|Posted by Eric Self on March 8, 2016 at 3:40 PM|
Of course these things seem to always happen on Sunday. Why is that? Maybe for the same reason your smoke detector wants to false alarm in the middle of the night, not the middle of the day. Murphy's Law.
We got a call from Vince at the local gun range. On a Sunday. It seems that one of the guys leaned against a metal building on the property and was nearly electrocuted. Or shocked. It depended on which story you heard and how far down the line it had travelled. Anyway, by the time the story got to Vince, and he and a few guys went to check it out, they found a bullseye-shaped hole the size of a quarter burning in the side of the service equipment. He called me. I said shut the breakers off. He said he HAD!
Did I say I was in Old Forge for all of this? So we got a hold of Adrian and set him in motion to the club, and got Central Hudson in motion too just in case. Vince shutting down the Main Breakers hadn't stopped much. We had to pull the meter. Adrian hit the shop on his way long enough to grab an Arc Flash Suit. Pulling the meter took care of the power problem, and he had found the problem before the line crew showed up. With the short repaired and the meter back in, we were able to get the service back up and running on the same day. Later in the week we could bat cleanup on the cause.
Well, what happened? And why didn't the breaker trip when the wire started shorting?
Good question. Gets us back to the good old Grounding and Bonding discussions in trade class. Here's what happened. The 400 amp main feeders were tapped coming out of the bottom of the Meter Pan, and split into two separate 200A panels. Picture it this way: three panels in line above a gutter or trough that connects all of them from underneath. The service feeds drop down off the roof in conduit, go into the meter, down out of the meter and into the gutter underneath. They travel the length of the gutter, and smaller service wires are spliced onto the main feeder wires that go up into the two 200A panels above the gutter. These service taps then hit main breakers. Make sense? I'll post a picture...
Anyway, the main feeders down in the gutter dead end into the side of the gutter instead of stopping a little short of the end. And these wire ends were secured with electrical tape. Maybe one or two wraps, pretty thin. Either way, the copper wires were actually touching the metal gutter through the tape and energizing it. And since the building was metal, and the gutter was screwed to the building, it was energizing the whole metal building too. That's how the guy got hit when he leaned against the building.
Why didn't anything trip? Well that was kind of a mess. First of all, the gutter wasn't grounded. At first look, the fittings were all metal, the gutter was metal, everything was metal. You would think there was a mechanical ground if nothing else, but the installers had used plastic sealed metal lock rings. Great for keeping the water out but it in effect isolated the gutter from the service. When the wire shorted to the end of the gutter, the only way to get to the service ground was back through the metal siding on the building. It was faulting to ground but arcing to get there. Kinda makes you understand why Building Codes are moving the direction of combination arc fault/ground fault breakers. In this case, there was no breaker yet, only the fuse on the transformer at the road. Don't you like to think that would have tripped eventually?
Fixing this mess wasn't terrible. We had to re-work the service grounds to properly ground the gutter. And we pulled a new wire to bond all the metal parts together, from the Meter to the Main Panels The hard part was getting the bonding bushing onto the EMT fitting after all the wires had been pulled. But we found these cool bushings that could be retrofitted. Weren't cheap but saved us a lot of time and money. They clamped around the threaded ends of the connectors and then screwed down tight as any standard looking bushing.
And as far as that burnt out bulls-eye on the gutter, we painted it gray again. Didn't want anybody to get any ideas down at the range...