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Old 11-22-2009, 01:38 PM   #1
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Sudden Dim Lights


My friend called me yesterday and said his lights went dim suddenly after he turned his dishwasher on for a while so I went over and took a look at it.

With all the breakers on I measured the voltage coming into the 100 amp main breakers from line to line, which was correct, then from line to ground on each, one line gets around 50v other one close to 200v.

SO I turned all the breakers off and measured it, and it gets correct voltage like that. I flipped breakers on one by one measuring voltage thinking there would be a problem with one breaker only, but three breakers out of 8 effect the voltage on the main lines.

If you leave the 3 off the voltage if fine, but if you turn either of those 3 on the voltage is effected measuring it at the main breaker.

I turned the main breaker off and tested for continuity on the (black) hot wires coming out of the particular three breakers that effects the voltage and it buzzes no matter which combination I use to touch them.


Would you guess those 3 wires are touching somewhere, or could it possibly have continuity because of the neutral wires all touching each other at the neutral bar?

Any ideas other than trying to trace those 3 wires down, or should I just tell him to call an electrician?

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Old 11-22-2009, 01:56 PM   #2
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Sudden Dim Lights


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Originally Posted by commandthepast View Post
one line gets around 50v other one close to 200v.
It's a bad neutral connection. The bad connection will have (200-50)/2 = 75v across it, instead of a few millivolts. The default fix to tighten the neutral connection coming into the load center while being careful not to short anything. There is an arc-flash danger.
The total voltage should add to the nominal 240v.
Sometimes the bad neutral is upstream of the load center.


Last edited by Yoyizit; 11-23-2009 at 09:44 AM.
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Old 11-22-2009, 02:06 PM   #3
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Sudden Dim Lights


This is usually a power company problem at the pole or along the service drop; call them immediately and tell them you have a loose neutral. Response is usually very quick.
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Old 11-22-2009, 03:03 PM   #4
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Sudden Dim Lights


All Right Thanks guys I told him to call the power company. Appreciate the help.

Just out of curiosity, why does it only effect the voltage on three circuit breakers and not all of them?
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Old 11-22-2009, 03:13 PM   #5
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Sudden Dim Lights


There is a load associated with those breakers that causes the fluctuation in voltage.
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Old 11-23-2009, 09:17 AM   #6
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Sudden Dim Lights


(for those eavesdropping) If you experience a problem like this immediately unplug all electronics including microwave ovens and including stoves and clothes dryers with electronic timers etc. (flip off the breaker).

Small loads, perhaps just a few lights, will work correctly given the ground to a ground rod and/or a water pipe together with a degraded or broken neutral out to the utility pole. Once the load is increased and the total load on one side of the 120/240 volt service is very different from the total load on the other side, then lights the side with the heavy load go dim and the side with the lighter load goes abnormally bright

You need to find out where the neutral was degraded or broken. All neutrals and grounds are tied together in the main panel. If the problem is upstream of that point, namely at the meter or in the line strung from the utility pole to your house, or up on the utility pole, the power company will fix it.

Many folks call the power company first if everything in the house is affected. If only some parts of the house are affected then the problem is somewhere in your house. If hot to ground on every other breaker in the panel is the same abnormally high value and on the remaining breakers it is the same low value then the problem is at the main panel or upstream. If possible it is a good idea to do some checking yourself but yopu need to be careful.
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Old 11-23-2009, 10:03 PM   #7
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Sudden Dim Lights


God, if that isn't a sticky, I don't know what is.
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Old 11-25-2009, 04:28 PM   #8
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Sudden Dim Lights


If the problem does turn out to be in your SE, or any other wiring that you, and not POCO own, then have them do a temporary disconnect so that you can safely correct the problem. Then, when it has been fixed (and probably should be inspected as well; POCO may require inspection before re-connect), have them come back and restore power.

I would surely feel much safer working on a killed SE than a live one!

FW
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Old 03-22-2010, 01:19 AM   #9
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Just to let everyone know, the problem was outside, where the big fat bare ground wire runs from the outside main box, to the inside breaker box. Water had leaked down into that line somehow and corroded it, it took the electrician 2 days to figure out what the problem was, but he fixed it and charged 550

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