Arc blast - is is something for “normal” people to worry about?

[Dan123456]

I’ve been getting a lot of videos recommended to me lately on the dangers of arc flash / arc blast and it is some scary shit 

All of these have been accidents in industrial settings or test facilities with 1000’s or 10,000’s of amps so I would assume it isn’t something “normal” people (I.e. anyone just dealing with single phase mains or high amperage DC) need to worry about but researching it isn’t quite as clear as I would like.

The most black and white number was from good old Wikipedia stating “Most 400 V and above electrical services have sufficient capacity to cause an arc flash hazard.”

https://en.m.wikipedia.org/wiki/Arc_flash

Does that simply mean, so long as you aren’t dealing with 400V+, it isn’t something you need to worry about?

Or can it still occur at lower voltages such as normal 240V mains or with say an 800A 12V car battery (but obviously with less power compared to the super nasty industrial accident type situations)?

[Someone]

Partly the voltage, more so the fault current, and actually the energy dissipated in a fault.

An automotive starting battery is already fairly dangerous, but not to the level of x00V with xxkA behind it (live working on CAT III/CAT IV). Remember those are the fault conditions, the nominal rating may be something benign like "just" 240V 20A.

[bdunham7]

The actual arc-flash phenomenon (the one that blows you up, not the spark in a light bulb as it burns out) isn't an issue for single-phase mains of any voltage or for low-voltage DC systems regardless of fault current.  You still can have a pretty big spark or arc and lots of molten metal flying around--like welding--but you won't be vaporized or fly through the air.

Typically this is an issue for 3-phase service panels or large equipment of 480V and higher, but there were recent revisions to regulations requiring PPE and precautions to be taken even in certain 208 or 240VAC three-phase panels if they had a very high fault current. 

So no, you aren't going to be arc-flashed into a pile of ash while repairing your toaster or replacing your car battery.   

[coppercone2]

people say that to mean nasty burn if they don't know the exact specifics and IMO I am happy that they at least know it can do something bad lol

usually to get a super severe burn from mains (not electiricution) you would need to have something like maybe wet gloves (steam burn) cupped around a short circuit (say pushing something in to a live board that requires alot of force and have that short out). like fucking with some kind of pump. Like those cotton gloves with the grippy dots that you might feel are the glove of choice for doing plumbing on a jacuzi pump.


But IDK that would happen if you have a circuit breaker malfunction. Because the maximum thermal energy is basically governed by the circuit breaker.

[T3sl4co1l]

Residential circuits are usually on the order of a few kA fault current.  At 120V, it's not too bad; at 240V, I don't think it's something that will jump out at you, but it can cause extended damage to circuitry -- burned traces, pads, damaged nearby components as the ball of plasma sinks current into anything conductive nearby.

The main thing is when fault currents are high (industrial 240V+ circuits), and fusing isn't proper, allowing tons of energy let-through.  Then the plasma cloud can grow from some inches to some meters, and seriously propel things.

Tim

[jonpaul]

The Arc detector breakers are NOT arc blast but Arc Flash,  AFCI Arc Fault Circuit Interrupter.

https://www.protoolreviews.com/what-is-arc-fault-circuit-breaker/

 AFCI have a poor record: Use of uP and conducted EMI sensing of are remotely.

False trip, fail to detect, etc. Also very costly.

Unfortunately local/NEC now required them in many unneeded applications including residential.

Another example of government trying to fix an imaginary problem and creating worse result.

To the NEC/State gov electrical code writers (politicians and lawyers not engineers)

PLEASE JUST STOP NAGGING US!

Jon

[Dan123456]

Few!!!

Awesome! Really glad I don’t have to add that one to the list of things that can go wrong in this hobby  Thanks so much all 

While reading I did some super quick and dirty math in my head based on the below numbers from that Wiki article:

“in a single phase-to-phase fault on a 480 V system with 20,000 amps of fault current, the resulting power is 9.6 MW. If the fault lasts for 10 cycles at 60 Hz, the resulting energy would be 1.6 megajoules. For comparison, TNT releases 2175 J/g or more when detonated (a conventional value of 4,184 J/g is used for TNT equivalent). Thus, this fault energy is equivalent to 380 grams (approximately 0.8 pounds) of TNT.”.

I was thinking divide by 1000 to go from 20,000A to 20A then divide again by 2 to go from 480 to 240 (again, super quick and dirty numbers). That still gave me the energy equivalent to ~150mg of TNT!

