Ok, so quick sitrep.
I installed a heatbed, power expander and external powersupply.
I've printed over 24h with this setup, then when I wanted to start another print (heating extruder, bed and turning on the fan) the power expander started to produce smoke...

I've attached some pictures, I really don't know what's going wrong so that's why I'm posting it here.

The back of the power expander.


The front of the power expander.

It is fried, a bin case, dead...
Get a normal and cheaper SSR rated for 30A or more and forget about it.