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Loop Impedance Measurement from a second Transformer

Bob Cowie
I have a situation where I have USA equipment at 120/208volts (Frequency is not an issue). The system is a standard TNC-S feeding out to Switchboards. One of these Switchboards feeds a Transformer of 230/400volt input with a 120/208volt output. I have obtained the maximum Zs values from the Manufacturer (adjusted for the amended voltage) however I need to establish how we test the system for compliance. Should the Earthing on the Transformer Primary be isolated to test and create a Star point at this location on the Secondary in which case it will presumably become a local "TT" system. As the Transformer Primary and secondary are Electrically isolated there is no route for the R1 value around the loop if the transformer is left earthed only via the Primary feeder.

Any assistance would be appreciated.

Robert Cowie
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  • Former Community Member
    0 Former Community Member
    From you saying the primary is 230/400V I assume this is a Yyn transformer. Don’t connect the primary neutral.

    If For a delta secondary only high leg (B phase) will give 120/208V, using the 208V high leg to (A&C) mid point neutral is frowned upon.

    For a Y secondary, as has been said, a grounded star point will give you a TN-S system.

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  • Former Community Member
    0 Former Community Member
    From you saying the primary is 230/400V I assume this is a Yyn transformer. Don’t connect the primary neutral.

    If For a delta secondary only high leg (B phase) will give 120/208V, using the 208V high leg to (A&C) mid point neutral is frowned upon.

    For a Y secondary, as has been said, a grounded star point will give you a TN-S system.

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