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
