Jo22 wrote on 2025-03-17, 23:11:

The antique light bulb method is okay,
in modern times I would at least install an RCD switch that disconnects the circuit very quickly if there's a short.

I think you mean RCBO not RCD, but if the PSU input shorts then the breaker will still need to go into overcurrent protection which is usually much slower to react than short to PE. Depending on all the cables, socket connections, etc. it's not always a very low resistance short and the breaker might need many ms (100+) to trip. If the PSU has a fuse that might blow first, and if soldered it's another part to replace now. Lightbulb will react with resistance rise (not full, but a lot) within a single half-cycle. And it will almost always protect the still working parts from being destroyed, plus you have a nice visual cue something is wrong.

Lightbulb will not protect you from being shocked, esp. if it's installed in a fixture with a plug that can be inserted backwards, and now it'll be on the neutral rather then than phase. It'll still work to protect the PSU though. For correct protection of the serviceman you'd need isolation transfromer I mentioned, there is no susbsitute for that. Correctly used lightbulb is still much better than nothing.