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Electrical short caused a fire here

Posted by baymee (My Page) on
Tue, Jul 12, 11 at 16:27

One of my old small outbuildings burned down. Fire company says it was from welded wires and they showed them to me. When they shut off the breaker in the house, it arced in both directions; it never tripped from the welded wires during the fire.

I unhooked the wire that went from a barn, (electrically located between my house panel and the outbuilding) to the outbuilding, at the barn's junction box and tried to turn on the power again. Same thing; arcing in both directions of the breaker.

So, I must have a short somewhere between the house and the barn, underground.

Could a short in the underground or barn wiring cause wires to burn beyond the short, such as the outbuilding, which is at the end of the run?


Follow-Up Postings:

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RE: Electrical short caused a fire here

No. One test is to cut the supply conductors right at the burned building, be sure the stubs do not touch anything and test with ohmmeter. Substituting another breaker is also an option.


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RE: Electrical short caused a fire here

Arcing and shorting are 2 very different things.

A short is when the live has a pathway to ground with virtually no resistance. The current spikes and it will trip the breaker.

Arcing is when electricity flows across a normally non-conductive medium - like air. That is the kind of spark that can cause a fire if it isn't contained. It will not necessarily trip a breaker, but it is still quite dangerous.


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RE: Electrical short caused a fire here

bus driver, I did both. I didn't do the ohms test, but I cut the conductors at the building and it still trips the breaker. I also substituted another breaker. Same thing.

Moving the breaker switch from off to on creates an instant short and trips the breaker. Moving the lever from tripped to off also causes an electrical arc at the busbar that can be seen and heard. Tried two breakers and the arc is the same.

So, something is wrong between the house and the barn that never tripped the breaker, but caused wires to burn at the end of the line in the outbuilding.


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RE: Electrical short caused a fire here

Perhaps the first short circuit was in the building and the faulty breaker did not trip. The resulting overheating in the entire circuit caused two conductors to come in contact in the supply to the building. That is one possible explanation. A short circuit in the supply would reduce the available current at the building, reducing the heating of the conductors at that point. If my theory has merit, the short in the supply is an effect, not a cause.


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RE: Electrical short caused a fire here

Likely just as much of a chance of another fire source damaging the insulation on the wires resulting in them shorting as that being the initiating event.

The feed lines to the building may also have been damaged by heat from the fire carried into the conduit (or even a large vehicle like a fire truck driving over a direct burial cable).

If the electrical short initiated things the breaker should have tripped.
An arc will not produce an over current condition necessarily, but a solid enough arc to weld wires together should have tripped the breaker.


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RE: Electrical short caused a fire here

I don't know, but all I know is that I use the barn every day, so I know the electric was fine before I went away. Come home and the building is gone and I have a dead short somewhere between the house and the barn.

Confusing for me.


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RE: Electrical short caused a fire here

My sister had the same conditions from her house to the garage.

GFI breaker in box in house kept tripping. (at one point in time an outdoor box was used necessitating a GFI. A splice by an electrician removed the receptacle box)

Anyway, I disconnected the wires feeding the garage but the breaker kept tripping. I disconnected the wires from the box at the house and the breaker finally did not trip. I measured the resistance between the two wires and found about 10k ohm. I figured the electrician's splice found some moisture.

I replaced the GFI with a standard breaker. Power was restored to garage. My sister called me the next day and said garden tomatoes were electrified and knocked her to ground. I brought over a Simpson 260 and found 85volts on tomato cage. The tomato cage when pushed into the earth cut through unprotected underground cable. The GFI tripped and saved her the first time it happened.

I would check for any new additions of fence or metal stakes between barn and house.


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RE: Electrical short caused a fire here

If you put your UF down at 18" this would be less of a problem. I've never sunk tomato cages ore than a few inches.


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RE: Electrical short caused a fire here

Or just use rigid or intermediate metallic conduit buried 6 inches.

You are not going to break them without very heavy equipment (think a backhoe).


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RE: Electrical short caused a fire here

The underground wire has been in since 1977 when I built this house. It goes under the stone driveway to the barn, unprotected. Maybe the wire is now compromised, but I can't think of any reason why the barn, at the end of the feed, would be affected.

