between their TNCS earth and the water pipe at a house. ? It could just means the water pipes, in parallel with the street PEN conductor, are actually a lower resistance than it  is.

    "Proper" faults, like loss of earth, loss of power or out of spec voltages that are associated with a significant fault PEN are recorded, and the faults are then diagnosed and normally addressed pretty sharpish, and there are figures for those, collated by the HSE.

    In an ideal world this information  would all be measured and telemetered in and associated with every MPAN but I fear it is not practical.

    Mike.

Reply

  • Not really, sadly.

    The DNO have no way to measure diverted current, as on anywhere that has TNC distribution, earth and neutral currents are indistinguishable from the substation end of things, coming in on the same wire, and on the load side there is no requirement for installers or users to even measure let alone report currents flowing in the earth bonding  of any level, and even if there was,  there is no organization to report it to that would keep a record.

    When there is no fault to correct what would you like the DNO do on receiving such a report  that there is 1 amp (or even 50) flowing between their TNCS earth and the water pipe at a house. ? It could just means the water pipes, in parallel with the street PEN conductor, are actually a lower resistance than it  is.

    "Proper" faults, like loss of earth, loss of power or out of spec voltages that are associated with a significant fault PEN are recorded, and the faults are then diagnosed and normally addressed pretty sharpish, and there are figures for those, collated by the HSE.

    In an ideal world this information  would all be measured and telemetered in and associated with every MPAN but I fear it is not practical.

    Mike.

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