[Lucien Nunes]

I'm too late to edit the previous post but I would add a comment about the relative safety of being connected to an isolating transformer or not, when not technically necessary such as undergoing maintenance.

A live-chassis set is always likely to be safer run from an isolating transformer, as this overcomes one of the intrinsic hazards of most such sets, namely poor insulation of live functional parts and poor protection against direct contact.

A class II appliance in good condition won't really be affected safety-wise by the transformer, as none of its protection relies on the supply having an earthed neutral.

A class I device, again in good condition, is also unlikely to be much more or less safe when isolated (especially if in the non-isolated condition there is additional protection in the form of an RCD) although the mode of protection is changed.

Situations where an isolating transformer does make things less safe are:

a) Where an appliance wiring error, in conjunction with an earth present at the outlet of the transformer, leads to the specific situation illustrated by Laurence in post #26 where the secondary becomes connected between earth and the casing of the appliance.

b) Where multiple devices are connected to one isolating transformer, and a fault in one or the wiring (first fault) defeats the protection by electrical separation at another, yet remains undetected due to the lack of ADS.

c) To a minor extent, where a large and complicated Class I device can have multiple faults in separate sections, as though they were separate appliances as with b) above. This might include a projection outfit comprising separate projector, transformer and amplifier units, each Class I but with increased possibility of broken protective conductors in the interconnecting cables preventing their casings from being held at the same potential.

The main one is b) and this is why an entire shack or bench should never be supplied from one transformer, and why public mains supplies are not floating (you can't have one customer's safety depending on another customer's installation being free of faults.) The same situation can arise with an IT (isolated from earth) portable generator, which should never be used to power the installation of a building that is designed for a TN supply without an N-E link being made. The problem can be controlled up to a point by separately protecting each device with an RCD. These will not trip on a first fault as they would with a TN supply, but if two loads are faulted to opposite potentials, both RCDs should trip in the event of a shock between them. However, an IT supply should not really be used in such situations.