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Neutral Earth connection

Former Community Member
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Interrogating an issue with a substation set up - The substation is a split one with two 11Kv to 400v cast resin transformers that supply each substation half. The set up incorporates back up generators that kick in on a power cut, I noticed that we have a UPS system installed that takes two supplies from the substation, one from each half. either one is on and the other supply stays as backup to the UPS system. What was picked up is that when there is a power cut and until the power is restored there is a load shed that disconnects the supply to the UPS. This UPS remains on feeding 400v three phase to the site, but the earth neutral connection is lost until one of the supply breakers closes back up again. means that the output is not referenced to earth for a few minutes till the gens spin up. I believe that through checking manuals and reading up this isnt a good scenario to be in in terms of sensitive equipment. 


I would be looking at providing a transformer in there along with a local earth neutral link so that this is maintained on all eventualities. has anyone come up with this sort of issue before ? it could also be addressed through neutral earth contactors at the appropriate places, Had looked at possible three pole devices on circuit breakers but that does not allow full isolation to be achieved and could also lead to possible paralleling of the neutral earth link at each transformer.
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    Depending on the design of the UPS, your PSSC may not be quite what it appears - electronically derived generation, rather than true rotating magnets, are not simple sources with a constant source impedance - rather the controls try to prop up the voltage to keep it constant as the  load increases, up to some critical point, usually a single figure multiple of the rated full load, when it all drops out of regulation and the volts then collapse rapidly with any small further increase in load.

    The upshot is that any breaker that is not 'small' compared to the UPS rating (say larger a 10A breaker on a 100A UPS supply) is not reliably going to do the instant trip thing, and you cannot really trust a loop tester.

    Earth fault relays configured as a set of jumbo RCDs to isolate zones in the event of earth current exceeding a few amps not clearing in a short time can sometimes provide a way out, but as you say it quickly gets very complex.

    (and any earth fault relay is only going to do its thing correctly if there is at least one NE link upstream of it, which is where we came in.)

    Switched NE links may be necessary, but consider what happens if one does not switch, and how that fault condition is detected.
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    Depending on the design of the UPS, your PSSC may not be quite what it appears - electronically derived generation, rather than true rotating magnets, are not simple sources with a constant source impedance - rather the controls try to prop up the voltage to keep it constant as the  load increases, up to some critical point, usually a single figure multiple of the rated full load, when it all drops out of regulation and the volts then collapse rapidly with any small further increase in load.

    The upshot is that any breaker that is not 'small' compared to the UPS rating (say larger a 10A breaker on a 100A UPS supply) is not reliably going to do the instant trip thing, and you cannot really trust a loop tester.

    Earth fault relays configured as a set of jumbo RCDs to isolate zones in the event of earth current exceeding a few amps not clearing in a short time can sometimes provide a way out, but as you say it quickly gets very complex.

    (and any earth fault relay is only going to do its thing correctly if there is at least one NE link upstream of it, which is where we came in.)

    Switched NE links may be necessary, but consider what happens if one does not switch, and how that fault condition is detected.
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