I initially used a DPST relay to switch both the live and neutral inputs of a computer, but someone told me it was a bad idea and that I should only switch the live input.

I live in Europe and here we have power strips with DPST switches. It is mandatory in some countries to install two pole switches in bathrooms. Plus RCD (EU equivalent to GFCI) is present on every plug (if the electrical installation was updated within the last 20 years that is).

But it's technically possible to plug your device into a socket that is not connected to earth (non polarized plugs)...

I know that the US system is completely different. So here's my question:

If I want to design a universal system with a relay that switches on/off mains power (ie 120V US AND 220/240V EU), what kind of relay is safest to use? 1 Form A (SPST) or 1 Form C (SPDT), 2 Form A (DPST) or 2 Form C (DPDT)?


  • 8
    \$\begingroup\$ Someone is confused between fuses and switches. It's a BAD idea to FUSE both live and neutral. If the Neutral fuse blows, it leaves the circuit live. It's a GOOD idea to SWITCH both live and neutral. That way you are protected even if Live and Neutral are swapped in your house wiring. (Rare, but does happen. Less rare with 2 pin plugs...). \$\endgroup\$
    – user16324
    Aug 11, 2020 at 10:48
  • 3
    \$\begingroup\$ @BrianDrummond it is really not about house wiring. Grounded mains plugs that can be plugged in either way do exist and are common in Europe. There is no guarantee whether the fuse inside a device is on live or neutral. \$\endgroup\$
    – Justme
    Aug 11, 2020 at 11:24
  • \$\begingroup\$ Some places have polarized plugs and some have unpolarized. When they're polarized, it's pretty normal to switch only live. \$\endgroup\$ Aug 11, 2020 at 13:35
  • \$\begingroup\$ If it's not safe to switch both live and neutral together then how would it ever be safe to unplug? It's the exact same operation. \$\endgroup\$
    – J...
    Aug 11, 2020 at 19:17
  • \$\begingroup\$ @BrianDrummond From a machine-directive standpoint: either you guarantee the neutral is at a safe voltage or you have to switch it off before commencing maintenance. Hence a lot of machinery having a 4P switch. 3 phases AND neutral. Single-phase brake relays are always DP as well. \$\endgroup\$
    – Mast
    Aug 11, 2020 at 21:09

1 Answer 1


Well, you don't have to blindly believe everything you are told on the Internet. Someone saying so can be wrong or just trolling.

Safest is to switch both live and neutral, because in some countries you can freely orient the mains plug in any orientation so you don't know which of the wires is live or neutral.

Another thing is if you plug the computer into old ungrounded plug, and only switch off the neutral wire by accident, you have a metal PC case connected to mains live via a capacitor, and that's dangerous.

Oh and old buildings do not necessarily have grounded outlets or RCDs so do not build anything that relies on the electrical system to be newly installed.

  • \$\begingroup\$ Ah, all your 4 pieces of advice are very good, Advice 2 is particular important. So DPST x 2 is the best choice. \$\endgroup\$
    – tlfong01
    Aug 11, 2020 at 9:17
  • \$\begingroup\$ DO it the same as power Strip DPST with RC snubber for OFF and MOV for ON with breaker., if there are motors and EMI sensitive loads nearby. \$\endgroup\$ Aug 11, 2020 at 11:43

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