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gbwillner 01-07-2011 05:52 PM

What caused this failure?
 
Background: I am finishing my basement (house built 1907). I am slowly rewiring everything, since most existing circuits are not grounded. The electrical box is upgraded to to 200A. The first floor outlets were on 3 different circuits (all ungrounded) that I replaced (grounded), and combined into one circuit- 20A parallel with 3 series circuits coming off one main branch point. These are running between the 1st floor joists. There was an additional ungrounded circuit for the kitchen that I combined to this circuit, but I could not properly ground it without tearing up the wall (at some point in the past the wall was furred and romex was installed, but the ground was cut. That cable then fed a kitchen light and several small applicance outlets.). At the time of the incident, I was running a plasma screen tv, a playstation 3, a few small a/c adapters (Roomba) etc. AND AN ELECTRIC HEATER (1200W).

Problem: So I was working in the basement. I planned to eventually set up the first floor outlet circuit entirely in series, but won't do that until the basement walls are built. Anyway, I notced that there was a burned piece of plastic in the ground. It was a twist-cap for splicing wires. I looked up and saw the plastic box containing the main branch point listed above was slightly black. I could see that the splice point of the neutral wires was glowing orange and the cap was missing (obviously it was what was on the ground). The hot and ground wires were unaffected. The neutral wires' rubber coating was melted about 5" for each branch up to the point where the wires entered the jacketed cable.
I quickly shut off the power, unplugged the heater and removed the brach leading to the ungrounded kitchen outlets, and turned the power back on. I was glad my house hadn't burned out.

So, what caused the failure? If I drew too many A from the box, why did the breaker not trip? Why was the neutral connection so physically hot that it melted the plastic twist cap?

Please help so that I may prevent this in the future.

THanks!

Saturday Cowboy 01-07-2011 06:00 PM

where are you located?? In the states there is NO series circuitry.

Scuba_Dave 01-07-2011 06:00 PM

Is it a MWBC - multi wire branch circuit ?

darren 01-07-2011 06:00 PM

Is this a multi wire branch circuit(2 hots and a neutral). If so make sure your hots are on two different phases, if the hots are on the same phase the neutral will carry the combine amps of both hots and will heat up the wire. You could check the neutral in the panel as well, if it melted at the joint and it may have melted in the panel as well.

The second option is that the joint was loose and that caused it to heat up. Remake the joint and make sure it twisted tight(yes you should twist the wires before the wire cap, although the box says other wise) before putting the wire on.

AllanJ 01-07-2011 06:19 PM

Do any of the sheathed cables have three insulated wires in them as opposed to just two?

The cables where the heated wire melted the insulation are in a somewhat unknown condition. If the insulation resolidified with the wire almost sticking through then there could be a short circuit. The short circuit could happen at a later date if the wire did not stick through completely now but breaks through a thin spot later. Ideally you need to snip off the section that melted and strip back the sheathing and stretch the cable so fresh wires is brought into the junction box. Sometimes the cable is now to short to stretch out.

If you determine that the heated wires are okay now, you need to disassemble them, scrape the ends clean and shiny, and then twist them back together and add the wire nut.

gbwillner 01-07-2011 07:12 PM

Quote:

Originally Posted by AllanJ (Post 564997)
Do any of the sheathed cables have three insulated wires in them as opposed to just two?

The cables where the heated wire melted the insulation are in a somewhat unknown condition. If the insulation resolidified with the wire almost sticking through then there could be a short circuit. The short circuit could happen at a later date if the wire did not stick through completely now but breaks through a thin spot later. Ideally you need to snip off the section that melted and strip back the sheathing and stretch the cable so fresh wires is brought into the junction box. Sometimes the cable is now to short to stretch out.

If you determine that the heated wires are okay now, you need to disassemble them, scrape the ends clean and shiny, and then twist them back together and add the wire nut.


Yeah I did what you said initially- I cut off the neutral wire until there was insulation and I scraped off the goo. I sounds like it may be a short since you pretty well describe what it looked like. Not sure how it happened since I try not to leave any exposed wire beyond the twist cap. Still not sure why it happened. It seems like the wire is OK now... and it's funny because none of the other splice sites were affected, only where the three branches come off the main. If it had something to do with the heater, the splice sites on those recepticles were fine. same with the kitchen.

As far as what others have said... sorry if I was confusing. There is only one circuit, and the RECEPTICLES are a mix of series and parallel. It was previously multiple circuits that I condensed since I don't need a 15A circuit to feed a single recepticle in the living room.

gbwillner 01-07-2011 07:17 PM

Quote:

Originally Posted by Scuba_Dave (Post 564984)
Is it a MWBC - multi wire branch circuit ?

No - sorry about the confusion...

gbwillner 01-07-2011 07:20 PM

Also, just to clarify, the power never went out as this happened, and all appliances were working fine. The neutral just got so hot the cap and the insulation melted.
As of now, I won't plug in the heater, or reconnect the kitchen group of ungrounded recepticles, until I think it's safe.

jlmran 01-07-2011 07:50 PM

You are still using the term "series", perhaps incorrectly. What do you mean when you say the recepts are a mix?

gbwillner 01-07-2011 08:20 PM

Quote:

Originally Posted by jlmran (Post 565114)
You are still using the term "series", perhaps incorrectly. What do you mean when you say the recepts are a mix?


One circuit branches into three. By that, the recepticles on those branches are in parallel. Within each branch, there are several recepticles, these are in series. Again, sorry for the confusion....

I just got home and inspected the wires. It looks like now there is oxidation on the copper of the ground wires (not insulated).
I remember that last week the P-trap on the drain on the second floor came apart leaving a big mess. There is a radiator next to the sink, and one of the pipes going down to the basement runs close to this junction.

I wonder if it's possible that water ran down this pipe and somhow got on the wires, causing the short. I never found any water in the basement, but if it was a slow trickle... The oxidation on the copper is weird. I also see it on the disconnected neutral wire (that leads to the kitchen appliances)......

mystery solved?????

darren 01-07-2011 10:20 PM

Even though you broke it into three different paths all your plugs are still wired in parellel.

As long as all the blacks are together and go to brass screws only and all the whites are together to the silver screw you have wired it in parellel.


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