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Zs measurement on IT Systems

Geof I.

I have a project where the customer has IT power distribution system (AC) - Normally in the pre-commissioning procedure I would add a requirement to check the Zs for each piece of equipment based on the trip curve and type of MCB on the incomer to the equipment (TN-x systems) ; However when I was thinking about this ....

Is there any benefit / is it possible to perform Zs measurement on IT distribution? My gut says there is no requirement / benefit, but my brain is nagging I'm missing something.

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    The first question to ask, is that is providing ADS, and whether this occurs on first fault?

    You can't, in effect, take a Zs measurement for "first fault", because there shouldn't be an "earth fault loop" at first fault, although there may be a high impedance connecting one of the line conductors to Earth ... it would, however, be meaningless, and a measurement of Id for first fault, along with knowledge of earth electrode resistance, is more appropriate.

    I think the answer to the question is best summed up by reading Regulation 643.7.1 c), which summarizes the requirements for verification of ADS in IT systems. This permits verification by calculation only for both determination of Id for first fault (and compliance with Reg 411.6) and for Zs for double earth fault ("second fault). Basically, you'd have to put a fault on the system to then measure Zs which could, of course, create hazards.

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    The first question to ask, is that is providing ADS, and whether this occurs on first fault?

    You can't, in effect, take a Zs measurement for "first fault", because there shouldn't be an "earth fault loop" at first fault, although there may be a high impedance connecting one of the line conductors to Earth ... it would, however, be meaningless, and a measurement of Id for first fault, along with knowledge of earth electrode resistance, is more appropriate.

    I think the answer to the question is best summed up by reading Regulation 643.7.1 c), which summarizes the requirements for verification of ADS in IT systems. This permits verification by calculation only for both determination of Id for first fault (and compliance with Reg 411.6) and for Zs for double earth fault ("second fault). Basically, you'd have to put a fault on the system to then measure Zs which could, of course, create hazards.

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