[GMB]

To clarify: my concern on safety relates to what happens if the TV or cabling tries to short the mains for some reason. The current transformer primary experiences 240V applied to a winding that was designed for a couple of volts, with the secondary thus trying to deliver many kilovolts to its circuit. It has to be able to experience this without causing a problem.

There is also the possibility of switch-on surge (mainly on a cold start) which must not cause any issues.

I also want a passive solution, so if nothing is happening then as little as possible is powered.

The much-advocated ready made solutions all require me to the build a PSU for the relay control and some seem to draw power themselves (why?). I am not, as perhaps people are assuming, trying to turn the mains on for the HiFi. My Hifi is super-clever in that its PSU stands by using a rechargeable battery and senses power demand from the preamp to initiate the start-up sequence. This allows different reasons to start it automatically. The TV is just the latest one - but brings the annoying problem of being double-insulated. The audio comes from its headphone socket and I have recently had reason to swap the cable to the PC for zoom talks - another double insulated thing. Plugging the audio jack generates a loud burst of mains hum - so it seems to need a lot better electrostatic isolation than managed so far.