MK intermediate switches

message from Dave Plowman on 10 May 2004

Spent the last few days installing CAT5 wiring, so took the opportunity
while the floorboards were up to add lighting etc switches beside the
computer which involved using intermediates. Used the MK grid system as I
needed more than one on one plate.
Of course, the old other switches didn't match anymore, so I replaced
these too.

And here's the point.

The single MK logic switches (bought from a shed) came with instructions.
Idly glancing as you do, it said to wire an intermediate 1&2 to L1 L2 on
the two way switches. But 1 & 2 as marked on the grid switch were the same
pole - ie with the switch 'off' RH1 (top) went to RH2 (bottom) and LH1
(top) went to LH 2 (bottom) - in other words the layout was as expected
for any intermediate I've ever seen. Wire it as they said (admittedly not
for a grid switch) and it wouldn't work.

Just to clarify, the grid switch was marked thus:-

__________
| |
| 1 1 |
| |
| 2 2 |
|_________|

I suppose it would be logical to make the terminals on an intermediate
the same as on a two way, but if they are changing things, would be an
idea to do it all at once...

BigWallop replied to Dave Plowman on 11 May 2004

<<snipped>>

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:

http://www.diydoctor.org.uk/projects/lights_and_switches.htm

Dave Plowman replied to BigWallop on 11 May 2004

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.

BigWallop replied to Dave Plowman on 11 May 2004

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.

Dave Plowman replied to BigWallop on 11 May 2004

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. ;-)

Dave Plowman replied to Dave Plowman on 12 May 2004

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

Dave Plowman replied to Dave Plowman on 12 May 2004

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

BigWallop replied to BigWallop on 11 May 2004

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

Dave Plowman replied to BigWallop on 11 May 2004

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.

Archived message: MK intermediate switches (UK D-I-Y Home Improvement)