Test isolation transformer

[crt_cowboy]

Hi,

I would like to check my isolation transformer to be sure it provides proper isolation.
From what I read one way to do it is to check resistance between the primary and secondary.

https://www.eevblog.com/forum/beginners/how-to-test-old-mains-isolation-transformer-to-make-sure-it-is-working-correctly/

When I measure resistance between primary and secondary I get no continuity / very high resistance.

When I measure voltage between the output of the transformer with the ground (from the mains), I still get voltage. Shouldn't I have low / no voltage if there is proper isolation ?

[CaptDon]

The transformer has stray capacitance between the two windings. Your meter is high impedance. You will always see some stray voltage and a tiny bit of leakage current. Think of it more as a capacitive divider with a few pf of coupling.

[bdunham7]

When I measure voltage between the output of the transformer with the ground (from the mains), I still get voltage. Shouldn't I have low / no voltage if there is proper isolation ?

You should measure the current, not the voltage.  Put a 10K resistor from the transformer output to ground and measure the voltage across that and then calculate the current.  You should have less then 500uA leakage current, which would correspond to a measurement of 5VAC.

[jonpaul]

Bonjour

Mains transformers are tested for ratio, resistance, hypot AC voltage P-S and hypot AC voltage P to frame or earth.

Certain special types have internal shields for very low capacity P..S.

if you test for resistance or capacity the results will not be a useful.

Please inform  us the make, model, a photo etc as the term isolation transformer is very broad.

Jon

[crt_cowboy]

This is the model variac Philips 2422 529 00007
see https://www.radiomuseum.org/r/philips_ringkern_stelltransforma.html

It has separate windings so on top of being a variac, it also act as isolation transformer.

I have tried to connect a light bulb between a secondary and the ground, and it did not light up. So I believe what I measured with the multimeter is more an artifact related to high impedance of the meter.

[Brad808]

What are you reading are "ghost voltages". You can read about it here; https://www.fluke.com/en-ca/learn/blog/digital-multimeters/dual-impedance-digital-multimeters

[geggi1]

Check the primary and secondary with an insulation resistor tester (Megger) with approxomately the dubble primary voltage.
If you got multiple Mega ohm between primary and secondary the transformer has an acceptable regarding isolation.
When you got the isolation tester up and running you should also chenck the insulation resistance between winding, chassis and ground connection.