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Offline onsk

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Home electricity mystery
« on: 13/03/2011 08:40:04 »
My own science education is very limited, so perhaps to others the explanation to this mystery would be obvious, but to me, this is a mystery.

I had a simple lamp attached to the ceiling of a room and to a switch on the wall. It worked normally: when the switch was in one position the light was on, when the switch was in the other position it was off.

I changed the lamp. The installation of the new lamp was straightforward: I simply connected each of the two wires in the ceiling to the corresponding wires in the lamp. To my surprise, as soon as both wires were connected, the light bulb started to work, although the wall switch was in the off position, and now, changing the position of the switch makes no difference: the light remains on! As far as I am aware, the two wires in the ceiling are connected only to that single wall switch.

Any idea what may be going on there?



 

Offline graham.d

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Home electricity mystery
« Reply #1 on: 13/03/2011 09:52:25 »
You need to have the wiring checked by someone who knows about electrical systems. Something, and maybe more than one thing, is wrong with the wiring or, possibly, the switch.

From what you describe there were 2 wires connecting the lamp. Was there only two wires? One possible cause is that there is also an "earth" wire there and that this wire was connected to the lamp intead of the neutral wire and that the switch has been wired incorrectly so that the neutral is switched rather than the live. In fact, I cannot see any way the light is now unaffected by a properly working and correctly connected switch.

It is also possible that the switch has coincidently got damaged. This seems unlikely, but maybe the switch has become fused to the "on" state and is not actually breaking the contact when switch off.

It is hard to diagnose this remotely. You should definitely have someone take a look at it though.
 

Offline onsk

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Home electricity mystery
« Reply #2 on: 14/03/2011 07:19:54 »
Thanks for you answer.

Yes there was a third, earth, wire, but I made sure to connect the new lamp exactly the way the old one was connected, to the two other wires. I'll try to figure out whether something was damaged in the switch.
 

Offline CliffordK

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Home electricity mystery
« Reply #3 on: 15/03/2011 01:44:40 »
Make sure you trip the circuit breakers when tinkering with the wiring!!!!!!!!!

There are two ways to wire a simple switch (not 3-way).

Lamp First, Switch Second
Switch First, Lamp Second.

To simplify things I'm ignoring all other fixtures that might be on the same circuit, as well as ignoring 3-way switches.

In the USA, you have 2 wires (Black (hot) White (neutral)) Plus Green/bare copper (ground).  We use 120V in which the Neutral is wired to the ground busbar at the box. 

240V wiring would be similar.

Anyway.

Switch First, Lamp Second.
--------------------------
Run the wire from the breaker box to your switch box.
Connect all white wires together.
Connect all ground wires together (with a pigtail from the grounds to the switch).
Connect one black wire to each screw on the switch. 
If you have more than one circuit, then connect them all to the first black from the box, and run a black pigtail to the switch.
Now, you just run the two black & white wires up to your junction box at the lamp, and connect the lamp wires to them (b&w).


Lamp First, Switch Second
-------------------------
Run hot wire from breaker box to your junction box at the lamp.
Run a single 2-wire from the lamp to the switch.
Somebody suggested that you should put some black electrical tape around the white wire on this one so you can recognise that it is "HOT".

Connect the White from the junction box to the white for your lamp.
Connect the Black from the junction box to the white going to the switch.
At the switch, connect the Black & White to the switch (also doesn't hurt to mark the white with black electrical tape).
Back at your lamp junction box, connect the black from the switch to the black from the lamp.

You can reverse the black and white going from the lamp to the switch, as the outcome is the same, just switching the two wires.

All ground wires should be twisted together, with a pigtail to your lamp if necessary.
------------------------------------

I'm presuming this second case is what you have.

and...  somehow you inadvertently removed the switch from the circuit.

If you accidentally put the switch white & black together with the breaker white and black, flipping the switch would instantly trip the breaker. 

If you have a big bundle of wires, determining which one is "hot" & which one is switched can be a pain.  Sometimes it is obvious.  Sometimes an electrical multi-tester can help you figure it out.  The ohm-meter part of your multi-tester can give you a good go-no go test for the switch.

 

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Home electricity mystery
« Reply #3 on: 15/03/2011 01:44:40 »

 

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