When to use an isolation transformer

[Gyro]

Yes, an isolation transformer with a standard UK mains socket on the output will have a floating secondary. The only ones I know of with center tap earth (ground) are the 110V (55-0-55) building site ones with the industrial sockets.

Normally the mains earth is carried through to the output socket earth, but this is sometimes disconnected for specific applications (the isntructions for my RS one mention it as a user configuration option).

[Seamus16]

No, check with it disconnected, there is no reason to power it up. You can also try connecting an incandescent lamp between any output terminal and earth ground, it should not light up. You may measure voltage from capacitive coupling but there should not be any significant current behind it.

I have tried it with a normal light bulb (checked it lights beforehand) and there is no light when connected between secondary outlet and earth (see pics). This is a case where cannot always trust what a voltmeter is showing. Todd Harrison did a video on this. When the light is connected there is no voltage showing. I suppose this is a case of phantom voltage.

[james_s]

The issue is that modern multimeters have a very high input impedance so they are very sensitive to capacitive coupling and picking up RF out of the air. You could solve this sort of situation by having a lower input impedance, say 100k across the meter in parallel with the probes.