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Field test procedure for Modified Differential Ground Fault systems

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  1. Biscuit is offline Junior Member
    Join Date
    Apr 2014

    Field test procedure for Modified Differential Ground Fault systems

    This guide from Square D is a great piece of reference material for testing modified differential ground fault systems. It includes test procedures for Main-Main, Grounded Wye Configuration; Main-Tie-Main, Grounded Wye, Non-Relayed Tie Configuration; Main-Tie-Main, Grounded Wye, Relay Tied Configuration.

    What you might find particularly helpful are the figures that show your test set and jumper connections. I thought this would be a good reference for experienced techs as well as a good learning tool for those who are still trying to grasp the concept of modified diff ground faults. Just passing it along, download in the attachments...
    Attached Files Attached Files

  2. #2
  3. Join Date
    Jan 2016

    Remove the bonding jumper(s)

    Siemens and GE both say to remove one or both of the bonding jumpers during the test of the MDGF. CH and Schneider do not. Electrically, I would think you would want them intact as it would be a real world situation. Is it possible that two of the largest electrical manufacturers have applied a MDGF wrong? Or perhaps someone out there can explain it to me?


  4. #3
  5. Join Date
    Jan 2016
    The goal of the test is to ensure the ground fault works when only one source is available. When you are using a primary injection test set current will go through all parallel paths. When you have a single source the current should only return to the neutral of the energized transformer. In order so simulate a single source situation you remove the opposite ground to neutral bond to allow only one path for your primary injection test current to flow through. Feel free to correct me if I am wrong. That is how I understood it after going through the plethora of GE ground fault testing procedures.

  6. #4
  7. Join Date
    Jan 2015
    You can find a lot of info on this topic online. Let me know if you need help and I will forward you some docs.


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