MK intermediate switches

An intermediate switch works in the same way as normal single way switches, only one switch opens when the other closes depending on the direction of throw of the toggle. A normal single throw switch works on the principle of one direction closes the circuit and the other direction opens the circuit, so if you have two single throw switches side by side and working in opposite directions from each other, when you operate them simultaneously you turn one switch "ON" as you turn the other switch "OFF" at the same time. This is how an intermediate switch works only it allows you to operate two side by side switches with only one finger.

This site page shows a good example of how they are wired:

That's not correct. It's basically two *two way* or changeover switches with internal connections between them.

Rather than draw out the circuit, here's a description of what it does. _________ | | | 1 2 | | | | | | | | | | 3 4 | |_________|

With the switch in position one, 1 connects to 3, and 2 to 4. In position two, 1 connects to 4 and 2 to 3. It may be used as a two way switch if required.

Then it isn't an intermediate switch then ? An intermediate switch is used to select the live path between the cross over in a two way switched circuit. The L1 (a1,b1) are connected continually, as is the L2 (a2,b2) so that the paths are not actually broken at the cross over position. The L1 and L2 paths are not interrupted they're only swapped between when the switch is thrown in any direction.

Yes it is. There are two terminal layouts used on intermediate switches, which I'll call 'modern' and 'old' - and they work like this:

Modern type

'up' 'down'

Old type

'up' 'down'

The 'up' and 'down' are arbitrary, just serving to indicate the two states of the switch. 1 and 2 relate to the two 'strappers' between the 2-way (SPDT) switches, and suffices a and b relate to the ends.

With the old type it's necessary, as shown, to cross over the connections to the ends of _one_ of the stappers, but this sort is rare nowadays.

With both types, as Dave rightly points out, it's entirely possible to wire incorrectly such that in one position of the intermediate switch the two strappers from each end are connected together, leaving no through connection. Because of this scope for confusion I usually suss out the connections using a multimeter first and then wire appropriately. (Which sounds to be exactly what Dave did in the first place.)

Then it's easier to wire and follow with the OLD type. :-))

Not quite sure what you mean by not interrupted. As you say, the paths are swopped. So at some point - no matter how brief - they must be broken.

I must admit to never having had a problem before, as all the ones I have used tend to mimic the layout of the 'same' two pole switches - ie one pole either side, so you simply run one strap to the top and one to the bottom. But then I always tended to use Crabtree.

To add confusion, I was breaking into two circuits - one for the lights, and one for the power curtains - both in triple and earth. And on the curtains one I had naturally used blue for the neutral... First fuse I've blown in ages. ;-)

Ah, well it does show the value of marking the ends with red or black sleeeving :-)

Heh heh. But by nature I was breaking into the middle of it...

"Every core of a cable shall be identifiable at its terminations and preferably throughout its length." [Reg. 514-03-02] ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Really. ;-) Yet another reg written by a theoretician.