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I'm designing a PCB that will control AC 240V electric tools. For now I decided to use a single double pole relay to switch live and neutral lines.

Is it also safe to use two separate single pole relays (one for each lines) that would be controlled by the same signal at the same time ?

2 relays would be cheaper than a double pole one.

I'm asking this question cause I'm thinking of the case where one relay would break and only one would remain good.

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  • \$\begingroup\$ Depends on your level of safety and if you even need to switch the neutral line... \$\endgroup\$
    – MadHatter
    Commented Aug 6, 2020 at 1:51
  • \$\begingroup\$ I think the two lines must be switched cause we never know if an outlet is not well connected...do you think there's a chance a relay could break? \$\endgroup\$
    – A.G.
    Commented Aug 6, 2020 at 7:31
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    \$\begingroup\$ Most things only switch line, hence why a poorly wired outlet is dangerous. Every wall switch in an American house only switches one of the two wires. Now relays can fail, so having two separate relays switch is probably not ideal. If safety is of high importance and there's a chance that someone can come in contact with one of the two wires easily, then switching with a double pole relay is probably preferred. \$\endgroup\$
    – MadHatter
    Commented Aug 6, 2020 at 13:08
  • \$\begingroup\$ Thanks for your comment. I'll keep the double-pole relay. \$\endgroup\$
    – A.G.
    Commented Aug 14, 2020 at 14:28

1 Answer 1

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I see the next advantages/disadvantages:

1 double pole relay:

  • + smaller area
  • + synchronous switching of live&neutral (if any appliance needs this ...)
  • + easier to control
  • - price
  • - more challenging PCB routing for high power
  • - if the relay fails, then ... you have no control

2 single pole relays:

  • + cheaper
  • + easier to route on PCB (especially on higher power)
  • + if 1 relay fails, you can still switch off the other one
  • - increased area on PCB
  • - there might be timing diffs between switching live&neutral (if any appliance is affected by this ...)
  • - you need to control 2 relays, not 1

Depending on this (let me know if you see other pros/cons or if you see it differently) you can choose your best option.

(Personally I need a latching relay for ~10A @ 230AC , and I think I will go on 2 SPST relays because of the disadvantages of the 1st option)

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  • \$\begingroup\$ A few month ago I finally modified my PCB for 2 single pole relays because it's cheaper and if one fails the second one will still disconnect the power source. \$\endgroup\$
    – A.G.
    Commented Apr 7, 2021 at 12:06

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