I bought a non-tech isolation transformer (500w), which I planned to cut the earth mains ground from the output, but leave the chassis grounded; however, when I opened it up I found the output outlets were grounded to the chassis and the chassis connected to the mains ground. So I just cut the ground. Now my isolation transformer is floating both on the primary and secondary coils. Naturally this means the outer case is no longer grounded and if there was a short within the transformer, touching the case would be bad. Otherwise, does anyone see if this will lead to a dangerous scenario when I am probing the DUT with a grounded oscilloscope? Assume that I plan to connect the black alligator clip of the oscilloscope to any arbitrary point in the DUT.

  • \$\begingroup\$ Thanks for the excellent responses. As to the keeping the chassis grounded, I didn't see a way to do that since the output outlets are mounted with metal screws to metal brackets to the chassis and this connects to the ground plug. I thought about trying to insulate it with rubber gaskets and such but I wasn't sure that a small breach could be prevented and what would be insulated today might develop a continuity later with disastrous consequences. \$\endgroup\$ – user13691 Oct 1 '12 at 22:57

I think what you did was close, but I wouldn't leave the whole chassis floating. I'm assuming this isolation transformer comes with a standard 3 prong outlet for the output side. Just disconnect the ground connection on this outlet, but continue to let the input side ground be connected to the chassis.

Generally you want as little as possible floating at some arbitrary voltage. There will be plenty of other things around at the normal ground potential, so the chassis of the transformer isn't going to make anything worse.

I agree with Dave in that I don't understand why isolation transformers come with the grounds connected. I got one that way too many years ago, but didn't expect it. I actually blew out a fuse before realizing that the grounds were connected together inside the box. I don't know what they think isolation means, but different from what I had assumed.

By the way, beware of inrush currents on a 500 W isolation transformer. All the ones I've seen are toroidal cores. These can retain residual magnetism depending on what part of the power cycle they were turned off at. If this happened to be at one peak and you turn it on next time near the other peak, there will be a very large inrush. I once blew a 30 A breaker with such a transformer, even though I had switched it on a number of times previously on the same circuit. Unfortunately this was on a weekend and the breaker was in a different locked room. Argh. Often you want to adjust the voltage anyway when you're debugging power circuits where you want isolation. In that case, put the variac before the isolation transformer instead of after it. If you bring the variac voltage up and down with the knob, there won't be much inrush.


You've actually done the Right Thing, although you need to be aware that the case of the transformer is now going to be at the same potential as the DUT ground, so any faults in the DUT (not just in the transformer itself) could cause problems. The better alternative is to use a 3-wire to 2-wire "ground breaker" adapter between the transformer and the DUT.

This is a bit of a pet peeve of mine. The whole point of an isolation transformer is to get a source of AC power that is completely isolated from (floating with respect to) the mains ground. Connecting the ground from one side of the transformer to the other completely negates this, and I don't know why they do it. The only reason you're using the transformer in the first place is because the DUT's ground is not already properly isolated from the line/neutral input!

At most, there should be a high-value resistor (e.g., 1MΩ) in parallel with a smallish capacitor (e.g., 1nF or less) connecting the two grounds. The resistor drains off static charges, and the capacitor provides a certain amount of RF bypass.


Now your scope ground can be on any potential (possibly deadly). It is always safer to use differential voltage probes. Take a look at this Tektronix technical brief.

  • \$\begingroup\$ You're thinking about the situation when the scope is plugged into the isolation transformer. I'm pretty sure that @John is plugging the DUT into the transformer. \$\endgroup\$ – Dave Tweed Oct 1 '12 at 21:43
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    \$\begingroup\$ Really, always!? You are assuming a narrow usage for isolation. Some don't even have anything to do with a scope, and differential probes have their own issues. Sometimes they are a good answer, but it's absurd to say you should never float a circuit to test. And yes, sometimes that will include floating the scope too, in which case you need to be extra careful, but to say this should never be done is overreacting. \$\endgroup\$ – Olin Lathrop Oct 1 '12 at 22:14

Yes, I know it's an old question, but there is a potentially fatal issue which has not been addressed and it is entirely to do with what the isolation transformer may be used for after you've finished doing what you need with a scope and a DUT.

This answer is for the part of the question which reads: "...does anyone see if this will lead to a dangerous scenario..."

For safety, all earth connections on the output sockets of an isolating transformer must be linked together, but must not be connected to the input earth of the isolating transformer.

Two operators could be using using equipment plugged into different output sockets of an isolating tranformer. Assume both operators are using metal-cased tools and the case of each tool is properly earthed to the earth pin on the flexible cord of the tool.

Scenario one of two: the output earths in the isolating transformer are not linked together: One operator has an unsuspected short circuit from phase to earth in his tool. The other has a short circuit from neutral to earth. Either operator may use his tool in perfect safety with this isolating transformer - right up until one operator touches the other and they both are electrocuted.

Scenario two of two: The output earths in the transformer are connected together: In the case of two faulty tools as described above, the operators are immediately alerted and kept safe because as soon as the second tool is plugged into the transormer, it blows the transformer secondary fuses. And as there can be only one reason for that occurrence given that both tools are known to be operating well individually, it will be instantly known that both tools need urgent repair.

A rare scenario, yes, but one which almost guarantees a fatal result when an isolating transformer is borrowed by a couple of tradespeople who are going to work in close proximity to each other.

So it is critically important to link all output earths in isolation transformers. If a problem arises when using an oscilloscope and DUT on a workbench, then do anything else to alleviate the problem but do not unlink the output earths in the transformer. Other people need to have implicit trust in the transformer when they use it.


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