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Thread: Inverter Output Circuit - Ground Detector

  1. #1
    Join Date
    Aug 2016

    Inverter Output Circuit - Ground Detector

    Hi guys,
    I have a 370V three phase ungrounded (ABC+EGC) inverter output circuit going into a step-up transformer (delta on 370 volts side). Am I supposed to install a ground detector for this application? I guess I am not sure about the definition of ground detector, because to my knowledge, the inverter itself will always be able to detect the ground fault and trip the equipment offline for protection. Does this inverter's self protection scheme considered as a "Ground Detector" as stated in NEC?

  2. #2
    Join Date
    Feb 2005
    South of Bawstin
    Good question. I have never seen a ground detector system installed for an inverter output circuit as would be required under 250.21(B) for ungrounded electrical systems.

    One could interpret that you are required to install ground detectors.

  3. #3
    Join Date
    May 2011
    Quote Originally Posted by hill900505 View Post
    ...Does this inverter's self protection scheme considered as a "Ground Detector" as stated in NEC?
    No. I believe you are confusing DC ground-fault protection with AC ground fault protection. The inverter may or may not respond to a ground fault on the AC side: it will only shutdown if the ground fault causes a voltage disturbance that looks like an unstable grid. And then even if it shuts down the ground fault will still be fed by the utility.

    You should also take a careful look at 705.32, including the exception. FWIW, my understanding is that ground fault protection is usually not installed on inverter outputs.

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