Confused by voltage measurement

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Old 04-24-17, 07:57 PM
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Confused by voltage measurement

My ceiling light connected to a dimmer on a couple on occasions stopped working and then come back the next day. The last time however it never recovered. I figured the dimmer died and got a new one to replace it. But before I put it in, I thought I would short the wires where the switch should be connected to make sure the circuit closes. And the light still would not turn on. I probed the wires in the switch box with a voltmeter and it showed something in the neighbourhood of 5V between the live and neutral wires. But then I also measured 120V between live and ground and approximately the same between neutral and ground! I do not understand how that can possibly be. I would expect to measure 0V for the neutral.

I am including the picture of the box. Note how it also acts as a juncture from where they probably distributed the lines to outlets in the room. You can see all black wire connected, but one of them feeds the lights and should normally be connected through the switch.

Can anybody explain this to me?

Thanks
Marko

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Old 04-24-17, 08:10 PM
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Welcome to the forums.

We see that problem here multiple times a day. You have an open neutral.

The neutral is open before that switch box and everything else connected from there on is also dead now with no neutral.
 
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Old 04-24-17, 08:35 PM
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Thank you for the welcome.

It has never occurred to me that a wire can disconnect on its own. Let me see if I understand what you mean here first - there is a broken connection between my probe point on the neutral wire and the neutral line in the electrical panel, is that right?

OK, so diagnosis was simple, how about the cure? There are three white wires bundled together here. One must be from the light fixture and it is obviously connected properly. One goes further away from the panel to some other light or outlet (which I do not care for in this case), and the third (one I am interested in) goes toward the panel (either directly or to some other intermediate connection I suppose). How can I identify the last one?

Marko
 
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Old 04-24-17, 09:13 PM
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You've got a perfect handle on the issue.

What I do is to identify everything on that circuit and work backwards towards the panel.
The most likely cause of the problem is backstabbed receptacle connections where the wires are just pushed into the back of the receptacle.
 
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Old 04-25-17, 04:03 AM
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As you are working back change all back stabbed connections to the screws.
 
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Old 04-25-17, 01:32 PM
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It has never occurred to me that a wire can disconnect on its own.
Usually it's the little spring clip in the cheap "builder-grade" receptacle that has simply worn out over time, although sometimes a large current like a vacuum sweeper will cause a weak connection to burn through.
 
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Old 04-25-17, 08:26 PM
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Here is an update...

I first turned off the breaker powering that light. Of course, I did not know which one it was so I was turning them of one by one until the voltmeter dropped. On a side note, it still showed 5V and not 0V, why?

Then I started hunting the outlets and lights that were off. My reasoning that these were the candidates for the disconnect, and if all tested negative it would have to be in the panel itself. I was able to find one dead outlet which came to the top of the list (not hard, it was the only one!) because it worked properly with the circuit breaker on. And indeed, out of four wires only one stayed attached to it when I pulled it out.

Now, I am not a licensed electrician to know this so I thought I would ask. From what I remember, the live (black) wire should go to the smaller pin in the plug, right? In that case this outlet was wired incorrectly. I am attaching the picture, the last black wire is connected to the left side in case it is not clear from the picture.

And for those who provided feedback here, the outlet uses only screws and not those "push in" holes.

Marko
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Old 04-25-17, 09:54 PM
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On a side note, it still showed 5V and not 0V, why?
Would bet you have a digital not analog meter. They tend to show false readings due to induced voltages.
From what I remember, the live (black) wire should go to the smaller pin in the plug, right?
Yes, black to brass.
 
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Old 04-26-17, 07:11 PM
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Yes, it is a digital multimeter.

Thanks for the help, the light is functioning now.

Marko
 
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Old 04-26-17, 07:33 PM
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And indeed, out of four wires only one stayed attached to it when I pulled it out.
And for those who provided feedback here, the outlet uses only screws and not those "push in" holes.
Those screws being loose were just as bad as the push-in terminals.
You found one with loose screws.... how many more are there ?
 
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Old 04-27-17, 09:51 PM
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I did not look at any other outlets to check if the connections in them are good. I have not notice any other malfunction.

Marko
 
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Old 04-27-17, 09:54 PM
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Hopefully it was a one-off problem. If not you'll be checking more of the devices.
 
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