OT Arc flash video

Some time back there was a thread on lock outs and working in live panels. Arc flash was mentioned several times. This showed up in my email today:

High-Speed Movies Reveal Behavior of High-Energy Arc Flashes | Machine Design

Sandia Labs has made a couple of high speed thermal videos of arc flash events, at least a couple can be seen on this link. Not an expert or an electrician myself, just think some will find it of interest.

Wow, that's quite an achievement. This should lead to a huge improvement in electrical safety. Thanks for sharing.

I have seen that type of behavior many times in the GE high current lab, although not for 4 seconds. Unless the protective device fails, I would expect the clearing time to be limited to 5 cycles or for 60 hz, or .08 seconds. This is for a molded case breaker. Fuses, such as class J, K and R are much faster, less than a cycle, typically more like 1/2 cycle.

Tom

When I was taking a safety training course I saw a great demonstration of the power of repulsion. The ends of a length of romex were clamped with a gurney (split bolt) and a high energy capacitive discharge was sent through the wires ( about fifty feet). The wires were ripped out of the romex with a sound like a gunshot. Very impressive.

Our volunteer fire department has been advised by the power company not to pull meters at residence fires. Grudgingly they said if you have to, stand to the side and hit the meter with an insulated shovel handle. Hate to break things but better than getting the flash in the face.
Many years ago a pole transformer near our backyard flashed over the terminals, it went one for about 5 or 10 minutes till the power company pulled the pole connection fuse. Everything in the house and the yard looked blue till they finally fixed it.

Back when I was an industrial electrician and about 20 years old I was troubleshooting a size 1 combination starter. I turned it on with the door open so I could test it and about 30 seconds later it flashed over phase to phase. Burned off my hair at the front, my eyebrows, eyelashes, and my pathetic beard, gave me a sun burn and really pitted my glasses. I was blinded and didn’t realize I blew the 400 amp 480 volt main breaker so all the lights were out. Somebody replaced the fuses in the starter with pieces of copper pipe taped to look like fuses!

Fast forward 30 years and an electrician at a jobsite gives me a Ferraz Shawmut training manual and they show a dummy exposed to a shorted size 1 combination starter as an example!

I mentioned to the electrician that panels farther away from the main service can be more dangerous than ones up close because wire resistance can reduce peak current so peak limiting doesn’t happen and it takes seconds to clear a fault. He said that wasn’t what he was taught in school.

I emailed Ferraz Shawmut about and got a chain of emails indicating I was correct and it was being taught wrong and they needed to revise the manual!

True!

The only good news is that it also reduces the fault current so that the energy level potential is lower also (which is basically saying the same thing as you did), so the power in any arc fault may be lower. However, the time of the arc before protection cuts in may mean that total energy let through is likely to be larger.... less energy, but longer time.

garyhlucas said:

I mentioned to the electrician that panels farther away from the main service can be more dangerous than ones up close because wire resistance can reduce peak current so peak limiting doesn’t happen and it takes seconds to clear a fault. He said that wasn’t what he was taught in school.

Click to expand...

That's no joke. When we went through a full and proper arc flash study of our entire system it is always the 480v drivers that are the biggest hazard. Not the 4160 or 7600, the 480.

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"Somebody replaced the fuses in the starter with pieces of copper pipe taped to look like fuses!"

Clearly an agent of Satan.

garyhlucas said:

I mentioned to the electrician that panels farther away from the main service can be more dangerous than ones up close because wire resistance can reduce peak current so peak limiting doesn’t happen and it takes seconds to clear a fault. He said that wasn’t what he was taught in school.

I emailed Ferraz Shawmut about and got a chain of emails indicating I was correct and it was being taught wrong and they needed to revise the manual!

Click to expand...

Depends on how you look at it.
A fault far from the over current device has the resistance of the wire consuming some of that current as heat, and its possible it will melt the insulation and short circuit much closer to the over current device. Looking at it the other direction your panel boards are rated for or "braced" for a specific maximum of current that your incoming service is capable of delivering in a fault situation. Common values are 10K amps for home and smaller commercial type panels with industrial switchgear going up to 100K or more. That rating means that equipment will withstand the magnetic energy of "XX" fault current before being ripped or blown apart. There are lots of variables to calculating that fault current with conductor size, length, and what they are made of (copper or aluminum) being the main factors. The power company and where their step down transformers are in relation to your panel location also determines available fault current.

Possibly the folks at F/S did not understand your question or they were not the actual engineering staff. If everything is sized properly and correct over current devices installed length of wire should not make but milliseconds of difference.

Something I like to point out when it comes to over current devices, they are not all made the same. Many of the lower quality makers DO NOT have short circuit protection built into their equipment, instead they rely on the heat generated by the over current to make the trip. All your Square D QO series, I-Line, etc (not homeline) have short circuit or magnetic trip as well as thermal trip. Some of the better Cutler Hammer line does as well. Most of the "other" brands rely only on thermal trip meaning it could take many seconds for them to trip in a short circuit situation.

If I can find them I have pictures of a couple 15kv switch cabinets we rebuilt on site after they blew up. These were directly down stream of 8800kva 34.5K volt to 13.8K volt transformers that had 200 amp cutouts on the highline side. Moisture from condensation was the cause. Sealed up better and with proper insulation to avoid tracking and arc over if moisture did get into the cabinets eliminated further issues.

Just went through another arc flash training session.There are plenty of videos of what happens when arc flash occurs.
If you are lucky enough to survive the blast impact and the burns if you breathed in any of the metal vapors then that will finally kill you.

FredC said:

Some time back there was a thread on lock outs and working in live panels. Arc flash was mentioned several times. This showed up in my email today:

High-Speed Movies Reveal Behavior of High-Energy Arc Flashes | Machine Design

Sandia Labs has made a couple of high speed thermal videos of arc flash events, at least a couple can be seen on this link. Not an expert or an electrician myself, just think some will find it of interest.

Click to expand...

Jeebus H!!!

I've always been warned about arc flash, heard it was bad news.
I once had a (crap) multimeter blow up on me, was on a shitty old 380v Italian machine while probing. That was pretty damned scary stuff.

I think I might look into training for my guys, as we have to turn transformers on and off with disconnects for certain jobs.
I cringe every time I hear that CHUNK of the disconnect flip over.

A little more to know. Peak limiting fuses can open a circuit in less than half a cycle if the current rises to the peak limit rating. It may be 10,000 amps but it lasts for less than 1/120 second. A fault that reaches ONLY 5000 amps and lasts for 1 full second delivers 60 times more energy. With a long run of wire keeping the current below the peak limit rating the fuse operates on the time current curve and delivers many cycles of energy into the fault which creates an incredible arc. This is why you need to do complex calculations to determine the arc flash hazard for any given equipment.