Double-wired light switch


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Old 11-25-07, 09:12 PM
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Double-wired light switch

I've just started to learn about electrical work, and today I mapped the circuits in my home. However, I have one light switch (a ganged device with one switch and one empty spot) that doesn't control anything. I removed the cover and got out my circuit tester (without killing the power at the service panel) because I wanted to see if the wires are hot. I'd like to figure out why I have this inactive switch in my house. I also did some research online about how to test for hot wires.

I believe that the switch is a simple dipole, as it has only two screw terminals on it (aside from the green grounding screw). I was able to determine that the wire at top is hot by touching one probe on my circuit tester to it, and the other to the box (which must be grounded, because the light came on on my circuit tester). However, I noticed something strange--there are two wires attached to the top screw terminal. When I tried to touch the probe to the second wire (still with the other to ground), I received a big flash, and the end of the metal probe of my circuit tester has taken some serious damage.

Now I'm really confused. For one thing, I'm feeling pretty nervous about working with electricity again! Also, I don't understand why I was able to touch one wire safely, but not the second wire. Shouldn't they be at the same potential when they're attached to the same metal terminal? And, why on earth would two wires be attached to the same terminal on a dipole switch? This doesn't match any diagram in my electrical book.

Any help is greatly appreciated. I can email a picture of the set-up to anyone who thinks that a picture would help.

Thanks so much!
 
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Old 11-25-07, 09:51 PM
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Are you sure you didnt hit against the switch casing by accident?

Not to be a wise guy but I have to ask.....This isnt an automotive circuit tester is it? A/C and D/C are totally different animals and MUST be treated as such.

What ckt is this on, or should I say what else is on the Ckt with this live wire. Is it near a staircase, which could mean basemet lighting, in the kitchen, which could be an appliance,

Bedroom, could be a switched outlet, the list goes on and on,
But more likely is something that doesnt work, and you havent noticed it, because.......Well,......IT doesnt Work...
Post more info as soon as you get it...Welcome to the forum....
 
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Old 11-26-07, 04:47 AM
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You must have accidentally touched the metal box at the same time as, and with the same probe that you touched the terminal with.

While it is not good that you created a short and caused a nice spark, I am glad that it has given you a fear of electricity. However, do not let this completely push you away from electrical work. Just remember to be very careful with testing live (I recommend pulling the devices completely out of the boxes) and never make any changes without shutting power off to the circuit(s) serving the box.

It is against code to place two wires under the same screw terminal on a receptacle or switch, so what you are describing is a code violation.* However, these are found all the timer with switches and receptacles that homeowners have replaced. The reason there are two wires to one terminal is that they bring power to the switch and also want to continue power out another cable in the box. This is NOT unusual.

* Note that backwired devices have plates held in position by screw that do allow two wires per terminal.

Possible reasons for the switch include:

1) A receptacle is half switched. Did you check both halves of every duplex receptacle?

2) A receptacle used to be half switched, but someone replaced the receptacle and forgot (or didn't know) to remove the tab on the brass terminal side of the receptacle, so now the receptacle is not switched and you have a switched that does nothing.

3) You have a switched receptacle hidden behind a piece of furniture.
 
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Old 11-26-07, 07:37 AM
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Thanks much for the replies. I think that you're both correct--I believe that the short was likely caused by my accidentally touching the grounded casing on the dipole switch.

Also, I do have half-switched receptacles (controlled by a different switch) in other parts of the same room, so I think that the idea that this switch may be intended to control such switches is a good lead.

At this point, I plan to kill the power at the service panel, pull the switch out and disconnect it, pull all of the wires apart from each other, turn the panel back on, and determine which wires are hot by touching each wire with the probe of my circuit tester with the other probe grounded on the box. Once I identify the hot wires in the box, then I can figure out which circuit they're on.

Are there any problems with my plan?

Thanks much!
 
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Old 11-26-07, 07:51 AM
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Make careful notes (possibly even take a digital picture) of how the wires are connected BEFORE you disconnect them.

If any wires are red, that is a clue that there might be a half switched receptacle involved.
 
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Old 11-26-07, 08:55 AM
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Many may disagree with me , and Im not saying that your wrong, But dont get into the habit of checking thru "THE BOX" for live wires.Most work today is done with plastic boxes, which leaves you high and dry when you encounter one. Try checking "Bare to hot" or " neutral to hot" instead.
Even one more possibility would be "Switch housing to hot" .

If this is going to be a hobby or "Home Maintenance" type of activity, Get an outlet tester, which will tell you at a glance that every thing is operating and connected in its proper place, and many have a test button for gfci receptacles.

Racraft..IM shocked....You didnt recommend The book "Wiring Simplified" to this new poster
It can be bought at Most home improvement or electrical supply stores. Well worth reading.
 
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Old 11-27-07, 07:57 AM
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Thanks again. I'll pick up a copy of the book that you recommend, and tackle this project again this weekend.
 
 

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