Today had a metal box come in contact with the tub of a panel. The boxes sparked when they touched each other. This panel was a temp panel that was just lying on the floor. The temp panel was a three wire panel and had the neutral bonded to the tub. In the other metal box was a oulet with the ground bonded to the box. Was the neutral bonding the cause of this.
metal contact-sparks
- Thread starter fireryan
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fireryan said:
I'm not familular with the "TUB" term? but curious, was the floor bare concrete? If so, a slab can have a lower resistance, depending on humidity, than the traditional ground rod at 25 ohm....The amount of surface area a slab is touching the ground is huge compared to a skinny 5/8x8 rod...at least that's what I've read...But one thing is certain there is a resistance difference in your bonding and/or grounding somewhere for sure..
dbuckley
Senior Member
- Location
- Canterbury, New Zealand
Yes, voltage drop across the neutral, when the boxes tocuh they create a parallel path.
I had a similar big surprise many years ago when my screwdriver on the neutral terminal in a large switch touched the metal box and sparks appeared, took me a while with a meter until I believed it!
I had a similar big surprise many years ago when my screwdriver on the neutral terminal in a large switch touched the metal box and sparks appeared, took me a while with a meter until I believed it!
another thought..I'd also check the integrity of that neutral connections back to the source, and varify that there is only one bonding point...........hope there is no ground fault current going on??? Need to get that neutral path resistance down....something loose? parallel bonding?
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LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
- Occupation
- Electrical Contractor
From what you describe, not if the receptacle was being supplied by that panel. But yes if it was not. (This all presumes no improper wiring or loose connections.)fireryan said:
There should be no potential difference between the neutral and the enclosure of the panel with the 3-wire feed and a bonding jumper, whether the voltage to earth is zero or not.
If the other receptacle is supplied from a different cable/cord, it's EGC should be at zero volts to earth. That means the panel enclosure, EGC, and neutral must not be at zero volts to earth.
So, yes, the neutral's voltage drop must be the cause of the voltage difference, and this epitomizes why neutrals should be isolated from ground past the main service disconnect.
Was that long-winded enough?
LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
- Occupation
- Electrical Contractor
Many moons ago, when I was a helper, we wired in a 480-208/120 transformer for fluorescent lighting in a warehouse. (Now that I think about it, why not use the higher voltage? Maybe it was Delta? Maybe we stepped up to 277/480? Who remembers?)dbuckley said:
Anyway, the guy I was working with hadn't bonded the neutral, and we also noticed the voltage between the neutral and the can: when we grounded it, one of the lights went out. I immediately realized that a ballast wire was pinched and making contact with the housing.
My reward was to be the one to climb up a ladder to check my theory. (We'd already disassembled the scaffolding.) I was right and pointed ou that we're supposed to bond the neutral to the EGC/electrode system. He called the boss and we put in the right stuff.
He gave me a bit more respect after that one, and others. He was an example of an electrician referred to as a mechanic. He knew hardware, but little theory. He believed 'charging current' caused a meter to spin for a moment when he plugged it in. I said it's a short. I was right.
I do remember where that building is, and I might get around to going there. I also remember how much of a pain it was to get the hood of the weatherhead on, with something like 350 al, and the conduit came out of the wall horizontally. They should make 45? weatherheads.
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