220V Heatbed + Crydom D2425 Relay + DuetWifi Wiring ... correct?
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That looks good.
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Are you using an ATX power supply, or other PSU than can be turned on and off?
If you are, i'd suggest putting a mechanical relay in series with the SSR, and connecting it's control signal to the duet's Power input. That way the duet can relatively reliably turn off the heater together with the power supply, should the SSR fail in a closed state. Or, alternatively, you could use a thermal fuse on the heated bed.I'm no professional electrician, but i believe you should also connect any metallic frame parts near the heated bed to the earth pin of that power plug, in case something damages the heatbed or wiring insulation.