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Old 01-11-2009, 09:08 AM   #1
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Unfused connectors MELTED House wires WHAT TO DO???


HI;

**I could really use some help from the experianced folks on this one, as the liceansed electrician was just lost in terms of knowing what wires are safe** Thanks in advance.

This is a true story about what happened to my dad and his house last night when working in his panel. This shows just how dangerous working around unfused service conductors is, even with your panel off. My dad is very lucky that there was no arc flash of any size. No one was injured, luckly, but it could have easily been a very different story.

My dad was trying to trace some circuits in his panel, with the main pulled. I don't know exactly how it happened, I wasn't in the basement at the time. Thank god he was using one of my klien insulated tools. He someone had a #4 awg bare wire that was connected to the water meter contact a unfused service conductor. This energized every metal part in the house. It melted through the clamp on the water meter cutting a hole in the pipe, flooding the basement.

I suspect that the circuits on other leg of the panel, not the one he made contact with became energized with 240V +/- any resistance from items on these lines.

As soon as I realized he had energized everything metal in the house, and wires were smoking and melting, I demanded he get away from the house and I would not even let him pull the meter because everything was energized. Calls to 911 and to the power co for an emergency dc at the pole were made.

It took the power co almost an house to dc at the pole and remove the meter. The entire time wires were melting and smoking. A number of the romex wires in the basement had the insulation melt and drip off the wires.

After the power co came and the fire dept cleared the house as safe, and the water got shut off at the streetWith power off at the pole ; I tested contunity between the neutral and hots. 4 or 5 of the circuits had continuity.

A liceansed electrican was called and came out. He grounded the system, to a old rod (one that was never connected to the electrical system) and rebonded to what was left of the water pipe. He did not replace the SE cable or anything else. He then had the power co restore power. I have all the breakers off, except for 2, where I put new wire in for the furnace and a utitilty outlet at the panel.

I asked the electrician, about how we can know if any of the wires in the house are safe or useable. I asked him about meggering he knew what it was but didn't have access to the equiptment to do it.

I know much is going to have to be rewired. This is a house from around 1917, very hard to fish wire in.

My question here is, If some of the branch circuits appear to look good physically, and I test the continuity on the wires with a sensetive meter, would that tell us the wire is safe to use? My concern is that the insulation may still be partially melted through, and there is a arcing hazard later on when there is a load on the branch circuit. Would Meggering actually tell us if they were safe to use?

The liceansed electrican just didn't know, and he was going to just turn on all of the branch ciruits that were not dead shorting. I was not comfortable with that at all, and I turn off everything, but the 2 circuits I did with fresh wire.

Do you think there is any chance of using any of the old wire safely? Do you think that the wires that were on the leg of the panel that got grounded out would be more likely to be ok, since they were not fed 240?

What would you guys do in this situation. It is a large 3,000sqft old house, very difficult to fish wires in. I bet this is going to be 10,000-20,000$ if the whole house needs a rewire due to the complexity of the wire fishing.

Any thoughts are greatly appriciated.

AGAIN FOR THE RECORD, AS I KEEP TELLING PEOPLE; UNFUSED SERVICE CONDUCTORS ARE VERY DANGEROUS, STAY AWAY FROM THEM IF YOU DON'T FULLY UNDERSTAND WHAT YOUR DOING!!!

Thanks guys,

Jamie

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Old 01-11-2009, 10:16 AM   #2
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Unfused connectors MELTED House wires WHAT TO DO???


I not replacing all the wires then meggering is the only test I would trust.
I would count on rewiring the entire house.
If you choose to go that way, your insurance will probably cover this repair.

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Old 01-11-2009, 10:56 AM   #3
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Unfused connectors MELTED House wires WHAT TO DO???


Only the wires with crispy insulation carried heavy current. If the current was that heavy, smaller wires that carried heavy current would already read as an open circuit.

I think that during the short, the voltage applied to the rest of your house was less than 1v.

The fusing current, and power, for a copper pipe containing water must be enormous. Maybe people on a physics website could calculate this. I think your panel can deliver 240kA so it would be some current less than this value.
#6 copper melts at 700A.

