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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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    I am a little unsure what you are trying to do. Are you trying to re-create an American style supply for testing, but happy to work with 50Hz? If so, then you want a delta connected secondary, with the centre-point of one of the windings earthed. This will give you two phases at 120V to earth, and one phase 208V to earth. The phase to phase voltages should be 240V in all cases. 


    Regards,


    Alan.
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  • 0
    I am a little unsure what you are trying to do. Are you trying to re-create an American style supply for testing, but happy to work with 50Hz? If so, then you want a delta connected secondary, with the centre-point of one of the windings earthed. This will give you two phases at 120V to earth, and one phase 208V to earth. The phase to phase voltages should be 240V in all cases. 


    Regards,


    Alan.
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