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I have a power supply that has only screw terminals for its AC mains input. Currently I'm using a power cord from a computer that I cut the female end off of and attached fork terminals to (live, neutral, and ground). It works fine but now I'd like to add a rocker-type switch to the cord so I don't have to unplug it whenever I turn it off.

Is it better to use a DPST-type switch to simultaneously connect/disconnect the live and neutral wires or is a SPST-type switch fine? If an SPST is ok, does convention and/or code prefer attaching it to the live or neutral wire?

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    \$\begingroup\$ Look at the plug. It has a wide blade and a narrow one. The wide blade is neutral, the narrow is "hot". Look at the power cord. If it's "zip" cord there is a rib on the (supposed to be) neutral wire that is (supposed to be) attached to the wide blade. If the power cord is round rubber-covered then there's a white (neutral) wire and a black (hot) wire. Get an inline switch, open it up. Note that there is provision for the neutral wire to just feed through, so you separate the neutral and hot wires (without stripping the wires) then cut ONLY the "hot", attach that to the switch terminals. \$\endgroup\$
    – Hot Licks
    Dec 11, 2014 at 22:19

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ALWAYS switch the live wire(s). If you only switch the neutral, then the entire circuit becomes live when switched off.

Switching the neutral also can add some safety in case the outlet is wired backwards. It would also allow you to give it 120-0-120 split phase later on (240v total), if it's rated for that.

Do NOT switch the ground, but you probably knew that.

In case you didn't know, neutral is actually the center tap of the utility transformer outside and is (supposed to be) connected to ground (dirt) in the breaker box and nowhere else. This is why it's okay to leave the neutral connected in most cases. However, don't be tempted to use ground to carry current just because it goes to the same place. Neutral is there because the current that it carries causes it to not be ground anymore once it leaves the breaker box.

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  • \$\begingroup\$ Plus GFCI outlets won't like ground current one bit. \$\endgroup\$
    – Ignacio Vazquez-Abrams
    Dec 11, 2014 at 20:04
  • \$\begingroup\$ Right. One purpose of ground is to provide a convenient place for faults to connect to and trip the breaker, instead of waiting for a user to figure that out. GFCI's are made to be particularly sensitive to unbalanced current between live and neutral (a few mA, even on a 15A circuit), which means that such a fault must have happened, even if it's not a dead short. They're not for overload protection. \$\endgroup\$
    – AaronD
    Dec 11, 2014 at 20:15
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    \$\begingroup\$ In Europe and many other places, where power plugs are not polarized, you must use a DPST switch. \$\endgroup\$
    – ntoskrnl
    Dec 11, 2014 at 23:04
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A SPST-type is ok and you should put it on the live wire (as usual), doing so also provide a protection for you circuit since that there will be no main voltage in the circuit if something short, as long as the switch is open.

In other word, placing the switch on the main will isolate your circuit from the main when the switch is open.

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