Firstly I'd say that there is seldom any reason NOT to have an RCD (GFI for you Yanks) on the bench feed, and since they are inexpensive and may save your life.. get one if the building doesn't already have them.
The main point to understand about work on opened high voltage equipment is that
earths are a danger to you. When the equipment case is closed the earth provides safety, but when it is open any earth in its vicinity does the reverse. To receive a shock calls for two connections, and the most likely way this will arise is through touching a live connection whilst resting your other hand on the earthed case of an item of test equipment. Or, whilst holding an earthed test lead.
To minimize this risk the one-hand approach should be used as much as possible. Avoid resting your other hand on metal. If a certain action requires two hands, consider cutting the power whilst it is done.
Isolating transformers are a huge improvement in safety when used correctly, particularly on SMPS work. Your main enemy here is the earthed outers of scope probes, and I really wish that the manufacturers would rethink this arrangement. On which topic, never use a
capless scope probe on PSU work. Unless, that is, you insulate the exposed metal ring close to the tip. Seen a good few blowups through this ring grounding-down a live connection.
Beware of 'isolating transformers' sold for building site use, which are not actually isolating transformers at all, but change a 240v supply into a 120v-0-120v supply. If one of these finds its way onto the testbench it can be extremely hazardous since the potential difference between the supply live and the '-120v' output of the transformer is 360v. (Seen an accident where this scenario left a guy with 'welding flash' to his eyes and an impressive hole in a PCB.)
Oh, and never trust equipment on/off switches. Pull out the plug when soldering or whatever, and place the plug where you can see it. The worst case here is a double pole switch on which the live side is welded shut and the neutral side operating. In this case the equipment will seem to switch off but will still be live.
![Evil >:D](https://www.eevblog.com/forum/Smileys/default/evil.gif)