Isolation transformer

But if one leg was to become accidentally earthed we then have reference to earth. Now the supply is not isolated from earth- on the second fault to earth I can understand if the fault is of low impedance the fuse should blow but if not it can then sit at a potential.

Quite true. However to get to that situation you need two distinct faults simultaneously - which puts you in a very similar risk area to normal ADS - what happens if the c.p.c. breaks and you then get a L-PE fault? yes the exposed-conductive-part just sits there at 230V waiting to kill anyone that touches it (presuming the circuit doesn't have additional protection, e.g. a ≤30mA RCD).

Similarly the double insulation approach can be lethal if both layers of insulation fail.

Ideally where you have more than one distinct exposed-conductive-part on a separated system you'd connect them all together with a protective conductor (unearthed) - so that if you did get two simultaneous faults to PE from different poles, you have something similar to a L-N short circuit which would then either take out the overcurrent protective device in pretty short order, or where the supply didn't have enough oomph, the voltage would likely collapse to something tolerable. The regs sort of require that - only when supplying a single item of current-using equipment can exposed-conductive-parts be left individually floating, where there are two or more items of current using equipment on the same separated system then the exposed-conductive-parts must be bonded together. They just seems to overlook the possibility of there being more distinct exposed-conductive-parts than items of current using equipment - e.g. where supply accessories or wiring systems are Class 1.

As with most things in life, electrical systems are never 100% safe under all circumstances - it's more a matter of keeping things 'safe enough' (which is of course a bit of moving target as society's attitudes to danger change)..

   - Andy.

But if one leg was to become accidentally earthed we then have reference to earth. Now the supply is not isolated from earth- on the second fault to earth I can understand if the fault is of low impedance the fuse should blow but if not it can then sit at a potential.

Quite true. However to get to that situation you need two distinct faults simultaneously - which puts you in a very similar risk area to normal ADS - what happens if the c.p.c. breaks and you then get a L-PE fault? yes the exposed-conductive-part just sits there at 230V waiting to kill anyone that touches it (presuming the circuit doesn't have additional protection, e.g. a ≤30mA RCD).

Similarly the double insulation approach can be lethal if both layers of insulation fail.

Ideally where you have more than one distinct exposed-conductive-part on a separated system you'd connect them all together with a protective conductor (unearthed) - so that if you did get two simultaneous faults to PE from different poles, you have something similar to a L-N short circuit which would then either take out the overcurrent protective device in pretty short order, or where the supply didn't have enough oomph, the voltage would likely collapse to something tolerable. The regs sort of require that - only when supplying a single item of current-using equipment can exposed-conductive-parts be left individually floating, where there are two or more items of current using equipment on the same separated system then the exposed-conductive-parts must be bonded together. They just seems to overlook the possibility of there being more distinct exposed-conductive-parts than items of current using equipment - e.g. where supply accessories or wiring systems are Class 1.

As with most things in life, electrical systems are never 100% safe under all circumstances - it's more a matter of keeping things 'safe enough' (which is of course a bit of moving target as society's attitudes to danger change)..

   - Andy.