Blowing up a 5000 amp fuse

[PStechPaul]

The details of the incident I described may not be totally correct, as it was probably at least 30 years ago. I have been on several such maintenance jobs and they have been performed during a planned shut-down, but there was usually an auxiliary source of power while the main feed was disconnected. Sometimes we used generators, but they are not quite up to the task of supplying primary injection breaker test sets that use 480 VAC at 200 amps or more. I think the accident occurred on one of the main circuits that could only be shut down at the pole or pad mounted distribution transformer. It may have been a "side job" that avoided the expense of having the electric utility involved. And the wrench might have been at least partially wrapped with electrical tape which is a common safeguard.

One of my first jobs with EIL in 1974 was at a huge government facility in Newark, OH. I think a few years later, at the same facility, a technician was seriously injured when he was racking in (or maybe out) when a cluster of contact fingers fell across the bus.

[J Tiers]

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In the course of designing and building electrical test equipment, I watched a film made by Bussmann Fuse Company, that showed full power destructive testing of low voltage (600 V) switchgear, and it showed how a molded case circuit breaker would typically be destroyed even when successfully clearing an overcurrent fault within its interrupting rating (typically 5,000 - 20,000 amp), whereas a similarly rated fuse would just quietly break the circuit with a puff of smoke. It might exhibit more damage at its interrupting rating of 100,000 to 200,000 amps, but the fault was still cleared without much drama.

Down the street from me there is a house that had a power cross to the 4160 volt feeder. The feeder has a fuse at the end of the block, size not known to me for sure, but I was told on the order of 100A, which seems reasonable given the area it supplies. The fuses are of the bare wire type, stretched over spring prongs in a ceramic housing. Probably not particularly fast fuses, not current limiting.

A tree fell, knocking one phase of the feeder onto the house drop, and onto the ground as well. I saw the results. The breaker box looked like a bomb went off in it. The mains and some of the branch breakers were hanging out the front, burnt. Obviously there had been a substantial arc flash, mostly contained in the box. Several outlets had had flashovers and the wall around them was blackened. I was amazed there had been no fire. pretty much every electrical device or fitting. wire, etc was damaged and it all had to be replaced.

All that through a 100A fuse, with part of the available current arcing to ground.

[wierdscience]

That was an epic blunder. All of that money spent on equipment and they just took a guess on the weight of the explosives. Another couple hundred bucks for a scale and this would not have happened.

Scary proposition, as I have noticed how far off human perception of things like weight and dimension can be from person to person.

[Willy]

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We could have switched the load out to other feeders if done very late at night when the load was minimal but the supervisor didn't want to pay anybody OT to do the job.

Damn!
Crap like this pisses me off!
That A-hole supervisor will gamble somebody else's life, not his own, in order to get the job done in a timely and cost efficient manner so that he looks good.
They all want you to be a team player but when the sh*t hits the fan they'll all point their fingers to the guy that can't talk anymore! He's had all of the training and signed multiple forms testifying so, we have it all on file.

Been in all too many similar scenarios.

[RB211]

Exactly! A power outage is an inconvenience to some people but it can be the difference between life and death to others who have to do the actual work.

I remember back when we had an insulator crack on the overhead 25Kv bus work that fed several three phase feeders in one of our 72/25 Kv substations. This was a big crack, totally through the insulator and when it was removed, it was in two pieces. It was also over speced for the working voltage being for 34.5 Kv IIRC. Our supervisor asked my crew foreman if we would do the replacement hot and he flat out and out refused, knowing there was no way to guarantee there couldn't be a flashover. (It was SOP to use a bucket truck with an insulated boom and insulated bucket liners and 40Kv insulated rubber gloves to work on 25Kv lines when we KNEW it was safe to do so.) Said supervisor just went to another crew foreman and asked him if they would do it. "Sure! No problem!"

Well, he got lucky and there was no flashover but if there had been, he would not have survived. The fault current would have been well over 7000 amps at 14,400V (nominal) to ground... I'm not sure if the Lineman in the other bucket would have survived that or not but I'm positive he would have been severely injured.

We could have switched the load out to other feeders if done very late at night when the load was minimal but the supervisor didn't want to pay anybody OT to do the job.

I'll stick to 240v

[Arcane]

There is always the option of cutting power to do the work. If you do not have arc flash gear, that's the only way to handle it.

Exactly! A power outage is an inconvenience to some people but it can be the difference between life and death to others who have to do the actual work.

I remember back when we had an insulator crack on the overhead 25Kv bus work that fed several three phase feeders in one of our 72/25 Kv substations. This was a big crack, totally through the insulator and when it was removed, it was in two pieces. It was also over speced for the working voltage being for 34.5 Kv IIRC. Our supervisor asked my crew foreman if we would do the replacement hot and he flat out and out refused, knowing there was no way to guarantee there couldn't be a flashover. (It was SOP to use a bucket truck with an insulated boom and insulated bucket liners and 40Kv insulated rubber gloves to work on 25Kv lines when we KNEW it was safe to do so.) Said supervisor just went to another crew foreman and asked him if they would do it. "Sure! No problem!"

Well, he got lucky and there was no flashover but if there had been, he would not have survived. The fault current would have been well over 7000 amps at 14,400V (nominal) to ground... I'm not sure if the Lineman in the other bucket would have survived that or not but I'm positive he would have been severely injured.

We could have switched the load out to other feeders if done very late at night when the load was minimal but the supervisor didn't want to pay anybody OT to do the job.

[J Tiers]

Ya think!!!

There is always the option of cutting power to do the work. If you do not have arc flash gear, that's the only way to handle it.

