There are various classes of power supplies and I've totally forgotten what the classes mean. But the one I used had isolation to 500 volts which coincides with the THCAD5's over voltage tolerance. eg. This 0.63 amp Meanwell.
oceancontrols.com.au/PSM-0173.html
(its only $20 or so).
This means is that both the THCAD5 and the DC side of the power supply are totally impervious to the arc voltage. So that means there is no need to turn off the AC power supply to protect anything. In fact, the component I wrote senses a full scale on the THCAD and enables an arc on pin which can only come on if some other voltage (like a plasma arc) is introduced into the sensing circuit.
PCW said leave the relay out and it took me a while before I was confident to do this until I really understood what an isolated power supply really meant.
When I first tested this, I just wired the power supply to another power point but when I wired it in properly I used a relay I had. It probably makes sense to drop this relay in an estop condition as the voltage is on the table and we want to isolate everything.
From memory, Islander261 had some unconfirmed third party reports that if the circuit was continuously left on, you could get some corrosion in the torch consumables on a water table so it was another reason I added the relay.
