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Old 11-15-2008, 12:59 AM   #1
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A story of a high amperage short


I was going to make this a reply in Jamie's thread, but I think that would derail it too far off topic. So here it is.

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I wondered what happens if you cause a fault on your main prior to your breaker
You get a free suntan.

Here's a story for you. At my previous place of work, a 100 ton A/C unit on the roof shorted out. Its 400 amp breaker failed to trip. The 1500 amp main breaker on the subpanel it was on also failed to trip.

Upstream from that were three fuses in the service equipment room, 1500 amps each, one on each phase. When the A/C shorted out, it blew one of these phase fuses.

That 1500 amp branch had a lot of critical plant functions on it, including the computer room. Since the computer room was dark, once everything was secured and powered down, I started following the maintenance guys around and helping them out.

We didn't have any spare fuses, of course. A 1500 amp fuse is not something you run down to Lowes and pick up. We sent someone to go get a replacement, 2 hours of driving time. He comes back with exactly 1 fuse, because they were $700 a piece and he was frugal.

The failed and burned 400 amp breaker is removed from the panel. The 1500 amp upstream fuse is replaced. Power is restored, and everything is working.

I start walking away from the room where the subpanel is, since everything is working. The maintenance guy goes to put the cover back on the panel that had the bad breaker in it, as I'm walking away.

When I get about 20 feet away from the room, the whole plant seems to shudder, and then the power goes back off. I run back to the room to see him on the floor behind the step ladder. He's OK but I think he may have needed a change of underwear.

While everyone is still trying to figure out what is going on, I get a meter and go down to the service entrance, and meter all three fuses. Every one of them was blown. No one believes me of course, until they come down and everyone sees it first hand. I think they were just in shock at that point.

Apparently when he went to put that cover back on, a piece of copper slag that had melted off the bus bar shifted and shorted out all three phases in that panel. Its main failed to trip yet again.

Another 2 hour drive to buy the last four 1500 amp fuses in a 100 mile radius, and another 2 hours of plant downtime.

The subpanel is checked out carefully for any more physical damage, the fuses are replaced, and we are finally up and running 6 hours from the first failure.

After this, management commissions a short circuit coordination study. The results come back but I'm not sure anything ever came of it.

So to answer your question Jamie, big short circuits are "self clearing". You don't want to be holding onto the tool when it explodes though.
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Old 11-15-2008, 01:49 AM   #2
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A story of a high amperage short


Sounds scary. I would not want to be right infront of such an incident....
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Old 11-15-2008, 08:02 AM   #3
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A story of a high amperage short


here are a few videos of arc flash situations.

the one title "racking a breaker" is a real good one that shows something like gig would have experienced.

http://video.google.com/videosearch?...%20flash&emb=0


In my area, there is a well known situation where an electrician was involved in such a situation. The power availble was considerably greater though.

They literally scooped the guys remains up with shovels.
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Old 11-15-2008, 12:35 PM   #4
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A story of a high amperage short


I found this arc flash protective clothing test video

http://www.youtube.com/watch?v=dA4V0hObZbU

Kind of interesting
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Old 11-15-2008, 01:04 PM   #5
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A story of a high amperage short


I spend quite a bit of time working on electrical systems like the one shown in the Google video that Nap posted. The big metal cabinet with the door open is called switchgear. This particular switchgear is most likely 15KV (15,000 volt) class. The actual operating voltage is usually either 12.5KV, or 13.8KV. It's possible that it's 5KV, operating at 4160 volts though.

Behind the open door is a circuit breaker. It's a bit different than the ones in your house panel. These breakers are about 3' X 3' X 3', and weigh about 350 lbs. Most of it is a mechanism to open and close (turn off and on) the contacts in the back. These contacts are enclosed in vacuum bottles. If you look at the back of the breaker, you'll see 6 jaws, 3 on top, and 3 on bottom. The vacuum bottles (contacts) are mounted between the top and bottom jaws. All breakers like this are 3 phase, the top row of jaws is the line (hot) side, and the bottom row is the load side.

These breakers can be installed and removed with the switchgear hot. Racking a breaker means positioning it, racking it in means to install it, racking it out means to remove it. If you look into the switchgear while the breaker is completely removed, you'll see a cubicle about 3' X 3' X 3'. In the back, there'll be 6 round holes, about 5" in diameter. These will be covered by a shield of some sort, because in these holes are busses that the breaker jaws connect to. The top set are hot. The bottom set are the load. There are rails in the bottom of the cubicle, and the breaker has wheels that ride on these rails.

