The mains input has a standard 2A circuit breaker, which should be able to sustain inrush current from the 400VA isolation transformer. Let's say it is a last resort for dramatic failure and also serves as a simple input on/off switch.
But 500W is a very dangerous power and a lot of damage can occur before the 2A protector trips. I want an electronic circuit breaker that reacts fast, at a current I dial at will (under 1A). Even at 10mA so I can deal with small PSU modules that can handle 1W.
This is quite out of the realm of the "electric devices and supplies" that equip your house and that you can readily buy at a nearby store. I'd love to use existing products for 1) speed of implementation 2) compliance with safety rules 3) cost (I don't think I can make cheaper than others).
There are various types of current sensors :
- Shunt resistors
- electromagnetic (direct relay-style, or indirect with a transformer)
- Hall effect
In addition to the manual switch, the current can be stopped by
- Static relay (or "optotriac") : fast but could "latch" in some corner cases
- electromagnetic relay : slower to disengage but sturdier and will not get stuck at 2A if well sourced.
Safety would dictate that both are connected in series, you never know :-P
Then comes the other uncomfortable question of : how do you power these switches ?