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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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  • 0

    how are you verifying the continuity from chassis of the system back to the origin - how is insulation monitoring for ADS performed?

    I'd expect  something akin to an R2 test at least.

    Mike

  • 0

    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.

  • 0 in reply to mapj1

    The problem overall is we are supplying equipment to be ship mounted. The ship supply is IT based and we will verify chassis to 'Earth' (Hull/Sea) via milliohmmmeter. 

    The ADS is not our supply and part of the vessel hull supplier and we have no knowledge of this system. My package designers have placed Type B and C circuit breakers in our equipment cabinets, which in pre-commissioning i would normally request Zs calc / measurement  to ensure trip time is maintained. 

    Obviously? Zs for an IT based system is probably not relevant as the fault path is pretty much restricted to L-L faults (shorts) within the end equipment cabinet. 

    I'm thinking  line- Line (or 2xR1 assuming equal lines) calc/measurement should probably be used in place of Zs for this type of system to ensure trip timing. 

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