Since there are 6 jaws, and each jaw takes quite a bit of force to connect it to the bus, these breakers are installed by means of a long screw, alot like a screw-drive garage door opener. This screw is turned by a crank, and you can see in the video the guy is cranking (racking) the breaker in.

In the video, when the jaws make contact with the busses, something goes horribly wrong. Either the back of the breaker is not clean (at 12,500 volts, dust frequently becomes conductive), something is wrong with the breaker itself, or the breaker was installed with the contacts closed (on).

Sadly, I'd be surprised if the guy survived the experience. An explosion that size will usually vaporize the flesh right off the front side of your body, not to mention all of the vaporized metal it sprays you with. Really sad.

Most of us that are experienced with 'high energy' electrical systems will stand to the side of a breaker when it's being turned on. There's really no way to stand to the side of a breaker when it's being racked in.

To get back to the original question, this system has thousands of times more energy than a house service. It's sort of a different kind of energy as well. If you short out a house system, it'll certainly make a big flash, and it'll send vaporized metal into your flesh (and eyes), and you'll likely need to visit the local emergency room, but you'll very likely recover, except you'll likely be blind.

Here's the 'different kind of energy'. A 240 volt arc is not self-sustaining. It vaporizes whatever is causing the arc, then it stops. Above about 370 volts, an electrical arc becomes self-sustaining. This means that the arc will continue until one of two things happens. 1) the source of energy is removed. Usually a breaker or fuses upstream kills the power. In the absence of that, 2) the arc continues until there's nothing left that conducts.

Notice in the video that the arc burns for a few seconds, and only stops when the lights go out. Had the upstream breaker failed to cut the power off, the arc would have continued to burn, and there'd likely be a pile of molten metal and ash where the switchgear once was.

All electrical systems demand a certain amount of respect, larger one deserve even more. I take a lot of safety precautions when working on systems this size, but when I rack in a breaker, i do it just like the guy in the video. I really don't know how this tragedy could have been prevented. I guess it's just a risk you live with when you work on stuff like this.

Rob

P.S. This is in reference to the Google video that Nap posted, though the Youtube one is pretty good also.

Last edited by micromind; 11-15-2008 at 01:16 PM. Reason: Added P.S. The other video was posted while I was writing.
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Old 11-15-2008, 01:07 PM   #6
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A story of a high amperage short


That one reason why I always double check the system after the fuse or breaker tripped espcally with larger system they will do hovac damge if not check before ya throw the switch.

My last one I ran into with 2500 amp main breaker yeah that control whole industrail building and have to replace the damged breaker it was destoryed so end up replace the whole thing the simple curpit was failed 200 amp subfeed breaker { by the way it is on 480 volts system so that will compound it } so the industrail company called me in for emergcy service call and working on that one all nite megger the whole system.

Not only the main breaker panel have to be replace but THREE other breakers have to replace and the POCO have to join in with me to replace their transfomer too { the short circuit burnted the connection at the transfomer }

Merci,Marc
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Old 11-15-2008, 05:41 PM   #7
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A story of a high amperage short


that CB on the rooftop might of been just a switch like Trane uses,and never claims it is a CB.aren't they suppose to do maint. on CBs that size in plants by cycling them...because they can actually weld together decreasing the sensitivity to trip on a dead short?
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Old 11-15-2008, 07:41 PM   #8
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A story of a high amperage short


Most of us that are experienced with 'high energy' electrical systems will stand to the side of a breaker when it's being turned on. There's really no way to stand to the side of a breaker when it's being racked in.

I remember of reading of a robot that can be used to rack in large breakers, but don't know anything else about it.

To get back to the original question, this system has thousands of times more energy than a house service. It's sort of a different kind of energy as well. If you short out a house system, it'll certainly make a big flash, and it'll send vaporized metal into your flesh (and eyes), and you'll likely need to visit the local emergency room, but you'll very likely recover, except you'll likely be blind.

