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Voltage too high troubleshooting

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Maybe I'm exhausted and am missing something obvious. I'm normally pretty confident in my troubleshooting abilities.

Got a report a specific light fixture keeps blowing light bulbs. Tried a 100w incandescent. Bright flash and it's dead.

Got out the meter. Measured 250v across the fixture. Killed one breaker at a time and found the following:

Breakers A, B, C, and D all have wires feeding the same conduit. Two neutrals, 4 hots in said conduit.

  • With breaker A on, the fixture reads 125v. Breaker A does not appear to operate any other fixtures.
  • With breaker B on, the fixture reads 85v. Breaker B operates some other lights and ceiling fans in the same section of the building. All of these lights and fans operate normally.
  • With breaker C on, the fixture reads 15v. Breaker C operates some other lights and outlets in the same section of the building. All of these lights and outlets operate normally
  • With breaker D on, the fixture reads 15v. Breaker D operates some other lights/fans/outlets in the same section of the building. All of these operate normally.
  • All four breakers off, 0v at fixture.

It's giving me neutral problem on a shared neutral, except that everything else is operating normally.
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I like ceiling fans & EMT
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Discussion Starter · #8 ·
A and B are probably sharing a neutral. C and D run close to A and B giving a false reading. Not enough info especially as we have no idea if the fitting is a incandescent.

Shared N could be lost any where in the chain so everything else working correctly just means its good up to that point.
Fixture is incandescent/edison socket.

Here's what I am not getting.

If the neutral is broken, wouldnt there have to be something on each leg acting weirdly? Otherwise there would not be a complete path.

In other words, if A and B share a neutral, and the neutral is defective, wouldnt something on B act abnormally?
 

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In other words, if A and B share a neutral, and the neutral is defective, wouldnt something on B act abnormally?
When troubleshooting we look at the numbers then look for what we would need to replicate these numbers then try to prove the theory wrong. I can thing of many ways especially if mis-wiring is taken into account but based on "it worked in the past and no one has touched it" then this is the most probable problem


Rectangle Font Parallel Symmetry Number
 

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I like ceiling fans & EMT
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Discussion Starter · #11 ·
When troubleshooting we look at the numbers then look for what we would need to replicate these numbers then try to prove the theory wrong. I can thing of many ways especially if mis-wiring is taken into account but based on "it worked in the past and no one has touched it" then this is the most probable problem


View attachment 166823
But then unknown load would not be working as any light bulb put into socket fails immediately, so the circuit cannot be completed.

There is nothing, that we can find, that is not working.
 

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Hackenschmidt
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Vanna said : buy a wiggy
You absolutely have to use a low impedance tester for this, a wiggy would rule out induced voltages that are non issues. But, this might be one place where a low-Z meter is even better. The T+ Pro would give you a digital readout of the voltage.

Think about it, there's something very wrong if you're not reading the voltage you should line to line or line to neutral.

You didn't mention what leg those breakers are on, I think that's the key. With open neutral problems, the L1-N and L2-N voltages will shift with the load on either side of the shared neutral, adding up to the L1-L2 voltage. If you're reading 250V at that outlet, that would happen with an open neutral if L1-L2 is 250V, and there are no loads currently on same side of the shared neutral as that outlet. The loads on the other side are just waiting for some load to complete the 250V circuit. When you turn on the light, that's what happens, the filament of the bulb is completing the 250V circuit. The voltage would drop across the combined parallel loads on the other leg, but you'd still have all that load going through that filament... the filament turns into a fuse.

Of course you have to be careful testing since you're messing with the voltage on other loads in the building.

I wonder what would happen with two bulbs in series at that outlet.
 

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Maybe I'm exhausted and am missing something obvious. I'm normally pretty confident in my troubleshooting abilities.

Got a report a specific light fixture keeps blowing light bulbs. Tried a 100w incandescent. Bright flash and it's dead.

Got out the meter. Measured 250v across the fixture. Killed one breaker at a time and found the following:

Breakers A, B, C, and D all have wires feeding the same conduit. Two neutrals, 4 hots in said conduit.

  • With breaker A on, the fixture reads 125v. Breaker A does not appear to operate any other fixtures.
  • With breaker B on, the fixture reads 85v. Breaker B operates some other lights and ceiling fans in the same section of the building. All of these lights and fans operate normally.
  • With breaker C on, the fixture reads 15v. Breaker C operates some other lights and outlets in the same section of the building. All of these lights and outlets operate normally
  • With breaker D on, the fixture reads 15v. Breaker D operates some other lights/fans/outlets in the same section of the building. All of these operate normally.
  • All four breakers off, 0v at fixture.

It's giving me neutral problem on a shared neutral, except that everything else is operating normally.
When you say breaker B is on, are the other breakers off?

I think the 15 volt readings are your meter or induced. The 85 volt on B tells me some voltage is dropping over a bad splice. With only A on, does the bulb work normally?
I would think it should dim.
 

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I like ceiling fans & EMT
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Discussion Starter · #17 ·
When you say breaker B is on, are the other breakers off?

I think the 15 volt readings are your meter or induced. The 85 volt on B tells me some voltage is dropping over a bad splice. With only A on, does the bulb work normally?
I would think it should dim.
The readings are one breaker on at a time.

I was not able to get a bulb to work with any combination of breakers. Not even a dim glow. That said, after reading this thread and rethinking, I want to try a known good bulb on the 125v setting again.
 

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I like ceiling fans & EMT
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Discussion Starter · #18 ·
Actually what I want to do is go back with a 240v bulb and see what happens with everything connected.
 

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I like ceiling fans & EMT
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Discussion Starter · #19 ·
Another thought. I have no idea how long it's been an issue. From the looks of it, the socket has been empty for years and they only recently tried a bulb in it.
 

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Another thought. I have no idea how long it's been an issue. From the looks of it, the socket has been empty for years and they only recently tried a bulb in it.
And the rest of the circuits are fine? Then I would start by looking for a mis-wired joint/open neutral close by. If it were further away something else would have issues.
 
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