Isolation transformer

As rightly noted, the shock risk is only present once you are touching both ends, either directly or via a fault path.

Also, it is possible to loosly ground the transformer - i.e. via a high impedance, and detect the voltage across that impedance, so that you can keep running with a single fault, but be alerted that there is a single fault problem, hopefully to be able to do something before it develops to double fault.

An early version of this was use on the London Underground, where the rail voltages were something like a nominal  660VDC but earthed 1/3 of the way up, via a resistive divider of warning lamps so the negative rail would idle at something like -220V and the positive one at +440V, but if one rail was then forcibly grounded by a fault, then the current flow would be limited by the lamp resistance, and the lamp for one side would go out, and the other come on extra bright -  an alarm then, but power would remain between the rails so that the trains could be kept running to the next station. (more recent sections are upgrading to 750V but the split and earth fault detection remains. Note that the real voltage varies wildly - by several hundred volts actually - as the trains either draw current for traction  or put it back into the line while braking, but that is a complication beyond this discussion. More here)

The modern insulation monitor on an IT system can be thought of as a descendant of the technique.

There are a number of ways that isolation transformers may be used, and the regs permit most of them, but do require a bit more thought than a simple TN-x supply.

Mike.

As rightly noted, the shock risk is only present once you are touching both ends, either directly or via a fault path.

Also, it is possible to loosly ground the transformer - i.e. via a high impedance, and detect the voltage across that impedance, so that you can keep running with a single fault, but be alerted that there is a single fault problem, hopefully to be able to do something before it develops to double fault.

An early version of this was use on the London Underground, where the rail voltages were something like a nominal  660VDC but earthed 1/3 of the way up, via a resistive divider of warning lamps so the negative rail would idle at something like -220V and the positive one at +440V, but if one rail was then forcibly grounded by a fault, then the current flow would be limited by the lamp resistance, and the lamp for one side would go out, and the other come on extra bright -  an alarm then, but power would remain between the rails so that the trains could be kept running to the next station. (more recent sections are upgrading to 750V but the split and earth fault detection remains. Note that the real voltage varies wildly - by several hundred volts actually - as the trains either draw current for traction  or put it back into the line while braking, but that is a complication beyond this discussion. More here)

The modern insulation monitor on an IT system can be thought of as a descendant of the technique.

There are a number of ways that isolation transformers may be used, and the regs permit most of them, but do require a bit more thought than a simple TN-x supply.

Mike.