How to Test Single-Phase Isolation Transformer

Not quite sure what you want to test - do you just want to know whether it's working (as a transformer), or do you want to test the isolation?

dear sir ;

thank you for rapid respond .

i want to know if its working as an isolation transformer .

regard

You can get an idea by using a PAT (Portable Appliance Test) tester to check for leakage between the primary and secondary, but this will only use (typically) 500Vdc. You will need an insulation tester (e.g. a "Megger") or high voltage generator to produce the 1.5kV, 5kV (or whatever the specification says) to do a proper proof test.

dear sir ;

thank you ;

i have a single phase isolation transformer 220v /15 kva , but when i mesure the voltage between the earth and neutral its 25 v .

so whate is the values that show me the transformer is good or not .

regard

You need to check leakage current from primary to ground, primary to secondary and secondary to ground all with the transformer disconnected from supply and load, and with a DC (NOT AC) test voltage.

The 25V earth to neutral that you're measuring (which is AC) tells you little, if anything, about the isolation.