I'm modernising an old bench saw, and I want to replace the switch it has with a safer one, so that the tool will not start if the power is disconnected and then reconnected. The usual way to do this with these sort of tools is a magnetic switch, basically a relay that has the coil connected to the output. The disadvantage of mechanical relays is that they can be accidentally triggered by a whack with a bit of lumber, and the cost. Industrial safety switches are too expensive for my home workshop. So I figured out this circuit using.

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Power is off until the start button is pressed, which then 'closes' the output side of the SSR. This energises the input side when the start button is released, so that it will latch on until the stop button is pressed. If power is interupted the SSR will switch off so that the machine won't start again until the user pushes the start switch. – Schematic created using Just mentally replace the mechanical relay with an AC-AC SSR in the circuit above. My question is: will running the output from the SSR back to the input work? It would on a mechanical relay, but I don't know if an SSR will work exactly the same. Also, will I need a varistor on this kind of SSR, or is one built in?

EDIT So this is the new circuit I propose. It has the start and stop buttons controlling the relay from a low voltage source, which has the benefit that I don't need to run mains wires around the cast-iron body of the saw to the switches, and I can add an indicator lamp to show that the power is on. Do NOT use Fotek solid-state relays from eBay or Amazon.

All of them are counterfeit. Sap Adobe Form Designer Download there. See this Underwriter's Laboratories warning notice: Not one picture of a Fotek relay on eBay or Amazon matches UL's picture of an approved relay. Most of the fakes fail at much lower current levels than their markings would indicate, and sometimes they fail in the ON state. The proper answer is a motor starter with contactor, but those are expensive. Consider something like this 30 amp 'Relay in a Box' unit: These come in an electrical box with UL approval, which is good around a saw where you need to protect electrical parts from sawdust.