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I want to add a simple inline switch to a small, mains powered lamp (~240V AC, 10W Max).

It's earthed; it has a typical 3-core cable with a 3-pin plug.

I bought a bunch of switches, expecting each switch to have 2-3 contacts per side, but they only have 1 contact per side. In other words, they're only capable of breaking contact for a single wire. They have grooves down the adjacent sides to run/hold the other 2 wires, but it seemed strange.

Is it fine to just open the active wire, as shown below?
Note: Brown = Active. Blue = Neutral.

This is the kind of thing I do all the time with 6-24V DC applications, but domestic appliances aren't really in my wheelhouse. Is there any reason this would be inappropriate or unsafe with 240V AC?

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  • \$\begingroup\$ Your idea of breaking all three wires is probably inadvisable. You should never break the earth connection. \$\endgroup\$ – Hearth Nov 29 '18 at 2:29
  • \$\begingroup\$ @Felthry That's not my idea. Did I say that? Although now that you mention it, I have to ask.. Why not? In this situation, how's it any different to pulling the plug out of the wall? \$\endgroup\$ – tjt263 Nov 29 '18 at 3:04
  • \$\begingroup\$ The right half of the image suggests breaking all three wires, rather than just one. And I suppose breaking the earth connection is okay, if you make sure to break the live and neutral too, first. \$\endgroup\$ – Hearth Nov 29 '18 at 3:25
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In the USA only industrial devices have to disconnect neutral as well as the hot wire. For residential use normally only the hot wire is disconnected, as neutral normally has a low voltage on it less than 10 VAC, which is not a shock hazard. European and Canadian rules may vary about disconnecting neutral but typically follow the same UL/cUL/TUV regulations.

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  • \$\begingroup\$ Does it even matter if it's active or neutral? I mean, it'll break the circuit either way, right? \$\endgroup\$ – tjt263 Nov 29 '18 at 1:40
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    \$\begingroup\$ @tjt263, if you just break the neutral, then the whole load circuit will still be a shock hazard to anybody who goes poking around in it. For example, changing a light bulb in a switched lamp. \$\endgroup\$ – The Photon Nov 29 '18 at 2:00
  • \$\begingroup\$ It's not the same circuit? \$\endgroup\$ – tjt263 Nov 29 '18 at 3:10
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    \$\begingroup\$ @tjt263 your comments re-inforce to me why the first thing I use on someone else’s circuit is my dmm - only dead electricians assume other people are competent... \$\endgroup\$ – Solar Mike Nov 29 '18 at 6:27
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    \$\begingroup\$ To clarify what ThePhoton and SolarMike were getting at, if you interrupt the neutral wire instead of the hot wire, all the wiring inside the device will still be energized. This is typically seen as bad practice since some people will assume that when the switch on the cord is off, the wiring inside the device is not energized. In particular with lamps, this presents a danger when changing bulbs. \$\endgroup\$ – Nate Strickland Nov 30 '18 at 0:50
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You should tell us your location, since this varies based on country. In the US, 240V residential service is actually 2 legs of 120 out of phase with each other, so both wires are hot and both must be switched. In much of the rest of the world, there is a true 240V hot and a 0V neutral, but plugs may be inserted in either direction, so you never know which is which and again, must switch both of them together. Even in places where hot and neutral are in theory well-defined (such as for 120V appliances in the US), it's still not a bad idea to switch both together, since it's not too uncommon to find outlets miswired with the hot and neutral reversed.

In short, whether this approach is safe depends on your local electric code, but to guarantee it's safe to use anywhere, get a two-pole switch.

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Without seeing the lamp, it sounds like it is something special as there is a ground wire as well. Normally, standard household lights work with 2-pole plug so it is enough to have the switch on one wire only. But in your case, who knows if there is an active power supply for a LED or fluorescent tube. And these power supplies have input filtering, and having a switch on one wire only is a very bad idea. Especially if ground connection fails. Remember that some plugs can be inserted in any way to socket so you get no guarantees which wire is neutral or ground. And some grounded plugs are compatible with ungrounded wall sockets so you can't rely on that either.

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  • \$\begingroup\$ It is an LED lamp. \$\endgroup\$ – tjt263 Nov 29 '18 at 13:28
  • \$\begingroup\$ Thank you, but it does not really help much. It could you please post a picture of the lamp or anything? For all we know, it might be a metal case outdoor LED lamp with switch mode power supply, or just a plastic tabletop lamp with E14 bulb socket so all the power conversion happens in the bulb. \$\endgroup\$ – user205616 Nov 29 '18 at 13:35
  • \$\begingroup\$ jaycar.com.au/10w-240v-led-work-light/p/SL2866 \$\endgroup\$ – tjt263 Nov 29 '18 at 14:15

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