That might not sound like much but would probably still be enough to take off a finger if you were unlucky 

[Berni]

The main difference is the maximum fault current.

These large industrial mains installations are built for delivering WAY more power for all the giant machinery in factories. As a result all the cabling is much thicker to have potentially 1000A on a single wire of a phase during normal operations. This means the resistance if the cables is really low, so when a short circuit does occurs the currents that can flow are massive potentially into the 100s of kA. The whole thing is made even worse by the fact that industry usually has dedicated transformers next to the facility. Those transformers are fed by some powerful high voltage lines that won't easily sag under load, so when you ask the transformer for a lot of current it will gladly provide it. Since the transformer is right outside the window, means that the already thick low voltage 400V cable run is also very short, making it even lower resistance.

This massive fault current is what makes things dangerous since as soon as that a spark forms the insane current starts flowing and dumping more and more power into the arc, due to the powers involved this grows into a large blob of incredibly hot plasma (so hot that it burns things near it to a crisp within miliseconds)

For residential mains installations this does not typically happen (even tho we do use 400V three phase for residential here) is that the transformer feeding it is typically far away down the street, might be a smaller transformer and the cables are much thinner since the normal power consumption of a house is only in the low 10s of kW. So in the event of a short circuit the fault current is going to be much smaller. Typically the normal residential fuses are rated for interrupting a 10kA fault current. The short circuit can still be spectacular but not the instantly giving you severe burns and setting all of your clothes on fire level of spectacular. But it could cause a cheap chinese non CAT rated multimeter to explode in your hand.

Another scary thing for arc flash are the giant UPS units that supply entire buildings. Those have a room full of lead acid batteries that will happily produce many thousands of amps at a few 100 volts. Being DC the arc might take a while to extinguish too.

[coppercone2]

Few!!!

Awesome! Really glad I don’t have to add that one to the list of things that can go wrong in this hobby  Thanks so much all 

While reading I did some super quick and dirty math in my head based on the below numbers from that Wiki article:

“in a single phase-to-phase fault on a 480 V system with 20,000 amps of fault current, the resulting power is 9.6 MW. If the fault lasts for 10 cycles at 60 Hz, the resulting energy would be 1.6 megajoules. For comparison, TNT releases 2175 J/g or more when detonated (a conventional value of 4,184 J/g is used for TNT equivalent). Thus, this fault energy is equivalent to 380 grams (approximately 0.8 pounds) of TNT.”.

I was thinking divide by 1000 to go from 20,000A to 20A then divide again by 2 to go from 480 to 240 (again, super quick and dirty numbers). That still gave me the energy equivalent to ~150mg of TNT!

That might not sound like much but would probably still be enough to take off a finger if you were unlucky 

no its a system with 240V of voltage and probobly 1-2000 amps of fault current for a small installation. The breaker in your house is rated for like 10kAmps though, if it works. It has a pretty big rating.

It has to do with the transformer outside of your house and how far you are from it.


20 amps of fault current is like what you would get from a rather small transformer. Like a 2 amp 1:1 transformer (200va)

[T3sl4co1l]

I was thinking divide by 1000 to go from 20,000A to 20A then divide again by 2 to go from 480 to 240 (again, super quick and dirty numbers). That still gave me the energy equivalent to ~150mg of TNT!

That might not sound like much but would probably still be enough to take off a finger if you were unlucky 

TNT isn't much more dense than flash powder, and those common firecrackers are something like 50mg.  And yeah, they can take things apart if you hold one too closely.

The main difference is they go off much faster.  Explosives that size are in the 10s of µs, deflagrants ~ms.  If you're talking a second of arcing, that's little more than a poorly-controlled welder.  Not something you want to stick your finger in, but not at all likely to turn it into the proverbial microwave-exploded hotdog.  (Cooked like a hotdog, perhaps...!)  Obviously, as fault current and voltage go up, the power level starts to look like a continuous explosion, and the potential for physical damage goes way up.  Which means a properly fused circuit, under arc flash, can still go off like a large firecracker (fuse clearing time: ~ms), and it's no accident those cabinets are made with sturdy hinges!

Tim

[SeanB]

Yes residential, with a breaker typically rated for 3-5kA of fault break current guaranteed, it is not too much of an issue, as most breakers will do a magnetic trip within 2 cycles with a short circuit. However remember that this also is on the assumption that the fault will occur more than 2m away from the breaker, allowing the wire impedance, along with the supply impedance, to limit current. So not really an issue, unless you are at the point of supply and have a fault there, or have solar panels, which will run into a short until the sun goes down.

