confirming that supply is isolated from main ground

Thread Starter


Joined Dec 10, 2011
I purchased a 240/480 to 120/240 distribution transformer, hoping it would serve as a cheap yet robust isolation transformer when connected in reverse (of course raising the voltage to 240V). Several transformer manufacturers claim that these may be safely reverse connected, with only minor consequences (larger inrush currents and slight voltage drops).

However, when I measure the voltage with my digital multimeter from mains ground to the output, I measure voltages of 120V and 94V depending on which output wire I use. Does this indicate that the supply is actually unisolated? My transformer has a shielded ground plane in the middle of the core--this shouldn't make a difference in terms of isolation, correct? I get the same result when I leave the center plane floating.

As a comparison I measured the voltage between GND and a rewound microwave transformer that I knew was isolated--this measured around 20V.

I would really appreciate if anyone could explain where these voltages come from and if my distribution transformer setup is indeed providing isolated power.


Joined Nov 9, 2007
This is too high a voltage (and probably current capacity) to get wrong.

Post a diagram of all your connections, when making these measurements.


Joined Mar 14, 2008
As Mike noted, you just need to measure the resistance between the output and input of the transformer (when unconnected of course). If the resistance is very high or unmeasurable then the transformer is isolated. The voltage you measure is likely due to capacitive coupling between windings.


Joined Jul 18, 2013
IMO you should never be using ground as a reference or do you mean neutral?
But measuring an apparent isolated secondary to earth ground any reading is usually due to the sensitivity of modern VOM's which is reading stray induced voltages.


Joined Jan 11, 2009
Connect first one side of the output then the other to a ground through a high value resistor. Measure the voltage drop across the resistor.

Be careful, a slip on the probes could be dangerous, might want to use clips and keep your hands off.


Joined Nov 7, 2013
Nothing to do with your question, but there may be other connection possibilities more suited to your purpose.

120 X 120

Or even 120 X 120 X 240 X 480

How will this be used?

Is it X1-X2-X3-X4
and H1-H2-H3-H4

Control transformer or distribution?
VA/KVA rating?

Thread Starter


Joined Dec 10, 2011
Post a diagram of all your connections, when making these measurements.
I attached a diagram (please excuse the MS Paint work...)

My meter gives no reading when measuring the resistance between primary and secondary (indicating open circuit), but I also tried russ_hensel's suggestion.

With a 220K resistor between ground and the output, I measured voltage drops of 9.4 and 7.4 VAC. However, I also measured the current through the resistor (which might eliminate the problem of overly sensitive voltage readings) which was 0.03 mA. By ohm's law 0.00003 * 220000 = 6.6V, so I assume this just due to capacitive coupling as crutschow said.

Would it be safe to assume at this point that probing circuits powered by this transformer with an oscilloscope would not cause issues? I have the transformer connected to a variac so I can do initial testing at safer voltages.



Joined Nov 30, 2010
This kind of transformer will protect the oscilloscope from an earth ground on the secondary side. Just be sure you don't install an earth ground on the secondary side out of habit.

That would be the habit of connecting "bond" (the green wire) to the chassis, and then connecting the common of the circuit you're testing to the chassis or neutral.


Joined Nov 7, 2013
I would suggest the opposite, although further testing is moot.
Transformer seems fine.

Connect a low value resistor or fuse to frame and/or 120 volt neutral.
(For a test only)

If transformer winding is isolated there will be no problem grounding a phase.
If transformer has a ground fault you will lose a 10 ohm 1/4 watt resistor or fuse in a puff of smoke.

Suggestion in my previous post would be to use 120 volt windings only for 120 to 120 volt isolation transformer.
Plus 240 and 480 if you felt comfortable at those dangerous levels.
Alternately use the 240 to 240 connections @ 120 volts for a current limited isolation transformer.

Should have enough isolation for testing at any voltage you are likely to encounter.


Joined Nov 9, 2007
I have the transformer connected to a variac so I can do initial testing at safer voltages.
That's a really good safe way to proceed.

Thank you for posting the pic, you can see from the replies that it was really helpful to the discussion.

MS Paint is just fine.

I assume from the red lines that you have earthed the electrostatic screen so I guess that the previous comments about induced pickup are correct.

Incidentally the current policy at AAC is against mains+ voltages so this thread is an object lesson in safety and how to discuss it.