I would imagine that insulation heated this hot would read higher megohms than normal.

Last edited by Yoyizit; 01-11-2009 at 11:16 AM.
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Old 01-11-2009, 11:46 AM   #4
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Unfused connectors MELTED House wires WHAT TO DO???


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Originally Posted by Yoyizit View Post
Only the wires with crispy insulation carried heavy current. If the current was that heavy, smaller wires that carried heavy current would already read as an open circuit.

I think that during the short, the voltage applied to the rest of your house was less than 1v.

The fusing current, and power, for a copper pipe containing water must be enormous. Maybe people on a physics website could calculate this. I think your panel can deliver 240kA so it would be some current less than this value.
#6 copper melts at 700A.

I would imagine that insulation heated this hot would read higher megohms than normal.
Thanks for responding.

So what would have determined which circuits took a hit and didn't? Would it have been a matter of what devices where on the circuit and there resistance?

Thanks
Jamie
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Old 01-11-2009, 11:55 AM   #5
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Unfused connectors MELTED House wires WHAT TO DO???


What you had was a short circuit from the unfused terminal in the panel down the #4 wire to the clamp and pipe. From the latter location some current will go to ground where the pipe exits the house but if there is another path, back to the panel ground, current will also take that.

Are (ground) wires clamped to pipes anywhere else in the house? If so, those circuits would need more investigation in case large short circuit current used the plumbing system and then used their ground wires to get back to the panel.

Are the melted wires just at the panel (physical as opposed to electrical heat damage) or does the damage extend several feet?

Meanwhile simply making the neutral or ground wires hot with 240 volts because a hot wire touched the panel grounding bar it is not going to fry the branch circuits. There must be a flow of current from one place to another for example the wire in question was grounded in one place and energized in another place.

The pipe got melted at the pipe clamp because there was not all that good a connection where the #4 wire was clamped on. Good enough for maybe 20 amps of current from a short from a breaker protected branch circuit but not for what may have been 1000 amps (limited only by the pole transformer fuse) from the unfused contact in your panel.

When testing for continuity to prove the existence of a short in a branch circuit, don't forget to unplug everything from that circuit. An incandescent light bulb will test only a few ohms since the resistance of the filament is very small when the filament is not hot.

Last edited by AllanJ; 01-11-2009 at 12:38 PM.
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Old 01-11-2009, 12:30 PM   #6
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Unfused connectors MELTED House wires WHAT TO DO???


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What you had was a short circuit from the unfused terminal in the panel down the #4 wire to the clamp and pipe. From the latter location some current will go to ground but if there is another path back to the panel current will also take that.

Are (ground) wires clamped to pipes anywhere else in the house? If so, those circuits would need more investigation in case current used their ground wires to get back to the panel.

Are the melted wires just at the panel or does the damage extend several feet?

Meanwhile simply making the neutral or ground wires hot with 240 volts because a hot wire touched the panel grounding bar it is not going to fry the branch circuits.

The pipe got melted at the pipe clamp because there was not all that good a connection where the #4 wire was clamped on. Good enough for maybe 20 amps of current from a short from a breaker protected branch circuit but not for what may have been 1000 amps (limited only by the pole transformer fuse) from the unfused contact in your panel.

It would take breaking/falling of (primary) wires up on the utility poles and carrying several thousand volts and their unwanted contact with the 120/240 volt (secondary) wires serving homes before entire home electrical systems would get fried.

When testing for continuity to prove the existence of a short in a branch circuit, don't forget to unplug everything from that circuit. An incandescent light bulb will test only a few ohms since the resistance of the filament is very small when the filament is not hot.

Finally, if you just flipped on the breakers of the seemingly undamaged circuits (after repairing the panel), the worst that would happen is that a 15 amp or 20 amp breaker would trip again if there was a short in that circuit.
I was concerned that something could arc and start a fire without opening a breaker, I thought that might be even a greater concern with the knob and tube. Since the house wiring is old most is not grounded. So if it was shorting to say a metal box due to melted insulation, then the breaker would not open if that box is not grounded. Thats my concern.

