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fordprefect 08-02-2006 03:47 PM

Switch installation question
Hi everyone!

First let me say that although I an electrical and computer engineering student (NTUA, Greece), I have been so focused in the "computer" part I have very little knowledge of electrology and even that is mostly theoretical. I do know how to use some very basic tools (multimeter, which I currently don't have, and, ahem, screwdriver) and once I put together a filter circuit that almost worked, but beyond that it wouldn't be exaggerating to say my only practical experience in such matters has been changing light bulbs.

Now, my problem is this:
I am currently staying in Chisinau, Moldova, which is a former Soviet Union country. In our kitchen there is a light as well as a ventilator. They are both controlled by what I believe was meant to be a common DPST switch.
When I arrived there was only a hole in the wall with three identical (well, one is a bit longer but I doubt that's significant) white cables in it. The light was turned on by sticking your hand in and connecting two of these cables (the short ones, actually). If you wanted the ventilator to function you connected the third (long) cable as well.
Not very safe, I thought, so with my limited knowledge I successfully (!) installed a two-button switch to control the light. I left out the third (long) cable, so the second button does nothing and there is now no way to turn on the ventilator.
I now want to connect the third cable too and make the switch fully functional.
This is my guess at what's going on inside the walls (maybe "neutral" and "hot" should be inversed):

Connections AC and ABC have been tested and BC is also presumed to be safe.
What I am concerned about and the reason why I am hesitating is this:
I have no way of telling which cable is C (it is logical it should be the middle one, but is that a rule?) and which is A, so it is possible A and B could be accidentally connected.
My question (finally!) is this:
Is it safe to connect AB (or BC, remember I can't tell) and see what happens?
If I understand correctly C is "hot" while A and B are "neutral" and you need two "hot" cables to get something wrong. In any case it seems to me there is no way to get a "wrongly" closed circuit. At worst I'll end up with an open circuit.
Is this correct? Should I proceed through trial and error or should I get a multimeter and/or call a professional?

Many thanks in advance and sorry for the long post!
Your help is greatly appreciated!

Square Eye 08-02-2006 04:55 PM

You can attach A and B. It will just be an open circuit if neither A nor B is the "hot" or neutral. This would be the simple way to go if it were not so dangerous. The multimeter, set on AC voltage, would show you where the power is with out connecting the wires together.

If you have a clip end on a multimeter lead,
attach it to the wire that you think is C and then check for voltage on A with the other lead. If you have voltage, one of these will be the "hot"

Then check the other wire without moving the clip. If you still have voltage, then you are clipped on the "hot".

If you have no voltage, then the wire you checked before is the "hot".

Think of all of the wires as being hot all of the time, because they complete a circuit. If you attach a wire or part of your body to any two wires, chances are high that you will become part of the circuit.

glennjanie 08-02-2006 08:28 PM

Hello Ford and Welcome to the Forum:
Square eye is a super duper electrician, I have seen his work and you can use his suggestion. A switch simply breaks the "hot" wire, forget about any neutral in this case. One short wire is bringing power to the switch, the other short one goes to the light and the long one goes to the ventilator. Your double switch should have 4 screws in the sides of it. You will probably find 2 of the screws are connected with a little break-away tab; this is the side the power comes in on. The other 2 screws on the other side will be seperated so one can be used for the light and the other to go to the vent. Does that sound clear as mud? Two more things; once you use the voltmeter, please turn the power off while you work on it and when you are finished "give her the smoke test". Turn the power on and, if there is no smoke, congratulate yourself; it there is smoke, reverse the two short wires.

fordprefect 08-10-2006 08:49 PM

Thank you both for your replies! I followed Square Eye's suggestion and got it right on the first try, meaning C was indeed the middle cable.

Square Eye 08-10-2006 09:14 PM


Thank you for keeping us up to date!

I'm glad you got it worked out and got that less than safe situation fixed!

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