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Electrical Contractor
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Discussion Starter #1
I was at my mobile home today and I was taking a dryer feed 10-3 into my new panel to temporarily feed my old sub panel and when pushing the wires through the romex connector it sparked with the grounds and hot wires touching the connector. It spooked me so I touched them again to see and it would keep happening.

All breakers were off and sub panel was disconnected from the main. And power is turned off at the meter.

Sub panel is grounded to water main and mobile home frame. I have not grounded the new sub panel to the water main and mobile home frame yet.

What could be doing this? I am really confused. Nothing there for voltage that I can feel. Could there be a small amount of voltage in the neutral? I did not have a meter with me will bring it tomorrow.
 

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Nothing there for voltage that I can feel.
Then it's probably it's less than 50v and the current through your fingers or body was less than 1 mA so your skin/body resistance was a max of 50k. Your hands must have been dry and not sweating.

The sparking seems to mean substantial current into the short circuit.

Tracking this one down should be fun. You'll probably need a clamp-on ammeter.
 

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:no:...ammeters only work when there is a load on the wire....an open ended wire doesn't have a load.
Yeah, he shorts the wires like he did before and sees how much current is delivered.

Knowing the open circuit voltage and the short circuit current he can figure the source resistance
http://en.wikipedia.org/wiki/Thévenin's_theorem
which then maybe gives him a clue to the source.

He can also use a toaster (~12 ohms) as a load instead of shorting the wires but then he needs some math to find the Thevenin equiv.

Voltage is half the story.
 

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Yeah, he shorts the wires like he did before and sees how much current is deliveredhttp://

:censored:absolutely not! You don't short wires to see what happens. Apparently you have a great deal of knowledge with electronics and device circuitry....but some of your advice is careless when working with AC house voltage and current. Very irresponsible advice.:no:
 

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:censored:absolutely not! You don't short wires to see what happens. Apparently you have a great deal of knowledge with electronics and device circuitry....but some of your advice is careless when working with AC house voltage and current. Very irresponsible advice.:no:
You're right, I don't short wires to see what happens.

However, the OP has already shorted wires to see what happens and has already put his hands on it, so we have some info about how dangerous this is or is not, and also how gutsy the OP is. That ship has sailed.

It's less than 120v. . .right?
And since the wires touching did not hurl small globs of molten copper through the air the current can't be that high - or some CB somewhere has opened.

This is also a good example of conditional probability as in
http://en.wikipedia.org/wiki/Bayes'_theorem
Before the OP did anything we had no idea what kind of voltage or current that wire was capable of delivering. Now we have a pretty good idea even without using instruments.

Decision Theory says to look ahead and reason backwards, in order to decide a wise course of action. By now some of you should at least anticipate what might my replies be to your statements.

I have an early day tomorrow - may I log off now? :)
 

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" Euro " electrician
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Yoyizit .,

I will advise ya real quick whatever you say s.v.p. .,,, jamais ever give anyone a advice which you feel not safe at all.

And I will never advise anyone to deberated short them out. Just say ., that was uncalled for that.

How many short circuit you dealt with it so far ?? if you understand it then just don't do it.

I just got done fixing one of my customer house not too long ago today a short circuit cause 4 splices fail right there that why I do not want anyone try to do a direct short circuit that is outright dangerous.

Merci,
Marc
 

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Electrical Contractor
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Discussion Starter #10
I think it was the ground from the old panel sparking when touching the neutral or ground bar. I did not spend any time checking into it because I did not have my meter.

I was kinda thinking a little bit of power coming from the neutral in the main service caused a spark when touching the ground going to the to the water main and mobile home frame.

First plan is going to be disconnect the ground going to the old panel and test for voltage between that ground and the neutral and ground in the new panel.
 

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The neutral wouldn't spark if connected properly. It looks like you have the right idea, checking it with a meter. Please exercise caution around the panel as well. Can you verify where the other end of the wire terminates?
 

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Discussion Starter #12
New sub panel is fed by the feed through lugs in my main service main breaker off power disconnected at meter. I had a meter in there before doing work to make sure there was not voltage there. Old sub panel that I decided to feed with with the dryer 10-3 feed is the current breaker panel for the home all breakers off. It does not have any other wire feeding it other than the 10-3 that I ran to the new panel today.

I get the 10-3 start to put it through the romex connector. I see sparks pull wire out. I am not talking arching and and flashing with metal flying all over. It was kinda like when you see static electricity in the dark kinda spark. I decided to put the wire though sparks again. Finish up wiring and go home.
 

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Discussion Starter #14
Yeah the only possible place for voltage to come through would be the neutral because it's the only thing connected to anything outside the home. I hope to be getting over there soon today.
 

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There is an easy way to check the neutral integrity.

By switching a 10A, 120v load on and off, the voltage change measured at the panel between neutral and the hot should be less than 0.5v and probably more like 0.2 v. Not all DVMs can see this change out of 120v.

If you have an elec. dryer that's not running the dryer plug will show the voltage at the panel without you having to remove the panel cover.
 

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Discussion Starter #16
I was over there with my meter and the most I could get with my meter is .06 between the old sub panel ground and the ground the ground bar in the new sub panel that is fed from main service connected to utility neutral and two ground rods. Checking between anything else resulted in no voltage at all.
 

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I was over there with my meter and the most I could get with my meter is .06 between the old sub panel ground and the ground the ground bar in the new sub panel that is fed from main service connected to utility neutral and two ground rods. Checking between anything else resulted in no voltage at all.
So that's why you didn't get a shock.

With 0.02 ohms conductor resistance you could have .06v/.02ohm = 3A flowing but .06v seems too low to spark in any case.

\/

"For air at STP, the minimum sparkover voltage is around 327 volts, as noted by Friedrich Paschen. [3]
While lower voltages will not, in general, jump a gap that is present before the voltage is applied, interrupting an existing current flow often produces a low-voltage spark or arc. "
 
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