Circuit for switched power using a relay

[BobA]

Just thought this should be posted for reference.  Note the duplex receptacle can be separated into 2 separate outlets by removing the tabs between the outlets on the line and neutral sides.   This allows one  to be powered by the AC at all times and the other to be switched by the relay.

Edit:  Yes I think I have posted this before and someone mentioned using an electrical box to contain it but I cannot seem to find it.

[Ed_McCarron]

I'm sure this will start a religious argument, but why switch the neutral?

Hypothetical scenario:  The hot contact on the relay welds, the load remains off due to the open neutral, but the devices are still hot with respect to ground.  Joe Schmoe sees the devices off, assumes they are dead, and starts poking around.  ZAP.

The NEC does not approve.

[BobA]

I switch the neutral since the contact is available and it is possible that using an improper plug (not polarized) that the hot and neutral could be reversed into the relay.

[MonMotha]

I'm sure this will start a religious argument, but why switch the neutral?

Hypothetical scenario:  The hot contact on the relay welds, the load remains off due to the open neutral, but the devices are still hot with respect to ground.  Joe Schmoe sees the devices off, assumes they are dead, and starts poking around.  ZAP.

The NEC does not approve.

I'm not sure what the NEC has to say about this, if anything, but I frequently see the neutral switched in this sort of situation.  I can think of only one reason not to (the reason you stated) and several reasons to do so (neutral is not necessarily precisely local ground, 240V operation has no neutral when doing 240V split phase, completely isolates the device from the utility side when off, mitigates against hazards due to improper wiring or reversed plugs, etc.).  My personal opinion is mostly indifferent.

In all cases, always unplug things before you go "poking around".

[Ed_McCarron]

I'm sure this will start a religious argument, but why switch the neutral?

Hypothetical scenario:  The hot contact on the relay welds, the load remains off due to the open neutral, but the devices are still hot with respect to ground.  Joe Schmoe sees the devices off, assumes they are dead, and starts poking around.  ZAP.

The NEC does not approve.

I'm not sure what the NEC has to say about this, if anything, but I frequently see the neutral switched in this sort of situation.  I can think of only one reason not to (the reason you stated) and several reasons to do so (neutral is not necessarily precisely local ground, 240V operation has no neutral when doing 240V split phase, completely isolates the device from the utility side when off, mitigates against hazards due to improper wiring or reversed plugs, etc.).  My personal opinion is mostly indifferent.

In all cases, always unplug things before you go "poking around".

I knew you'd chime in.

240, 480, three phase -- those are totally different beasts.  I assume this is plain old residential 120v, with an earth bonded neutral.

If I had a switch on a neutral, my inspector would bounce it so quick your head would spin.

That said, there is an exception in Article 404.2B that states

A switch shall be permitted to disconnect a grounded circuit conductor where all circuit conductors are disconnected simultaneously, or where the device is arranged so that the grounded conductor cannot be disconnected until all the ungrounded conductors of the circuit have been disconnected.

Then again, thats mainly for residential wiring.  No one is going to check inside your arcade cab.  I've seen it on PC power supplies -- they used to always switch the hot and neutral.  Not sure if that still happens.

Re:  Unplugging -- Yes.  Do it.  Then check with a meter.  Then brush the feed with the back of your hand.  Then measure it again.  Then proceed.

[BobA]

Have to agree 100% with the unplug before you stick your fingers in. Always unplug, measure for voltage before touching, better to be safe then sorry. 

[MonMotha]

240, 480, three phase -- those are totally different beasts.  I assume this is plain old residential 120v, with an earth bonded neutral.

240V split phase = earth bonded center tap/neutral, 120/120 commonly seen in homes.  There are unfortunately too many terms for it.  Not to be confused with a "real" polyphase supply like the 3 phase (208V, 240V, 480V, etc. and HV) commonly used for transmission and in commercial/industrial environments.

You are definitely not permitted to switch ONLY the neutral.  That's universally bad.  Switching both seems fairly common practice in devices.  Obviously not so much in fixed wiring.

My experience has been that the NEC is full of "once upon a time, in some freak occurrence, this resulted in somebody getting killed or a house burning down" types of rules that have little basis in actual reality.  That's not to say that the NEC is garbage: quite a bit of it is good electrical practice, but there seem to be some elements that are based upon hearsay and fear.

Note that I'm not an electrician, and you should always follow the NEC when required.

I'd probably opt for switching both: switching just the hot requires only a single failure that is relatively common to potentially result in a dangerous situation (reversed hot/neutral) while switching both requires a double failure one of which seems less likely (broken relay with hot always engaged and the user doing something dumb as a result of misinformation).

FWIW, you might add a suitably sized fuse on the hot side before the relay.  This will prevent an overload of the relay from resulting in a dangerous situation as I doubt your relay is rated for 15-20A (and even if it is, a fuse never hurts) and will also catch the (unlikely) failure of the two poles of the relay shorting together.  The diode on the relay coil is not strictly required in this situation since there is no local switching component to protect, and the SMPS output is inherently protected from small inductive kickbacks, but it doesn't hurt, and I'd probably leave it there.