# Ac Flash Event



## Salty9 (Jul 13, 2009)

Circuit breaker arc flash event. The operator was wearing safety gear and survived but spent a week in the hospital.


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## jeremyjs (Sep 22, 2010)

what exactly are we looking at. I see a handle and a big melted mass of something. What is it supposed to look like?


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## Salty9 (Jul 13, 2009)

Something similar to this except smaller without the remote operation. As a WAG I would say it was equivalent in amperage to 10 Shivas.


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## PStechPaul (May 1, 2012)

I replied to the thread about the deliberately shorted fuse (crowbar) o-s***t emergency shutdown, with some videos on youtube of circuit breakers exploding due to exceeding their maximum interrupting rating, which can be as "little" as 10,000 amps. Fuses, OTOH, commonly come with 100k and 200k IR, and are also so fast-acting that they actually open before the current even reaches its peak value.

I have designed and worked on circuit breaker test equipment which uses low voltage primary injection up to 100,000 amps, and I have seen movies comparing the result of circuit breakers vs fuses, and total destruction as shown in the picture is a very real possibility at currents much above the ACIR, even at "low voltage" 480 VAC. But sometimes such major arc flash events are caused by a conductive object falling across energized bus bars. 

Early in my career I worked on an all-day breaker testing job at a large industrial site in Ohio, where we unracked, cleaned, tested, and re-racked hundreds of metal-clad breakers in cubicles similar to those shown in the second picture. A few years later, another crew was working in the same facilty, and one of the stab clusters came loose from the breaker and fell across the bus, causing a serious arc blast, and severe injury or perhaps a fatality to the technician. 

This is a crew I worked with in another installation. IIRC everything was shut down except 120 VAC from a small generator:























There is much more energy available from grid-tied systems than most EVs. Industrial breakers are commonly 1200 to 4000 amps on a three phase 480 VAC system, and fault currents can easily exceed 20 or even 50 times that. So you can have 10 MW of power concentrated in less than a cubic foot, which can create temperatures of thousands of degrees which can turn metallic conductors into vaporized, conductive plasma. Fortunately, upstream fuses will usually blow within a few milliseconds to limit the total energy, but sometimes a lesser fault will result in a considerable delay and much more overall energy.

EVs are more likely to be limited to about 10x battery capacity, so a major fault may have perhaps 1000 amps at 600 VDC, but that's still 600 kVA and that's a lot of power. Food for thought when considering safety systems. High interrupting capacity, peak limiting fuses are essential as ultimate backup protection.


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## subcooledheatpump (Mar 5, 2012)

Yeah serious electrical installations, and assemblies (switches, breakers, transformers, generators) are very dangerous. I have no idea if this is true or not, could be totally made up, or it could be actual fact, but, I've heard a similar story of a worker racking a breaker, ended up misaligning it and shorting the bus. The other workers said only his shoes were left.


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## Tesseract (Sep 27, 2008)

PStechPaul said:


> I replied ... with some videos on youtube of circuit breakers exploding due to exceeding their maximum interrupting rating, which can be as "little" as 10,000 amps. Fuses, OTOH, commonly come with 100k and 200k IR, and are also so fast-acting that they actually open before the current even reaches its peak value.....


What you say about fuses usually being able to interrupt a higher fault current than breakers is true, but this is not as important in EVs as you might think. The fault current from the typical EV battery pack is going to be much lower than what even the smallest service transformer (aka - "pig") will dump when shorted.

For example, Rebirth Auto built a 60 x 200Ah battery pack for a Volvo V70. Each 200Ah cell had an internal resistance (determined by comparing the voltage drop at 200A and 400A) of 700 microohms, giving a total resistance for the 60 cell series string - not including interconnects - of 42 milliohms and a projected fault current of ~4600A. A 160VDC breaker with a 10k AIC* rating would be more than sufficient for protecting such a battery pack.



* - note the proper terminology is AIC which stands for "amperes interrupt capacity"; "IR" is usually understood to mean "voltage drop".


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## dragonsgate (May 19, 2012)

Please explain shiva to me. I just tried looking it up but the only thing that happened is I am converting to Hinduism.


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## Tesseract (Sep 27, 2008)

dragonsgate said:


> Please explain shiva to me. I just tried looking it up but the only thing that happened is I am converting to Hinduism.


Shiva is the Hindu god of transformation and destruction... seemed appropriate for a 1.2MW motor controller


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## toyolla2 (Jun 21, 2010)

Here is my story and it's another way these accidents happen.

I was taking over final commissioning of a press house with 3500 Amp bus bars.
On site I was given training on how to trigger the main isolator so that when I was the last to leave I could "Power Off". As insurance against fire etc when left unattended overnight I was told. The decision to do this was made by the production manager who had no authority nor electrical training it turns out.

In fairness my own company never thought to give training to me either, since we didn't supply or install the component. And we treated this 6MVA switching device as if it were a glorified light switch. It was switched on and off sometimes several times per day. And this went on for a couple of years.

Then we had a the fire in the building electrical distribution room. Nothing to do with our isolator fortunately. The electrical contractor was using aluminum bars which should have had its fasteners retorqued as annual maintenance. Enough said.

However while the regional inspector was overseeing the restoration it was suggested to get the main isolator for the press also checked out. The manufacturer gets the call and his rep turns up and starts dismantling it for access.

Inside was a horror show. Some of the key mechanical linkages showed wear to the extent of imminent failure. What were we doing to cause this much internal damage obviously the rep wanted to know.

It was explained that it had been operated, and guessing here, around 3000 times in the last three years. To which we were then reminded that an isolator is not switch. Once it's been turned on it should stay on. Otherwise it should not be operated more than about one time every six months.

Of course breakers are available at the power level we were using to do this type of switching action but they command a price around ten times greater.


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## dragonsgate (May 19, 2012)

Ok thanks for clearing that up for me. I sort of knew about Shiva. In fact I have a painting around here somewhere. I did it many years ago and read up on her/him. Obviously it didn’t sell cause I still have it. I have a hard time keeping abbreviations and symbols in my head so I figured something new had been slipped in with out telling me. Shiva is appropriate though. Electric explosions can be exciting if done correctly. Right after I got out of the Navy in 1970 I worked at a pulverized sand mill for a short time while I was searching for the first rug on that ladder to success. They had a giant barrel that looked much like what you see on cement trucks except it was about twice the size of a ten yarder. The mixer was driven by a 440 volt open frame motor with a forty eight inch dia.armature. On the second week there I was standing about twenty feet away when a bearing gave out and the armature hit the field. Luckily it was on the night shift so I got to enjoy the full effect.


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