Isolation transformers, use of.

[Mark Hennessy]

The BBC document is a good guide but does not quite cover all of the points we need to know when using an isolating transformer.

I think it does cover the points that *we* need. The document is quite clear in its scope.

The OB unit transformers are not wired to provide isolation at all in the mode we are using  them for as they are configured to prevent unwanted tripping of an RCCD in a venue that is providing power and for nothing else and operators will be trained to use the equipment accordingly.

Their use on OB vehicles is outside the scope of that document - which it says in the introduction - but I covered that in post #16. But only really to give an example of why things are not always obvious.

What we need is a fully floating secondary that can float about the chassis of the unit under test regardless of the connections in the plug and the secondary terminals of the isolating transformer must both be considered live while the chassis is connected to a local earth usually the chassis of a test instrument.
Both terminals of an isolating transformer can be live at the same time if the temporary earth is connected to the negative of a bridge rectifier as is found in some later model valve TV sets and virtually all SMPSUs. In this case the secondary of the isolating transformer floats about the negative terminal of the rectifier and will be live at both ends with respect to the chassis of the test instrument and the item under test.
This is no problem if only one unit is connected to the isolation transformer and is exactly the reason for using one in the first place.

Yes, I agree with all that, and that is exactly what the BBC EGN covers. So I'm slightly confused