Main breaker problem

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McLintock

McLintock

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Location
USA
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Electrician
Went on a service call to check the power in the garage, the garage door installer say he had 99 volts at the outlet. So by the time I got there the main breaker in the panel was off.

Read voltage between the two legs coming in and read 247v and between one leg and neutral 123v and same on other. Turn the breaker on and read the bus bars and 247v between the two and 99v on one and 148 on the other, no loads on at this time all branch circuits off. Read the two lugs before the main and get the same readings as on the bus, turn the breaker off and readings go back to correct, turn breaker on and readings are off.

Question is, is the main on this panel bad or could a bad neutral be causing this?


“ shoot low boys their riding shetland ponies”
 
Did you read across the breaker (while on) to check for a VD through the breaker? I agree with the others, signs of a bad neutral, but with no loads those are interesting readings.
 
I agree the readings are of a bad neutral, but getting those readings with no load as throwing me in a loop. In the morning i am going to bypass the breaker and just put lugs on the bus, did not have much time to investigate the problem the other day.

Probably is a bad neutral, but I’ll try this to see if it may work, and not tear up her yard.


“ shoot low boys their riding shetland ponies”
 
McLintock said:
I agree the readings are of a bad neutral, but getting those readings with no load as throwing me in a loop. In the morning i am going to bypass the breaker and just put lugs on the bus, did not have much time to investigate the problem the other day.

Probably is a bad neutral, but I’ll try this to see if it may work, and not tear up her yard.


“ shoot low boys their riding shetland ponies”
Click to expand...
Its clear there is a neutral issue. A bad breaker could not casue 1 leg to be higher than normal. Don't waste time trying to bypass breaker.
 
The neutral is "so open" that the meter is reading an imbalance even with no branch breakers on.

That points out an issue with using a high-impedance voltmeter for troubleshooting.

Since the service is useless without load, it doesn't matter what is read with the power off.
 
Definitely not a main breaker issue.
Bad neutral.

Do you have a good ground rederence that's not in that panel? You can verify your hot legs that way.
 
Even an incandescent night-light bulb would be enough load to minimize phantom voltage readings.
 
I did not read deeply enough. I just assumed that the 99/148V with the main on was caused by system loads, and did not catch the part about all branch breakers being off.

The uneven voltage with the main on means there must be a load or leakage current.

However prior to wondering if there is something else wrong with the system my guess would be that some intelligent breakers are consuming control power even when off.

Jon
 
winnie said:
I did not read deeply enough. I just assumed that the 99/148V with the main on was caused by system loads, and did not catch the part about all branch breakers being off.

The uneven voltage with the main on means there must be a load or leakage current.

However prior to wondering if there is something else wrong with the system my guess would be that some intelligent breakers are consuming control power even when off.

Jon
Click to expand...

This panel as 3 branch circuits all 20 amps, could one of those be leaking voltage when off, if this is the case why would the 20v be on the other leg?

Would the bad neutral be in the garage, not the feed coming from the panel in the house?


“ shoot low boys their riding shetland ponies”
 
The only way to get a higher-than-normal line-to-neutral voltage is a bad neutral.
McLintock said:
Would the bad neutral be in the garage, not the feed coming from the panel in the house?
Click to expand...
The neutral problem must be on the source side of wherever you're measuring the imbalance, not the load side. You want to check for the voltage imbalance (while some loads are on) as close to the source as you can.

If you measure proper voltages leaving the house, and improper voltages at the garage, the problem must be in the feeder, or at one end of it. Is there a separate ground? You'll also measure voltage between the neutral and earth.
 
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(Note that I edited the above post moments ago)
McLintock said:
Ok, but why the correct readings with the breaker off?
Click to expand...
Because what is now a floating neutral, capacitance to the world tends to make it float around zero volts. Your meter is such a small load, it doesn't force the neutral away from zero volts the way real loads do.

A solenoid tester, which is what I use, would show the problem even without other loads on.
 
The other option is rather that a floating neutral (completely open) you have a high resistance neutral (break with slight conductivity).

For example a splice that has failed and is full of water.

Or soil as your neutral conductor, from the ground electrode at the garage the one at the home or at the transformer.

With no load at all you have just enough connectivity to give a good voltage reading, but apply some load and your voltage changes.

The fact that this voltage changes with the main on but the branches off means that there must be some current flow, which could be leakage through a breaker or could be power going to a breaker, eg to a GFCI or AFCI breaker.

Jon
 
Something I came across earlier this week, some 120 volt glass crushing machines would attempt to start after pressing the start button, coil would pull in & would hear the hum of the motor, then it would drop out, put a meter on it & would read 119 -121V, push start & it would collapse to 55V, started a 240V 3Ø machine & it fired right up, checked the panels the machines were fed from, felt it was a neutral problem, finally went to the main panel outside, checked it, the meter was adjacent to the main panel opened the door to main breaker in the meter can, it was sparking to the corrugated metal roofing used as fencing as the door was lifted, after the door & dead front was removed, looked at the neutral lug & have never seen one so loose, it was so loose that it would make one think the cable was stuffed in the lug & left as-is, but that is doubtful as the service was installed in the early 1980's, after tightening it & restoring power all was well, no more sparking & machines worked normally.

For those who a curious, glass crushers have a conveyor move glass bottles being recycled towards a crusher to break them before they drop in a bin, the starters are interlocked so the crusher motor has to be started before the conveyor to avoid material jams.
 
LarryFine said:
Because what is now a floating neutral, capacitance to the world tends to make it float around zero volts. Your meter is such a small load, it doesn't force the neutral away from zero volts the way real loads do.
Click to expand...
That does not explain the different readings using the same meter with the main on and off and all other branch breakers are off.
 
So I plugged my heat gun in to an outlet and it went down to 25v on that leg and 215v on the other one. With a light bulb in it goes down to 115v. So looks like I have some digging in my future


“ shoot low boys their riding shetland ponies”
 
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