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I have a home workbench and do minor refurb work with old turntables and monitors (CRT) etc. That implies potential exposure to high voltages (mains or above) at power supply capacitors, etc. I have seen many recommend that having a 1:1 isolation transformer on the bench is a critical safety measure.

There are a few questions floating around about this here. I understand that by doing this you're no longer connecting to earth ground directly, but I'm perhaps less well educated on the principles of electrical engineering and trying to understand which specific "I touched this component, at this time" scenarios are improved by using the isolation transformer, and which ones are not.

It sounds like there's some sense that you could in theory poke at various components while the device is powered on and as long as you only touched in single places and didn't accidentally bridge circuits you would be safe, but this doesn't sound intuitive to me. I mean, I'm in danger normally of shocks towards ground even with shoes on, aren't I?

Does any of this matter if the device is powered off? (E.g. as a large cap discharges)

Links or other pointers welcome, of course. Thanks.

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Isolation means the first ground fault is free

Normally, mains supply is isolated except there's a neutral-ground equipotential bond at the service point (facility supply), whose job is to assure the supply doesn't float thousands of volts above earth, and return fault current to source (the supply transformer not earth).

This N-G equipotential bond essentially serves as "the first ground fault", using up the free one. Meaning a second one can complete the circuit, a-la

transformer -> hot -> you -> safety ground -> neutral-ground equipotential bond -> neutral -> transformer circuit complete

When you install an isolation transformer, you are removing that first ground fault. This is thought to improve safety because now your errant touch will be the first ground fault. I will confess this has saved my life. Now, you're a ground fault, but assuming everything else is working properly, it goes

Transformer -> Isolated hot -> you -> safety ground circuit incomplete

Or

Transformer -> Isolated hot -> you -> safety ground -> neutral-ground fault in appliance itself -> neutral -> transformer circuit complete

So an isolation transformer is by no means a silver bullet. You can also have this happen

Transformer -> isolated hot -> you -> isolated neutral -> transformer circuit complete

An isolation transformer will do nothing for you touching hot and neutral at the same time.

A GFCI does this better

Now, consider a GFCI device. Now, you complete the circuit alright, but this causes hot and neutral currents to be unequal. SNAP.

It still doesn't protect you from grabbing both hot and neutral at the same time; at that point you're just another load. However, if you're also in contact with safety earth through low-resistance contact, then hot will flow more current than neutral since it's farther away from ground, and again SNAP.

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  • \$\begingroup\$ Thanks. Is the implication here that receptacles at a workbench should be GFCI outlets (possibly in addition to use of an isolation transformer)? \$\endgroup\$ – Ben Zotto Mar 11 '20 at 16:04
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It sounds like there's some sense that you could in theory poke at various components while the device is powered on and as long as you only touched in single places and didn't accidentally bridge circuits you would be safe, but this doesn't sound intuitive to me. I mean, I'm in danger normally of shocks towards ground even with shoes on, aren't I?

That's the main reason. A normal supply has a grounded neutral. If you accidentally touch a live wire, you provide a path for the current through the earth and back to the neutral.

With an isolating transformer, the secondary of the transformer has no connection to ground. Even if you touch a live wire, there's no path for the current.

But the isolation transformer does nothing to save you if you accidentally touch the live and neutral at the same time.

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An isolation transformer will help protect you from Line to Ground shocks. With an isolation transformer, both mains inputs are floating and negligible current will flow from single mains input and ground.

If you never ground yourself, this isn't very helpful. But, it is very easy to ground yourself! Any test equipment connected to the mains is grounded. The metal cases are grounded and the signal input/outputs (scope ground clips!) are grounded.

It will not protect you from many other types of shocks. Floating line to floating Line will still shock you. Any caps charged to a high voltage can shock you, etc.

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