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TNC

Jam
Evening all.


My client has taken on maintenance at a site with private transformer feeding what appears closest to a TNC arrangement, and buried unarmoured cables. As might be expected from the opening sentence there are many other issues both in design and installation. Fortunately it’s not routinely manned.


An opportunity to replace the main switchboard has arisen, and in so doing revise the protection. On the other hand while the site owners have been strongly advised by various parties (which will include myself), it’s not in my remit to instruct complete replacement of the entire installation and I have to allow for the fact that it might not happen, at least not at the same time.


Given the increased risk of faults I would like to improve the protection. RCDs are out because they’d trip on normal neutral current... but am I right I thinking that a TP or TPN CB with calculated ground fault function (ie LSIG) and no neutral CT would act in a similar but less sensitive fashion? as most of the loads are balanced it might then be reasonable to determine a limit neutral current and then set the ground fault to above that.


Is there something else you would suggest?

If TNC could become TNS by removing some connections and converting some equipment to delta, the unarmoured cable would still remain. Given site history I’d be inclined to add a CT on the main earth conductor to a more sensitive (10s of A) earth fault relay. But while I might know, or be able to find, the electrode impedance, assuming a zero impedance fault strikes me as optimistic when trying to see if it’s sensitive enough to catch a buried cable fault. A 15Ohm phase-soil fault would leave 5Ohms for the electrode (which in this case is reasonable) to give 20A for the relay to pick up. Is that even likely?


Thanks in advance
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    OK, that is not as immedately dangerous  as it sounded at first. Clearly a broken neutral in the distribution would be very bad, as it is for the DNOs so a similar level of caution with inspections and double crimping of any PEN joints  and all the rest,  should be exercised.

    If this was NZ or OZ it would be quite normal, it does mean only the final circuits after the N-E split can be RCD protected, and the submains cannot be.

    If this was connected to the UK domestic LV  network and shared that substation with other users, s it would be a very  clear violation of the ESQR.

    I appreciate it is a private network, and the combined cables are in the distribution and not diirect to loads, so  it may be OK.but  I consider it on thin ice however. Section  8 clause (4) A consumer shall not combine the neutral and protective functions in a single conductor in his consumer’s installation "

    Is pretty solid  and
    35.Any generator, distributor, supplier, or meter operator or any agent, contractor or sub-contractor of any of the foregoing who fails to comply with any provision of these Regulations which applies to him, any person who fails to comply with regulation 18(3), 21, 22 or 25(1) and any consumer who fails to comply with regulation 8(4) or 34(2) shall be liable on summary conviction to a fine not exceeding level 5 on the standard scale

    Requires it to be take seriously. Unless there is a grant of exception by the secretary pf state, but I imagine they would have mentioned that if there was.

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  • 0
    OK, that is not as immedately dangerous  as it sounded at first. Clearly a broken neutral in the distribution would be very bad, as it is for the DNOs so a similar level of caution with inspections and double crimping of any PEN joints  and all the rest,  should be exercised.

    If this was NZ or OZ it would be quite normal, it does mean only the final circuits after the N-E split can be RCD protected, and the submains cannot be.

    If this was connected to the UK domestic LV  network and shared that substation with other users, s it would be a very  clear violation of the ESQR.

    I appreciate it is a private network, and the combined cables are in the distribution and not diirect to loads, so  it may be OK.but  I consider it on thin ice however. Section  8 clause (4) A consumer shall not combine the neutral and protective functions in a single conductor in his consumer’s installation "

    Is pretty solid  and
    35.Any generator, distributor, supplier, or meter operator or any agent, contractor or sub-contractor of any of the foregoing who fails to comply with any provision of these Regulations which applies to him, any person who fails to comply with regulation 18(3), 21, 22 or 25(1) and any consumer who fails to comply with regulation 8(4) or 34(2) shall be liable on summary conviction to a fine not exceeding level 5 on the standard scale

    Requires it to be take seriously. Unless there is a grant of exception by the secretary pf state, but I imagine they would have mentioned that if there was.

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