There is atleast 15-20 feet of nm/ romex going across the room that is just melted, it had no direct contact with the fault in the panel, other than being connected to the neutral bar.

I am going to pull some more suff apart this afternoon and physically how far from the panel stuff has melted.


If I connect the hot and neutral on a circuit at the far end, and test from the panel, I should see a very very small ohm reading, even with a somewhat long run, correct? Would I just estimate the distance of the run and multiply it by 0.000078 per foot? and see if I get close to that reading on a ohm meter? How accurate of a test would that be?

I may have to buy a Fluke 1587 or similar.

Thanks
Jamie
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Old 01-11-2009, 12:57 PM   #7
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Unfused connectors MELTED House wires WHAT TO DO???


The only way I know of to judge the integrity of any cable is to megger it.

This test applies a high DC voltage (for house wiring, I'd use 1000 volts), and measures the current on the return. A couple of milliamps would be acceptable. The idea here is that if a circuit can stand 1000 volts and not leak more than a few milliamps, it'll stand 120 volts with much less leakage. Usually, we megger at twice the insulation rating.

I would first unplug everything in the house, remove all light bulbs, turn on all switches, and megger each circuit at the panel. Anything that loads a circuit must be disconnected. Smoke alarms, etc. First test the circuit with a basic ohm meter, if it shows open then megger it. If it doesn't read open with an ohm meter, it certainly won't pass a megger test. Megger from hot to neutral, hot to ground, and neutral to ground. Grounds do not need to be disconnected.

Any circuit that reads less than a few milliamps is considered safe, it's pretty common to read less than 1 microamp.

Rob

P.S. The only circuits that should have been affected are ones that connected to things that were grounded by other than the electrical system. If the furnace is gas and its wire is cooked, check the gas piping carefully; it carried substantial current.
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Old 01-11-2009, 01:27 PM   #8
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Unfused connectors MELTED House wires WHAT TO DO???


I'm sorry to hear about your troubles Jamie. At least now you have a real world example of what we have been telling in theory.

As others have said, megger, megger, megger! As far as the K&T circuits, it is unlikely they took any damage, because they are ungrounded and the current that did the damage was flowing on ground wires, otherwise the water pipe ground would not have caused any damage except at the water clamp.

Your old man was quite lucky he didn't get his face burned off!
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Old 01-11-2009, 02:01 PM   #9
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Unfused connectors MELTED House wires WHAT TO DO???


The actual current flow here is likely a lot less than you think.

As an example, lets assume the POCO transformer is 50 KVA. That's actually pretty good sized for a house, but somewhat normal for a bunch of houses fed from one transformer. The typical impedance for such a transformer would be around 4%. At 240 volts, 50 KVA = 208 amps. 208 amps divided by 4% = 5200 amps. This would be the current that the transformer could produce if the 240 volt side were shorted out at the terminals and the the high voltage side were maintained at rated voltage.

This 5200 amp figure is at the transformer, and doesn't account for current losses in the line from the transformer to the house.

In reality, most houses that are fed overhead, and not far from the POCO transformer will come out between 1000 and 4000 amps of short-circuit current. If the feed is underground, and the transformer is close to the house, figure 2000-5000 amps. It's extremely rare to have a single-phase service that can produce more than 10,000 amps.

It's amazing what a few thousand amps of current out of control can do though. Imagine what can happen in an industrial building with a 480 volt service capable of 30,000 amps!

Rob
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Old 01-11-2009, 02:19 PM   #10
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It's amazing what a few thousand amps of current out of control can do though. Imagine what can happen in an industrial building with a 480 volt service capable of 30,000 amps!

Rob
Oh, brother! I wish I had some pics of some of the burn downs I've seen. The one that sticks in my mind in particular was when a couple of maintenance guys ran a lift up into a 12,470 V bus. It just showered molten metal what seemed like forever. The guys weren't hurt very badly because they were getting the hell out almost the time they realized what they had done! They had some pretty good burns but the lift was toast.
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Old 01-11-2009, 02:25 PM   #11
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Unfused connectors MELTED House wires WHAT TO DO???