If you are living close to the transformer, or, like me, with 80m of distance, though it is 200m of cable, and the cable is 35mm to the street, where it goes up to 50mm, and there are a trio of 200A fuses in the street cabinet, and another trio of 400A fuses in the substation across the road. Then 20m of 16mm copper, which means the breaker in the meter room has to be rated for 10kA breaking, as anything smaller will not survive. Same for the breakers in the house, as they also rely on cable length to limit current, though the regulations allow you to get by with 3kA rating. The 10kA meter breaker is fine to protect the cable to the house, and the breakers inside are fine for the cable in the walls.

But I have seen socket outlets that have shorted out, and which have blown the cover off, and deformed the steel pressed plate, and also light switches that did the same, with a black skid mark up the wall, and similar inside, with a nice copper coat from the arc. However the breakers, even if some of them were 50 year old Hymag thermal only breakers, did operate to disconnect, though often the breaker was faulty and would not reset. Faulty cords do make a nice bang, and burn a length of the insulation, but generally should trip. However cheap ones, with CCC conductors, often are high enough resistance that a short will not trip a 20A breaker, and will cause a fire.

Guess that is the finding that will come from the last China Mall fire, seems to be a common thing, where the building is wired with Chinese cheap cable, and cheap breakers, with non approved fittings all over, where the cable breaks, and the breakers do not break the circuit. Seems to be a near annual thing, as all of the malls have had a fire at some point in the last decade, size and damage depending on how fast the fire department gets there. 2 closed forever, building condemned, due to the structural damage, and not only from the fire.

[Psi]

but it can cause extended damage to circuitry -- burned traces, pads, damaged nearby components as the ball of plasma sinks current into anything conductive nearby.

You forgot "taking a sizable chunk of metal out of your side-cutters and turning them into wire strippers" 

[Berni]

I also seen a glass fuse in a cheep chinese power supply completely explode from the fault current when that PSU died and shorted out its own input.

All that was left behind was 2 wires coming out of the PCB pads where the fuse was attached to, the rest of the glass fuse was in the form of tiny sand sized pieces all over the insides of the PSU.

This is why fuses have a fault current rating. This one was obviously not sufficient for the amount fault current my wall outlet was willing to provide. It did also trip the 16A breaker in the electrical panel. But i guess being a cheep chinese PSU i can be glad it even had a fuse in the first place.

[SeanB]

but it can cause extended damage to circuitry -- burned traces, pads, damaged nearby components as the ball of plasma sinks current into anything conductive nearby.

You forgot "taking a sizable chunk of metal out of your side-cutters and turning them into wire strippers" 

got those..........

[coppercone2]

oh yeah the old story for fault current in 240V

a fuse exploded in a device (20 amp) that lead to a big patch of metal copper on the PCB and the chassis lid (pizza box)

Nice shiny copper. Total short circuit on the PCB traces, scraped off with a chisel screw driver, and later thrown away.

I would say something like a big splotch of copper about 1.5 inches with soot around it

[Psi]

but it can cause extended damage to circuitry -- burned traces, pads, damaged nearby components as the ball of plasma sinks current into anything conductive nearby.

You forgot "taking a sizable chunk of metal out of your side-cutters and turning them into wire strippers" 

got those..........

Everyone accidently cuts an actively powered mains cable once.
If you're lucky you learn the lesson using your budget cutters and not on the lindstroms.

[NiHaoMike]

Another scary thing for arc flash are the giant UPS units that supply entire buildings. Those have a room full of lead acid batteries that will happily produce many thousands of amps at a few 100 volts. Being DC the arc might take a while to extinguish too.

An EV battery is basically the same, just with much higher energy density.

[Psi]

Another scary thing for arc flash are the giant UPS units that supply entire buildings. Those have a room full of lead acid batteries that will happily produce many thousands of amps at a few 100 volts. Being DC the arc might take a while to extinguish too.

An EV battery is basically the same, just with much higher energy density.

I think they at least have fuses per cell or per module.
At least cars are Lion or LiFe and not LiPo.  The amount of energy you can pull from a shorted LiPo is insane.

[tom66]

Another scary thing for arc flash are the giant UPS units that supply entire buildings. Those have a room full of lead acid batteries that will happily produce many thousands of amps at a few 100 volts. Being DC the arc might take a while to extinguish too.

