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

1 Answer 1


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.

  • Thank you for your answer. How much current will flow through my body if I touch to the terminal of 175V and the ground? Is it dangerous?
    – Morio
    Apr 11, 2020 at 15:30
  • If you undress a polyester Pullover, you get in contact with 4000V, sometimes 10000V or more. In the dark and dry air, you will see many sparks. So it is NOT the voltage, it is the current that is dangerous. And the current depends on the source resistance in series with the body in series with the ground path back to the source. To measure it, just connect a linear device (toaster, heater) to the source and earth to measure the current and voltage.
    – xeeka
    Apr 11, 2020 at 15:44
  • I have add the circuit
    – Morio
    Apr 12, 2020 at 18:24
  • The winding between A and B does have a capacitance to Earth. Even if that capacitance is rather small, its impedance might be in the range of the resistance of the measurement test equipment thus forming a voltage divider. If R is small enough, the voltage at A, B should be very small. At 230V, the total impedance must not be smaller then ca. 20kOhm to avoid dangerous currents through the body - the bigger the better, e.g. 300kOhm. A digital multimeter has an input impedance of 2MegaOhm or more, and that is the reason why those high voltages can be measured.
    – xeeka
    Apr 12, 2020 at 18:56
  • What is the size of R in the diagram?
    – xeeka
    Apr 12, 2020 at 18:58

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