Quote:
If I connect the hot and neutral on a circuit at the far end, and test from the panel, I should see a very very small ohm reading, even with a somewhat long run, correct? Would I just estimate the distance of the run and multiply it by 0.000078 per foot? and see if I get close to that reading on a ohm meter? How accurate of a test would that be?
The main problem here is not the open wires. They will be easy to find. The problem is the cables with wire intact but insulation damage that arc or short in the future. That is what a megger test should find. It is a test with each wire disconnected. High voltage is applied between the wires testing for arcing and current flow.
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Old 01-11-2009, 03:40 PM   #12
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I'm sorry to hear about your troubles Jamie. At least now you have a real world example of what we have been telling in theory.

As others have said, megger, megger, megger! As far as the K&T circuits, it is unlikely they took any damage, because they are ungrounded and the current that did the damage was flowing on ground wires, otherwise the water pipe ground would not have caused any damage except at the water clamp.

Your old man was quite lucky he didn't get his face burned off!
Well, I have an update. I went over and did further investigation into what happened. The wiring in his house is sure a jumbled mess. I found that there is actually only one piece of romex that in all twirled around in the ceiling that melted. I also checked the old 4awg grounding cable that was removed and replaced to the water meter. The 4awg cable had almost 0 resistance, and it appears to be intact (it is in a mc type jacket).

After talking to my dad, and looking at where clamps were and such. I finally figured out what happened.

That 4awg wire shorted to leg of the panel, but the bond to the water meter stayed in tact, and was the path of least resistance to ground for the current (I assume it was the least, because the other choice would be to flow through the house wires, which are mainly ungrounded).

Upon closer examination once things were cleaned out, the water pipe was not actually severed as I first thought happened, It appears that the leak is not where the grounding clamp was, it was where the copper pipe jointed the lead pipe. So the current flowed through the copper 4awg to the copper pipe to the lead pipe -- lead pipe had more resistance, pipe heated up, and the joint loosened?? or caused pressure to build in the pipe and broke the connection? Possible?

As to the romex NM that was all burnt, I figured out that that cable much have been in direct contact with the ground wire to the water meter, which, must have become very very hot, which melted the romex outher sheathing, then caused a short in the cable.

Upon inspecting the box more, I highly suspect that the wires in the box are damaged from the heat that was produced by the fault.

In other areas of the house, I can find no evidance of any damage to the wires, other than in the panel and the one 20' piece of romex that is torched that was physically touching the 4awg ground wire.

I installed a utility outlet next to the panel and pluged in some extension cords. All appliances and computers tested thus far and just fine.

So, at this point, I suspect that he only damaged the cabled where they come into the panel and that one piece of romex

Do you think this sound right? Don't you think I would have found some evidance of damage elsewhere in the house or blown electronics if any amount of the current ended up flowing back into the branch circuits?

Thaks very much
Jamie
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Old 01-11-2009, 04:16 PM   #13
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Unfused connectors MELTED House wires WHAT TO DO???


Homeowners should NOT be messing around in their panels.

I am glad that Dad is OK but...... what a dumbass.

I don't want to say that you encouraged him because of your new interest and experience with electrical but....

Again, I'm glad he's OK. Just have him call/pay someone to take care of his electrical. It aint rocket science but you NEED to know what you are dealing with.

I think that your assesment of the damage was probably correct. Tell Dad to do whatever it is that he does so he can be here to enjoy his grandkids.
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Old 01-11-2009, 04:47 PM   #14
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Quote:
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The typical impedance for such a transformer would be around 4%. At 240 volts, 50 KVA = 208 amps. 208 amps divided by 4% = 5200 amps.
A book by Ray C. Mullin on resi. wiring puts the impedance at the panel at 1 milliohm. Maybe this was a typo.
I guess I'll measure it sometime.

If you have an AM radio in the middle of the house tuned between stations with the volume turned up, you have a whole-house arc detector. You could test it by slowly pulling out the plug of an incand. lamp that is on. Do this just once.
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Old 01-11-2009, 05:20 PM   #15
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Unfused connectors MELTED House wires WHAT TO DO???


Those nasty service conductors...always watch what your doing!

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