Crazy sub panel voltage Resolved

[710ltd2002]

I did some continuity testing on the faulty circuit today and traced the problem to a receptacle that shop lights plug into. Once I pulled the recep out of the gang box, the fault went away.

It appears the problem was under the strain relief. When I installed it, I crushed the black and white wires together so tightly that it split the insulation. The photos show the tiny hole in the black insulation, and you can see the crush mark on the romex white casing. (The wires were all enclosed up to that point. I cut the casing open so that I could inspect the wire that had been under the clamp.)

After the repair, I checked the box and circuitry. No more faults. Lesson learned: wire is more delicate than I think, and I don’t need to go he-man tightening the strain reliefs.

Thanks again to everyone for the help.
Dave

 

[Ted in NE-OH]

Why did it not pop the circuit breaker? There still might be a problem somewhere.

 

[Janicholson]

Why did it not pop the circuit breaker? There still might be a problem somewhere.

I agree. A breaker should have tripped with a dead short! Possible still failed neutral! Jim

 

[710ltd2002]

Why did it not pop the circuit breaker? There still might be a problem somewhere.

I can’t answer that. Maybe because it was such a tiny short it wouldn’t allow enough amperage to flow? There was no sign of melting on the black insulation nor was the paper twist around the ground wire charred.

I am confident that was the only fault in the circuit. I began before the light switches with the ohmmeter looking for hot to ground continuity. Nothing until that one switch was closed. The wire ran from the switch to a junction where it splits to feed two runs. I disconnected those nuts and checked each wire individually. The hot to ground short was in the 20 feet or so of romex than ran to a single receptacle. Once I loosened the strain relief and pulled the recep out, the short disappeared. After the repair, I closed every active breaker in the panel and checked the two hots and neutral. All read as they should.

Thanks for the discussion on this. I only know so much, and I certainly want to learn more.
Dave

 

[710ltd2002]

I can’t answer that. Maybe because it was such a tiny short it wouldn’t allow enough amperage to flow? There was no sign of melting on the black insulation nor was the paper twist around the ground wire charred.

I am confident that was the only fault in the circuit. I began before the light switches with the ohmmeter looking for hot to ground continuity. Nothing until that one switch was closed. The wire ran from the switch to a junction where it splits to feed two runs. I disconnected those nuts and checked each wire individually. The hot to ground short was in the 20 feet or so of romex than ran to a single receptacle. Once I loosened the strain relief and pulled the recep out, the short disappeared. After the repair, I closed every active breaker in the panel and checked the two hots and neutral. All read as they should.

Thanks for the discussion on this. I only know so much, and I certainly want to learn more.
Dave

Also remember I was getting only a tingle from the light switch box, not a jolt.

I’m interested to hear all theories.

 

[Ted in NE-OH]

I can’t answer that. Maybe because it was such a tiny short it wouldn’t allow enough amperage to flow? There was no sign of melting on the black insulation nor was the paper twist around the ground wire charred.

I am confident that was the only fault in the circuit. I began before the light switches with the ohmmeter looking for hot to ground continuity. Nothing until that one switch was closed. The wire ran from the switch to a junction where it splits to feed two runs. I disconnected those nuts and checked each wire individually. The hot to ground short was in the 20 feet or so of romex than ran to a single receptacle. Once I loosened the strain relief and pulled the recep out, the short disappeared. After the repair, I closed every active breaker in the panel and checked the two hots and neutral. All read as they should.

Thanks for the discussion on this. I only know so much, and I certainly want to learn more.
Dave

I think you have an open neutral feeding the sub-panel

 

[710ltd2002]

I think you have an open neutral feeding the sub-panel

How do I check for that? Pull each neutral off the bar individually and test for voltage to ground?

Dave

 

[MarkB_MI]

Thanks for the update. I agree with those who say you still have a problem. I don't think your ground is properly bonded to neutral.

 

[chuckthin]

After you fix short try plugging in a high load like a 1500 watt room heater or something
and see if you have a voltage drop.

 

[Stan Coryell]

I can’t answer that. Maybe because it was such a tiny short it wouldn’t allow enough amperage to flow? There was no sign of melting on the black insulation nor was the paper twist around the ground wire charred.

I am confident that was the only fault in the circuit. I began before the light switches with the ohmmeter looking for hot to ground continuity. Nothing until that one switch was closed. The wire ran from the switch to a junction where it splits to feed two runs. I disconnected those nuts and checked each wire individually. The hot to ground short was in the 20 feet or so of romex than ran to a single receptacle. Once I loosened the strain relief and pulled the recep out, the short disappeared. After the repair, I closed every active breaker in the panel and checked the two hots and neutral. All read as they should.

Thanks for the discussion on this. I only know so much, and I certainly want to learn more.
Dave

The "ground" wire in your panel is not connected to the source (the transformer at the street) and you have zero ground fault or phase to ground over current protection. Remember, the ground rod is there for lighting protection. As you found out this can be extremely dangerous.

