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.
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.
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
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!).
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.