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House Wiring: Looking At Light Switches

A light switch is a light switch, right? You just flick it and the light comes on . . . Of course - but it is very obvious that many people think what goes on behind the architrave or wall is a black art. Magic, even!

By Ross Tester

And then there are those two-way or even three-way light switches which let you turn the light on and off from two or three locations. If single light switches are magic, two-way switches must be the stuff of sorcery to some!

There are many areas of a home where two-way light switches make a lot of sense – any room or hallway, for example, where you can enter and leave by different doorways. But there is a right way and a wrong way to do it! More on this shortly.

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The back of a standard light switch mechanism (in this case an HPM brand). The four terminals are C (common), switched terminals 1 & 2 and the non-connected "loop" terminal. In a normal light, terminal 2 is seldom used.
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And here's a photo of the same thing. Terminal 2 is covered by a thin shield - if needed the shield is easy to remove.

But before we go on, a warning (again!): even though the following is completely legal in New Zealand, it’s not legal in Australia unless you have an electrician’s or electrical contractor’s licence.

As we mentioned last month, you might have a PhD in electrical engineering, or a lifetime of experience in electrical repair or assembly but that counts for nought without that ticket!

OK, so we’ll assume you’re in NZ and want to replace a light switch.

The standard light switch

First of all, let’s have a look at a standard mains light switch. "What’s to look at," you ask? Well, quite a lot when you look into it (pardon the pun!).

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Fig.1: for those not familiar with switch types, here are the four most common. The simplest, which simply makes or breaks a connection in one wire, is an SPST type - this is the switch you'll most usually find inside appliances. Light switches are almost always SPDT types, even though the majority of the time they are used in SPST mode. SPDT can switch one wire to two different circuits.Where both active and neutral need to be switched at the same time, a DPST switch is used. Double pole switches are certainly not the largest available - you can easily get four pole (and more) switches. But where large numbers of circuits need to be switched simultaneously, a relay or contactor would normally be used.

We have shown both a photo and a drawing of the back of a light switch. As you can see, there are actually four screw terminals on the back – why, when for a standard switch function you need only two terminals.

That’s true, but those four terminals give you quite a bit more flexibility than a simple off/on function.

For a start, a light switch is a "double throw" device – this simply means that you can have it switch between two different circuits if you wish (see Fig.2). It has a "common" terminal (labelled C) and two switched terminals, usually labelled 1 and 2, either of which can be connected to the common terminal depending on which way the switch is positioned. You don’t need to connect to both switched terminals – in fact, in the vast majority of lighting installations, only one switched terminal (and the common) is used, effectively making it a "single throw" switch.

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