I'm designing a controller for a small process heater, switching 220volts at 20amps. Since the controller uses a ramp/hold pattern with a lot cycles, an SSR is recommended for reliability. Question is: should it have a two pole relay? the 220volt is the north american "two phase" system with 220v line to line and 110volt to ground. The risk is that with only one pole the heater has 110volts to ground at all times. Certainly mechanical contactor designs are two pole in my experience, say for HVAC compressors, but adding a pole to an relay is relatively cheap, not so for an SSR
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