Brickeyee, how does emt hold up 6" under the ground? I could run pvc through the grassy areas and switch to metal under the road. I really like having power at the barn.


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RE: Electrical short caused a fire here

Thanks for the tips. I didn't originally install the cable. I was going to reinstall the cable. I went to a local rent-a-center and they no longer rent trenchers as the ground here in NE PA is mostly almost all rock 6 inches down due to glaciers and I do not want to dig 300 feet of trench by hand.


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RE: Electrical short caused a fire here

If this is a single GFCI-protected branch circuit of 20A or less you can put it at 12" (direct buried). Otherwise it needs to be at 24" if direct buried.

As brick says Rigid or Intermediate can go at 6"
Nonmetallic conduit (and EMT if it suitably listed for underground installation) can go at 18"

You can knock 6" off those (other than the Rigid / Intermediate) by putting it under 2" of concrete.


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RE: Electrical short caused a fire here

I guess that is one way to keep the varmints out of the garden ;-)


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RE: Electrical short caused a fire here

"Brickeyee, how does emt hold up 6" under the ground?"

Not deep enough and not allowed.


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RE: Electrical short caused a fire here

$15.28 for 1/2" X 10' Rigid conduit at the Despot. Is that in the ballpark for pricing?

Can't find anything about Intermediate conduit.

And you said 6" is OK for that. I don't think the existing UF is even 6" down and it's been there for 35 years.

I'll have to call the digging number. The underground utilities cross my path, but if I remember right, they were down about 24" when I laid them.


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RE: Electrical short caused a fire here

Rigid conduit is 6 inches of cover.


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RE: Electrical short caused a fire here

Thanks. I guess I'll go that way.


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RE: Electrical short caused a fire here

It is a decent trade off in labor to dig the trench.

A 7 inch deep trench (to make sure you have 6 inches of cover) is bot even a full shovel blade deep.

If you remove the sod carefully it cab be dropped right back into the hole and watered in.


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RE: Electrical short caused a fire here

Careful with the conduit size. If this barn is a few hundred feet from the house, even 1 20 amp circuit could be a very difficult pull. Especially if the wire is over sized for voltage drop. If this a 50 foot run with 1 or a multi-wire 20 amp circuit, 1/2" may be all is needed but no room for upgrading the size later.


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RE: Electrical short caused a fire here

It's about a 100' run to the barn. I don't have any heavy loads and it's worked fine since 1977.

By the way, I found the feed to the burned shed. It's on the opposite side of where I thought it was, so the reason I couldn't reset the breaker is because I cut the wires beyond the welded wires.

Forget about using a shovel in my soil. It's very rocky (small rocks) and almost as hard as concrete. I have to trench.

Why the different trench depths for various conduits? A 6" deep PVC pipe in the lawn will never see any weight. A 6" PVC pipe under the road will see heavy weight.


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RE: Electrical short caused a fire here

"A 6" deep PVC pipe in the lawn will never see any weight."

Until a tractor is driven over the line.


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RE: Electrical short caused a fire here

Is grey PVC OK above ground instead of EMT? Do rodents chew on that?

Also, how well does intermediate rigid conduit hold up at the 6" level in moist soil? Can I assume that THHN wire is used?


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RE: Electrical short caused a fire here

PVC installed above ground has to be rated for sunlight exposure.

THHN is illegal underground (unless it is also dual rated THWN which much of it is).

You need to be careful with IMC that your soil is not too corrosive and that any fittings/screws are approved for such use.


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RE: Electrical short caused a fire here

"Also, how well does intermediate rigid conduit hold up at the 6" level in moist soil? Can I assume that THHN wire is used?

Very well.

It is galvanized steel pipe.

THWN must be used in in ALL buried conduit.
It is a wet location by definition.

Every underground conduit IS going to fill with water, if only form condensation in the conduit.


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RE: Electrical short caused a fire here

It looks like I'm in pretty good shape. The description on the spool of wire says MTW or THWN or THHN or AWM. The PVC conduit says sunlight resistant.

I might have to use a Rigid pipe compression coupling because I won't be able to thread the last section of pipe together. Sound good?

Is any sealer used on the threaded couplers?


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