Electrical – Voltages of floated outputs of an isolated transformer

electrical

First of all, I wish all of you good strength and patience to overcome this bad situation with coronovirus

I am electrical engineer in the electricity company in Cyprus. I confess that I have no experience and knowledge in this matter.

I look forward to your kindness in answering my questions below:

Why do I measure voltages of the output terminals (secondary of isolating tranformer to earth ground? there is no connection to the earth…!! there is something missing….!! I did't understand…!!

I have an isolated transformer 2kVA 800V/230V. I am measuring as below:

  1. between two terminals (not directly on the transformer output but on a MCB, 1m distance): 232V

  2. between one terminal and earth: 175V

  3. between the other terminal and earth: 49V

note: the primary of this transformer 800V/230V is the two Phases output (secondary) of a three phase transformer 22kV(Delta)/800V(Star).(the neutral – site of 800V – is grounded).

other question: If I have a small generator 230V without connection to earth will i measure voltage between the outputs terminals and earth ground? if yes … why..???

I need your help to undertand this measurments.

I will appreciate it

Thanks in advance

Best regards from Cyprus
Kyriacos

circuit 1

circuit 2

Best Answer

That there is no galvanic connection to earth does not mean there is no capacitative connection to earth. E.g. the transformer windings build a capacitance if the core is earthed. Wires could run close to grounded parts etc. This capacitative source impedance should be rather big, i.e. if a device of 20 or more Watt is connected to a phase and Earth, the voltage must drop.

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