KV Meter

One meter is connected on the generator side of the breaker, and the other is connected on the bus side of the breaker.

When the breaker is closed, in theory the two voltmeters - grid and generator - receive the same voltage. In theory, they should read the same.

In practice, each voltmeter has an accuracy tolerance and the potential transformers [PT] which convert the kV level at the switchgear to 100/110/120V nominal (according to which standard practice of design was used) also have an accuracy tolerance.

The tolerance for ordinary switchgear voltmeter and potential transformer is about +/- 2% each, so you might see 2+2 = 4% difference in reading.

Practising engineers have to be sensible, since it is most important that the voltages are really equal when the breaker is closed I suggest....

1. with breaker closed, read generator and grid voltmeters, say you see G and B kV.
2. Calculate the "best estimate" of the actual voltage, (G + B)/2 = E. Or use the reading of the most accurate voltmeter/PT set available on the bus as E [ e.g. accuracy class for voltmeter/PT of 0.5 (%)].
3. Adjust the full scale adjustors of the two meters so both read E kV.
4. If you have basic moving iron analog pointer meters, you can must adjust the "zero" screws to correct the reading at normal voltage.
5. Attach labels to the panel, writing that you have adjusted the meters to read the same at normal voltage, - else someone is sure to see the zeros are not right when the meters are de-energised and undo your good work.
6. For each meter, include their readings before and after you adjusted them.
7. Tape over the adjusters as an extra precaution.
8. Record your adjustments in your own notebook and the log book.

If the voltmeters are actually fed by AC to DC transducers, do not adjust the transducers - they are usually more accurate than any "pointer" type panel meter. People who adjust transducers (especially MW) so remote instruments read the same as local panel meters cause designers to "tear their hair out" since a lot of the cost of a MW transducer is in the ability of its maker to calibrate and certificate it with a high accuracy.