Testing a switch results in arc

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  #1  
Old 01-02-06, 08:21 AM
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Testing a switch results in arc

We have just purchased a "new" old house and there is a switch by the sink that doesn't seem to control anything. There are also two wires in a metal conduit hanging under the sink which I'm assuming is for a garbage disposal (currently not present, but looking to be added). Anyway, I was going to use my trusty circuit tester (two leads and a bulb) to check for power and when I touched one lead to one terminal on the switch, I got a blinding arc which blackened the tip of the lead and the side of the switch.

What went wrong? How can I properly test for power at a switch? And while I'm at it, test the wiring for the garbage disposal?

Thanks,
Brian
 
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  #2  
Old 01-02-06, 08:24 AM
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It seems quite likely that the switch controls power to the line under the sink for the disposal.

Don't know what you did. But somehow your tester shorted hot to neutral or ground.
 
  #3  
Old 01-02-06, 08:29 AM
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How can I properly test for power at a switch?
I think a "blinding arc" is pretty good evidence that you do have power.
 
  #4  
Old 01-02-06, 08:34 AM
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You probably shorted the switch terminal to the metal box. You should have tested with the switch pulled out from the box, not with the switch installed.

The proper way to test for power is with a two wire tester. Depending on the wires in the box, you would test between the switch wires and the neutral, or between the switch wires and the ground.

You are possibly correct in guessing that the switch most likely was for a garbage disposal. Generally garbage disposals stay with the house when you move, but unless it's specifically stated in the contract anything can happen. When disconnecting the unit they should have turned off the circuit breaker and properly capped the wires. They were foolish not to since they could be held liable if you or someone else were hurt or killed on the unconnected and not capped wired.


Your short may have tripped the circuit breaker. Make sure that the breaker is off and then carefully remove the screws and pull the switch from the box. Examine the wiring. Turn the breaker back on and carefully check for power at the switch leads using a two wire tester, using the metal box for the second lead. Then with the switch on and off check for power at the two wires under the sink.

Since this house is new to you, you also need to identify each and every circuit. Your goal is to determine what each circuit breaker controls each and every receptacle, light and appliance in the house, and to know for certain what each breaker controls. Spend a few hours doing this with a helper. Make sure that you make detailed notes and post them near the main panel. Do this even if the main panel is already marked. Rarely are the markings 100 percent complete and accurate.
 
  #5  
Old 01-02-06, 09:01 AM
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Thanks for the prompt responses.

As far as I know, I didn't short the tester lead to anything, but I could be mistaken. I should have taken the switch out of the wall first.

As for the wires under the sink, they aren't capped, but they are separated and wrapped with electrical tape. If there ever was a disposal there, it was removed prior to us ever viewing the house. There is a tag hanging on the wires as well. Problem is that it has been painted over so I can't clearly read what is written on there. The part that makes me nervous is that it looks like 220 might be written on the tag. I have access to a multi meter and I need to find out what is going on here.

Finally, a circuit map is most definitely needed. It is my next order of business.
 
  #6  
Old 01-04-06, 09:55 PM
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I found the breaker that controlled the switch and shut it down. I pulled out the switch and it has one black wire and one white wire pushed through the back and a ground wire that is clipped to the side of the box. At this point I could tell that I shorted it between the terminal screw and metal frame of the switch.. I turned the power back on and tested the switch. I was a little skittish after what happened last time, but I tried white wire to box and the lamp of the tester lit. Black wire to box and nothing.

Also, the wires in the metal conduit under the sink are red and white. Since they are not white and black as I was expecting I was too nervous to touch the tester to the wires.

I'll see if I can get some pictures posted.

Thanks for your help.

Brian
 
  #7  
Old 01-05-06, 04:47 AM
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Brian,

The wires in the switch box are a switch loop. The white wire should be always hot. The black wire would be hot when the switch is on, open otherwise. Open means not connected.

When dealing with AC the color of the wire is not necessarily a give away as to the voltage. A red wire means the same thing as a black wire. Sometimes a red wire will be used for a switched circuit.

Go ahead and test between the red wire and the white wire. Do this with the switch in question off and also on.
 
  #8  
Old 01-05-06, 11:31 AM
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Thanks for the feedback.

I decided that it was time to learn more about electrical systems so I purchased the Home Depot Wiring 1-2-3 book and was just reading about the switch loop. I kind of figured that could be the case.

Now my last question will be about grounding at the disposal. I only saw the two wires (red and white) out of the conduit. What about the ground?

Thanks,
Brian
 
  #9  
Old 01-05-06, 11:45 AM
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The conduit itself provides the ground, or at least it should. You can also add a green ground wire to make sure that the ground is provided.
 
  #10  
Old 01-08-06, 08:15 AM
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I finally got a chance to test the wires underneath the sink and they are definitely controlled by the switch. Good news.

Now, is there a way to tell if the conduit is grounded properly? Obviously, I don't know what is going on behind the wall, and I didn't notice similar looking conduit down at the box.

When you say I can add a green wire, what do you mean?

Thanks,
Brian
 
  #11  
Old 01-08-06, 10:16 AM
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The conduit should be grounded. To test, look for voltage between the hot wire (should be the red one) and the conduit.

By adding a green wire I mean that you could add a green piece of 14 gage THHN wire (12 gage if this is a 20 amp circuit). You would add the wire to the two already in the conduit. At the other end (which you will have to find) you would connect the green wire to the ground wires already there.
 
  #12  
Old 01-22-06, 09:28 AM
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I've finally gotten back to this project (it has been low on the priority scale).

I did test between the red wire and the conduit and it is grounded.

So if I'm understanding properly, this setup should be fine? I bought a metal screw in connector to attach the conduit to the disposal base.

If for some reason this is not a proper ground (is there even such a thing as good and bad grounds?) do I run a risk of having my metal sink electrify?

Thanks much for you help.

Brian
 
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