You have three options:

Add a ground wire between the sub panel and the main panel to satisfy the ground and install GFCI breakers in the sub panel. (This is closer to code compliance, but may not completely satisfy code)

Just install GFCI breakers in the sub panel. (This will give you ground fault protection)

Tie the ground to the neutral bus in the sub panel. If these were tied together, your ground fault would have caused the breaker to trip, preventing you from getting shocked or preventing a fire. THIS IS A CODE VIOLATION. It would be unsafe if the neutral between the sub panel and the main panel is lost. You would end up with EXACTLY the same condition as you just had. Keep in mind, electric clothes dryers, sub panels, and all generators have/ are wired this way. This is probably how I would fix it in my barn.

 

[kencombs]

Also remember I was getting only a tingle from the light switch box, not a jolt.

I’m interested to hear all theories.

Maybe the tingle was because black was shorted to the bare ground and the only path to earth was the ground rod whose impedance is too high to carry more current. If it were mine I'd use GFCI recepts in that building until the feed could be updated to 4 wire.

 

[Janicholson]

After you fix short try plugging in a high load like a 1500 watt room heater or something
and see if you have a voltage drop.

Correct! A faulty neutral will not allow a load to operate. I believe it is still broken, or the neutral would never have shown voltage. A common error in electrical circuit analysis is that a loaded circuit reacts to measurement vastly differently than an unloaded one. Todays meters have such low impedance that they can fool the user. Jim

 

[drsportster]

I did some continuity testing on the faulty circuit today and traced the problem to a receptacle that shop lights plug into. Once I pulled the recep out of the gang box, the fault went away.

It appears the problem was under the strain relief. When I installed it, I crushed the black and white wires together so tightly that it split the insulation. The photos show the tiny hole in the black insulation, and you can see the crush mark on the romex white casing. (The wires were all enclosed up to that point. I cut the casing open so that I could inspect the wire that had been under the clamp.)

After the repair, I checked the box and circuitry. No more faults. Lesson learned: wire is more delicate than I think, and I don’t need to go he-man tightening the strain reliefs.

Thanks again to everyone for the help.
Dave

Takes a helper about six months to learn to properly fold wires into a box .

 

[drsportster]

A meter tends toc ause confusion which most electricians overcome by only using a Wiggy. Keeps it simple 120/240 etc Hot or Not

 

[buickanddeere]

As previously stated . An open neutral back to the main panel . Lousy ineffective ground at pony panel .

 

[John T]

Still could still be a Neutral problem or a poor Neutral Ground Bond HOWEVER if alls well now congratulations.

John T

 

[miangus]

The "ground" wire in your panel is not connected to the source (the transformer at the street) and you have zero ground fault or phase to ground over current protection. Remember, the ground rod is there for lighting protection. As you found out this can be extremely dangerous.

You have three options:

Add a ground wire between the sub panel and the main panel to satisfy the ground and install GFCI breakers in the sub panel. (This is closer to code compliance, but may not completely satisfy code)

Just install GFCI breakers in the sub panel. (This will give you ground fault protection)

Tie the ground to the neutral bus in the sub panel. If these were tied together, your ground fault would have caused the breaker to trip, preventing you from getting shocked or preventing a fire. THIS IS A CODE VIOLATION. It would be unsafe if the neutral between the sub panel and the main panel is lost. You would end up with EXACTLY the same condition as you just had. Keep in mind, electric clothes dryers, sub panels, and all generators have/ are wired this way. This is probably how I would fix it in my barn.

Agree, you are depending on the ground rod for your safety ground and there is too much resistance for it to provide enough current flow to trip the breaker. The sub-panel may have been live due to the short you found.

 

[miangus]

Correct! A faulty neutral will not allow a load to operate. I believe it is still broken, or the neutral would never have shown voltage. A common error in electrical circuit analysis is that a loaded circuit reacts to measurement vastly differently than an unloaded one. Todays meters have such low impedance that they can fool the user. Jim

Agree, I spent a week tracing some overhead issues in the tri-plex to my barn a few years ago. The readings were as expected until loaded then the reading exposed a high resistance connection in the neutral. The meter read 120-volts line to neutral on both legs , loaded with a 150 watt light bulb it went to 90 volts on one leg and 130 volts on the other leg. The circuit was feed from the house and had service entrance cable in the house and up the riser. I think it may have taken a lightening hit but somehow most of the stranded aluminum wrapping wire which carries the neutral in the service entrance cable were melted in-two leaving a small number to carry current and create the high resistance neutral.

 

[buickanddeere]

Thanks for the update. I agree with those who say you still have a problem. I don't think your ground is properly bonded to neutral.

Neutral properly bonded to ground where ?
The ground system is to have ZERO to do with the load current carried on the neutral system.
A better ground is not a solution for an open neutral between the supply and the pony panel .

 

[JMOR]

does not sound "resolved" to me