[RB211]

I wonder if all this really belongs to a machining forum... First we're invited to see sutured hands, now we are enjoying vaporized bodies... Do we really need to continue? How about someone showing a photo of his hemorrhoids being fixed? Or a colonoscopy video?

Videos that demonstrate what can go wrong in the name of safety? They most definitely belong in a machinist forum.

[RB211]

It asks me if I'm over 18 (yes) and do I want to see adult content. What in the world are they doing? Adult content? Anyway, I'm not opening it, not having any record that I've been watching 'adult content' on my PC

You do you.

[Arcane]

That "bit of tape" might not have been so great..... Impact on an edge might well cut through the tape with the same result.

Tape needs several layers to begin to be "protection". And usually there is some other material wrapped on before the tape, when covering anything that has potentially sharp edges.

Better a sleeving of something thicker and tougher, such as heatshrink tubing on the handle, possibly two layers of different types. Something fibrous would be better against cut-through.

You just don't want to be near a couple foot diameter ball of plasma at several thousand degrees.

Ya think!!!

[J Tiers]

That's a prime example of what I said about s h i t happens and "nothing can go wrong".

A bit of electrical tape around all but the jaws of that wrench would have prevented it from shorting out the bus bars.

That "bit of tape" might not have been so great..... Impact on an edge might well cut through the tape with the same result.

Tape needs several layers to begin to be "protection". And usually there is some other material wrapped on before the tape, when covering anything that has potentially sharp edges.

Better a sleeving of something thicker and tougher, such as heatshrink tubing on the handle, possibly two layers of different types. Something fibrous would be better against cut-through.

You just don't want to be near a couple foot diameter ball of plasma at several thousand degrees.

[Arcane]

I have done some work involving maintenance and testing of such high current metal clad switchgear, where we removed breakers from cubicles, cleaned, lubed, and performed visual and electrical testing, which included hipot insulation testing, contact resistance tests, and primary injection testing which involved long time, short time, and instantaneous trip tests at currents of typically 3x, 6x, and 10x rating. Trip times are typically about 90 seconds at 3x, 0.5 seconds at 6x, and 0.05 seconds at 10x.

My friend Ken, for whom I worked quite a few years, was doing overnight maintenance at a large night club in Baltimore called the Power Plant. His older, highly experienced partner was using a wrench to tighten some bus bar connections when it slipped and fell across two lines of the incoming feed, which was probably 480 VAC and 2000 amps or so. It welded in place across the bus bars and caused an arc flash and fireball that persisted for a few seconds, after which an external fuse eventually cut power. Ken saw him drop the wrench from maybe 10-20 feet away and just had time to raise his arm and turn away. He was badly burned but survived, while his partner was critically injured and died after a few days of pain in the hospital.

In the course of designing and building electrical test equipment, I watched a film made by Bussmann Fuse Company, that showed full power destructive testing of low voltage (600 V) switchgear, and it showed how a molded case circuit breaker would typically be destroyed even when successfully clearing an overcurrent fault within its interrupting rating (typically 5,000 - 20,000 amp), whereas a similarly rated fuse would just quietly break the circuit with a puff of smoke. It might exhibit more damage at its interrupting rating of 100,000 to 200,000 amps, but the fault was still cleared without much drama.

That's a prime example of what I said about s h i t happens and "nothing can go wrong".

A bit of electrical tape around all but the jaws of that wrench would have prevented it from shorting out the bus bars.

[Richard P Wilson]

Anyhow, here’s what an Arc flash of the power they are playing with will do to your body. Don’t worry, nothing gruesome to see as the body is atomized.

It asks me if I'm over 18 (yes) and do I want to see adult content. What in the world are they doing? Adult content? Anyway, I'm not opening it, not having any record that I've been watching 'adult content' on my PC

[MichaelP]

I wonder if all this really belongs to a machining forum... First we're invited to see sutured hands, now we are enjoying vaporized bodies... Do we really need to continue? How about someone showing a photo of his hemorrhoids being fixed? Or a colonoscopy video?

[PStechPaul]

I have done some work involving maintenance and testing of such high current metal clad switchgear, where we removed breakers from cubicles, cleaned, lubed, and performed visual and electrical testing, which included hipot insulation testing, contact resistance tests, and primary injection testing which involved long time, short time, and instantaneous trip tests at currents of typically 3x, 6x, and 10x rating. Trip times are typically about 90 seconds at 3x, 0.5 seconds at 6x, and 0.05 seconds at 10x.

My friend Ken, for whom I worked quite a few years, was doing overnight maintenance at a large night club in Baltimore called the Power Plant. His older, highly experienced partner was using a wrench to tighten some bus bar connections when it slipped and fell across two lines of the incoming feed, which was probably 480 VAC and 2000 amps or so. It welded in place across the bus bars and caused an arc flash and fireball that persisted for a few seconds, after which an external fuse eventually cut power. Ken saw him drop the wrench from maybe 10-20 feet away and just had time to raise his arm and turn away. He was badly burned but survived, while his partner was critically injured and died after a few days of pain in the hospital.

In the course of designing and building electrical test equipment, I watched a film made by Bussmann Fuse Company, that showed full power destructive testing of low voltage (600 V) switchgear, and it showed how a molded case circuit breaker would typically be destroyed even when successfully clearing an overcurrent fault within its interrupting rating (typically 5,000 - 20,000 amp), whereas a similarly rated fuse would just quietly break the circuit with a puff of smoke. It might exhibit more damage at its interrupting rating of 100,000 to 200,000 amps, but the fault was still cleared without much drama.