Your speaking of generating a arc flash like this when both legs of the panel connect and cause an arc correct? You don't generate that large of an arc with just one leg of the panel comming into contact with ground do you? I've seen an arc from a 120v leg to ground, such as a loose outlet hitting the box, and it can scare you, but isn't all that large of a fault.

So I assume the level of damage you speak of is only if you arc both legs of the panel?

Jamie
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Old 11-15-2008, 08:16 PM   #9
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A story of a high amperage short


Quote:
Originally Posted by jamiedolan View Post

Your speaking of generating a arc flash like this when both legs of the panel connect and cause an arc correct? You don't generate that large of an arc with just one leg of the panel comming into contact with ground do you? I've seen an arc from a 120v leg to ground, such as a loose outlet hitting the box, and it can scare you, but isn't all that large of a fault.

So I assume the level of damage you speak of is only if you arc both legs of the panel?

Jamie
The arc you see on a 15 or 20 amp 120 V circuit is limited in magnitude by the quick action of a fuse or circuit breaker and the impedance of the circuit. If the wire were tied directly to the bus in the panel, what you see would be quite different. And different still if it were at the transformer.
The thing about short circuits is that for the same resistance, the amount of available energy goes by the square of the change in voltage. So, if the blast energy at 120 volts is x, then the blast at 240 will be 4x. At 480, 16x.

The impedance of the circuit and distance from the source makes alot of difference, too.
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Old 11-15-2008, 11:51 PM   #10
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A story of a high amperage short


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The arc you see on a 15 or 20 amp 120 V circuit is limited in magnitude by the quick action of a fuse or circuit breaker and the impedance of the circuit. If the wire were tied directly to the bus in the panel, what you see would be quite different. And different still if it were at the transformer.
The thing about short circuits is that for the same resistance, the amount of available energy goes by the square of the change in voltage. So, if the blast energy at 120 volts is x, then the blast at 240 will be 4x. At 480, 16x.

The impedance of the circuit and distance from the source makes alot of difference, too.
So even if I had just hit one leg of the hot with a neutra or ground wire, I could have had a arc large enough to seriously injur me? ,

I think next time I have to do something like that, the meter head is going to be removed.

Jamie
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Old 11-16-2008, 12:17 AM   #11
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A story of a high amperage short


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So even if I had just hit one leg of the hot with a neutra or ground wire, I could have had a arc large enough to seriously injur me? ,

I think next time I have to do something like that, the meter head is going to be removed.

Jamie

You might get some burns and collect some copper, not too bad if your face isnt right there.

Depends on the size of the wire too.
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Old 11-16-2008, 12:20 AM   #12
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A story of a high amperage short


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You might get some burns and collect some copper, not too bad if your face isnt right there.

Depends on the size of the wire too.
It was AL for 100 A service, maybe #1 in size would be my guess. I think I will wear a face shied if I ever had to do something like that live again.

Jamie
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Old 11-16-2008, 02:06 AM   #13
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A story of a high amperage short


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here are a few videos of arc flash situations.

the one title "racking a breaker" is a real good one that shows something like gig would have experienced.
Ack no it wasn't that bad. The partially melted busbar was still serviceable on most of the slots (at least temporarily) after they cleaned it up.

Pretty cool video though.
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Old 11-16-2008, 02:12 AM   #14
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A story of a high amperage short


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that CB on the rooftop might of been just a switch like Trane uses,and never claims it is a CB.aren't they suppose to do maint. on CBs that size in plants by cycling them...because they can actually weld together decreasing the sensitivity to trip on a dead short?
No there wasn't a CB on the rooftop.

It was A/C -> 400 amp 3ph CB -> 1500 panel main -> 1500 amp branch fuses -> 5000 amp service fuses

400 amp failed to trip, melted busbar. 1500 amp CB failed to trip, 1500 amp fuse blew.

As for periodically cycling circuit breakers... I've never heard of that. If you are supposed to, I doubt they do.
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Old 11-16-2008, 02:16 AM   #15
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A story of a high amperage short


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The impedance of the circuit and distance from the source makes alot of difference, too.
Right. I did the calculation in another thread, 100 feet of 12 gauge is 0.15 ohms.

So in a dead short on 100 feet of 12 gauge you'd have max 800 amps, assuming the power company has a magical superconducting transformer for you.

If you short the lugs in your main panel on the other hand, you have a much lower resistance connection there. Much bigger flash.
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