Im using a mains heated Silicon matt on mine.... Ive put three levels of protection and control. A mechanical relay, that would fail open (ie off)
This is the the first layer, this is triggered by PS_ON, PS_ON will also kill power when thermal runaway is detected in software. This could happen if a therm has failed or if the SSR fails and is stuck on, The actual PWM power control is done by the SSR from the heat bed pin.
Using an SSR for the PWM protects the mechanical from wear, as these are designed to have fast power cycles, its basically a power transistor...
After that, a thermal fuse on the bed, if the software crashes and for some strange reason PS_ON is stuck on and heat signal is also stuck on (very unlikely but in the realms of possibility) the fuse will trigger when the temp hits around 140 with the fuse ive got.
So in this setup its incredibly unlikely a mains heat bed will cause a fire. Im also thinking of just adding a power fuse in the line as well to protect against possible shorts if anything damaged the power line in there... Though there is going to be 2 fuses, one on the kettle socket and one in the plug socket...
I was just thinking a lower rated fuse just for the bed. maybe 4 or 5 amps... My bed is 3 amps max... So anything above that would be a short or something else failing...
