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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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    Ah yes, good point, I had briefly forgotten that there are in fact far too many ways of doing 3 phase in the USA.  ?  (let's not even mention the options on 277V L-N / 480v  phase to phase. At least here it is always 3 equal legs, and usually 230V /400 unless it  is 400/690..)


    You mention 208v, so consider the drawings below in one case the phase phase voltage is 208 and in the other it is 240.


    In both cases line marked 'N' in these drawings needs to be earthed.  Only the upper kind, with 3 equal leg voltages can be generated  from UK 3 phase with an auto transformer. With a double wound system the world is your oyster..


    e399bc558b07e0e5719a9b4cb47f3ba2-huge-a011608a.jpg

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  • 0



    Ah yes, good point, I had briefly forgotten that there are in fact far too many ways of doing 3 phase in the USA.  ?  (let's not even mention the options on 277V L-N / 480v  phase to phase. At least here it is always 3 equal legs, and usually 230V /400 unless it  is 400/690..)


    You mention 208v, so consider the drawings below in one case the phase phase voltage is 208 and in the other it is 240.


    In both cases line marked 'N' in these drawings needs to be earthed.  Only the upper kind, with 3 equal leg voltages can be generated  from UK 3 phase with an auto transformer. With a double wound system the world is your oyster..


    e399bc558b07e0e5719a9b4cb47f3ba2-huge-a011608a.jpg

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