Isolation transformers, use of.

[Skywave]

The RCD is useless in your setup. If it's on the primary of the TX, it won't "know" about any earth leakage faults on the secondary. If it's on the secondary, unless somewhere on the secondary winding is earthed before the RCD there cannot be any imbalance for it to detect.

I disagree. The RCDs is on the secondary winding. Let me explain in more detail.
The feeds from the transformer's 'L' and the 'N' - which go to the RCD and thus to the O/P socket -  I shall call phase 1 and phase 2. Each phase is 120-v. RMS w.r.t. the 0v. centre-tap of the transformer, which is fed to the transformer's output 'E' pin and thus to the 'E' pin of every socket outlet. In the event of a leakage current, phase 1 or phase 2 to 'E' - (same as the test item's metal casing) - there will be a current imbalance and the RCD will trip. On test, this has been proven to happen.

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You may regard your own safety as optional  . . .
Please omit the sarcasm: thank you.

. . . . but let's make it abundantly clear that yours is not an example to follow.
I'm not saying that it is. What I am doing, in effect,  is using an isolation transformer in a method which is not the conventional way of using such a device. But that method is not 'unsafe'.

For those without specialist knowledge, the guidance given by Mark and in the BBC document are the safest methods. RCD on bench, isolating TX for working with a single specific item when you want to connect earthed test gear.

Which I have just dealt with immediately above. And, over the many years, that traditional method is what I have always used when working with a mains-powered item which has one 'side' of the incoming mains connected to the metalwork which the user / tester may come into contact with.

There is no panacea: there are always ways to hurt or kill yourself.
I'm not saying that there is a panacea. Moreover, this topic is about using an isolation transformer, not general electrical safety.

Al.