Help with nonfunctioning circuit

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  #1  
Old 12-29-06, 07:49 PM
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Help with nonfunctioning circuit

The light in my laundry closet stopped working suddenly. I thought the light bulb burned out but a fresh bulb did not work. So I checked all the connections - all good. Using an inductive power indicator I verified there was power to the wall switch and to the bulb socket. I then did a continuity test from the switch wiring to the light fixture. All checked out fine. I tested the light fixture. There was about 50 ohms resistance so I put in another fixture - no change. I put in a new wall switch - no change. Finally I used a multimeter to check the voltage at the switch and the fixture. There was a slight variation in voltage around zero to 1 VAC at both locations. The inductive power indicator shows power to the bulb socket but the voltage is so small as to be negligble. I tested the meter on a working circuit and it showed 120 VAC so the meter is good.

The line from the switch to the light is at the end of the wiring run. It appears everything else on the circuit, receptacles and lights are working normally. I am at a loss. Any ideas?

Thank you.
 
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Old 12-29-06, 08:37 PM
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So, you have a voltage tester. Thats a good thing.

With the light fixture removed, you have a black and white. Be sure these are seperated. Turn the switch on and read the voltage between them (wht/blk).
This should be 120V +-. If not move back to the switch.

Now we must know what you ave for wires in this box.

Simply: If there is a ground in there. With the sw. OFF, Find the wire with 120v. Now turn the sw. ON. the other wire connected should show 120V +-.

If both test are good, shut the power off and reconnect the fixture.
Human error happens.
 
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Old 12-29-06, 09:06 PM
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Get a wiggy

What you need is a Wiggington type solenoid tester. It will tell you if that circuit is hot or not. It takes about seven milliamperes to give a voltage indication on a Wiggy. Phantom voltages that can deceive a inductive tester will not give a false indication on a Wiggy.
--
Tom Horne
 
  #4  
Old 12-29-06, 09:36 PM
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1 VAC btwn the black and white wires

Thanks for the inputs. The digital voltmeter shows only less than 1 VAC on the supply wiring to the switch. What I can't figure out is since so little is being supplied, why do all the other outlets and lights on the same circuit breaker have 115 VAC? I am stymied.
 
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Old 12-29-06, 09:41 PM
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Originally Posted by djulio View Post
Thanks for the inputs. The digital voltmeter shows only less than 1 VAC on the supply wiring to the switch. What I can't figure out is since so little is being supplied, why do all the other outlets and lights on the same circuit breaker have 115 VAC? I am stymied.
At what two points are you applying the probes?
 
  #6  
Old 12-30-06, 03:22 AM
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Digital voltmeters are not appropriate to test ac circuits, unless you know what you are doing.

It sounds like you have an open circuit. Look for a disconnected wire at the receptacle or light just before this light, or just before it's switch.
 
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Old 12-30-06, 11:26 AM
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Is this new construction or a little dated? This will help with the perspective. Wiring does not change by itself ( i know this is kinda duh but its for everybody not just the OP and people who know this) The situation you describe is this (light works one day and then the light doesn't work the next day. you were correct to check the bulb first. If the car won't start check for gas.) Wiring is like that. It doesn't change by itself usually there is a catalyst to change something. Racraft was right to tell you to check a plug or another switch before the laundry room.

Here what i think might have happened. If new construction: Some electrician wired the house and installed the receps with the stab ins in the back of a recep before the laundry room and someone in your household recently plugged something into it and pulled out the plug and when doing so loosened the stab in (tenuous) connection. If this is the case you should go through the house and change all the receps to screw terminals as opposed to the notorious stab-ins
 
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Old 12-30-06, 05:20 PM
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Thanks - have work to do.

Hornetd - I applied it to the light socket first, then the switch, then the supply wires to the switch (across the white and and black wires).

racraft - I am trying to find the right one so far everything I have checked is working fine. I have to move some furniture around to see if I am missing a receptacle or something somewhere. If it is an open circuit, why does any voltage appear at all at the switch? The small voltage disappears when I turn off power at the circuit breaker.

Sthrnamp - Not new construction but I think I may have identified the catalyst. The light fixture had water marks in the globe. A year or two ago, I had an upstairs toilet over flow. Looks like water travel into the light fixture. However, since I replaced the light fixture and switch with new ones, and the continuity check from the switch to fixture wiring was perfect, it probably has nothing to do with it. I will keep checking other components in the same circuit for the type of stab in connections you mentioned.

Thanks agin for all the input.

Dale
 
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Old 12-30-06, 05:28 PM
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Dale,

Do not use your digital voltmeter on ac circuits until you learn how to use it. The voltage you are reading is phantom voltage. Ignore it. It is indicative of an open circuit. Use an ANALOG meter or use a simple two wire tester for all your testing.

You must check everything else on the circuit. This does not mean by plugging something in and seeing if it works. That result is inconclusive. This means by physically opening the box and making sure that the wires physically connect to the device or to each other (to whatever they are supposed to be connected to).
 
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Old 12-30-06, 05:40 PM
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Analog on the job!

Racraft - I am breaking out my analog mulimeter. OK - I will open everything up and check connections. Thanks!

dale
 
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Old 12-30-06, 06:11 PM
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What do you have for wires in the switch box? In the ceiling box?

I read black and white, This is a switch loop and you will not get voltage between them.

Amp (hello) is right, wireing does not change itself.
 
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Old 12-30-06, 06:26 PM
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Just to add while wiring does not change itself small critters can change it. Unlikely and only to be considered as a last resort but still possible.
 
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