An EV battery is basically the same, just with much higher energy density.

Surprisingly the prospective short circuit current of a typical EV battery is less than that a 240V domestic mains supply.  It is DC though, which does pose unique risks. 

It does surprise me how many precautions are taken around EV systems compared to domestic electricians. 

[jonpaul]

See IEEE 1584-2018 std.

Great video   IEEE 1584-2018 Arc Flash Incident Energy Calculation Method using ETAP



REAL arc flash incident :



Attention! Risque! Bon chance!

Jon

[T3sl4co1l]

but it can cause extended damage to circuitry -- burned traces, pads, damaged nearby components as the ball of plasma sinks current into anything conductive nearby.

You forgot "taking a sizable chunk of metal out of your side-cutters and turning them into wire strippers" 

[BrokenYugo]

Another scary thing for arc flash are the giant UPS units that supply entire buildings. Those have a room full of lead acid batteries that will happily produce many thousands of amps at a few 100 volts. Being DC the arc might take a while to extinguish too.

An EV battery is basically the same, just with much higher energy density.

Surprisingly the prospective short circuit current of a typical EV battery is less than that a 240V domestic mains supply.  It is DC though, which does pose unique risks. 

It does surprise me how many precautions are taken around EV systems compared to domestic electricians.

If I had to guess, you probably want to overkill the new safety training on a high energy 400+V DC system to people (mechanics) used to playing with 12V DC, which is pretty hard to hurt yourself with, nevermind die instantly. Must compensate for the known complacency of the audience.  Ever see a Fluke meter with low voltage automotive fuses hacked into it? I have.

[David Hess]

You forgot "taking a sizable chunk of metal out of your side-cutters and turning them into wire strippers" 

I ground off the melted end of my diagonal cutters, as shown below.

Just last week I tried to find replacements for these cutters which I have had since I was like 6 years old, but Diamond tools is long gone.  I think they ultimately became part of Swanstrom Tools but wow, Swanstrom are expensive.  Inexpensive tools are $15, expensive ones are $30, and Swanstrom are 4 times that.

My survey of respectable tool makers last week found Knipex (best), Tekton (most economical), Klein, and Williams.

Everyone accidently cuts an actively powered mains cable once.
If you're lucky you learn the lesson using your budget cutters and not on the lindstroms.

The second example below shows that some of us are slow learners.  The pliers are not Lindstroms, but they are at least Swiss.  The conversation went something like this:

"Shouldn't you turn the power off at the circuit breaker?"
*ZOT!*
"I took care of it."

[Siwastaja]

At least cars are Lion or LiFe and not LiPo.  The amount of energy you can pull from a shorted LiPo is insane.

You realize the first of these words is an animal, the second one is something some of us have and others (maybe me included) not, and the third one doesn't exist - none of the three have anything to do with batteries. There is no such a thing as LiPo and as an imaginary device, it isn't any more insane than lithium ion batteries; high power density variants of which you are probably thinking about.

[Siwastaja]

It does surprise me how many precautions are taken around EV systems compared to domestic electricians.

On the other hand, it has to be - it would be super dangerous without. On the other hand, EVs are self-contained devices where all large loads are internally accurately current-controlled so there is no concept of "inrush", except in form of bus capacitance, which is dealt with a separate precharge circuit.

In commercial AC, one can plug whatever random stuff in, so the distribution has to provide enough inrush capability for large motors etc., and on the other hand protect the wiring using user-replaceable fuses, which then need large enough short-circuit current (small enough impedance) so that the fuses actually blow quickly. There is no such a thing in EVs - the only occasion where the main pack fuse would blow is total destruction of the drive inverter caused by a design error, or an external destructive reason like a serious crash. Therefore the fuse can be of quick acting type and dimensioned carefully, quite close to the actual limits, without concerns of nuisance blowing because the current consumed can be exactly controlled - in all other cases except the one where the vehicle is lost cause anyway, and fuse needs to blow to prevent fire.

(Well, quick charging is another case where large currents go through the fuse. That is why the DC chargers need to be designed seriously well, and why all the handshaking is going on in the cable. And sure enough, there are cases where a faulty DC charger has "destroyed" the poor little vehicle, basically by shorting out itself and blowing the pack fuse, and since this is not meant to be user replaced, you need an expensive repair.

[tom66]

I think they at least have fuses per cell or per module.
At least cars are Lion or LiFe and not LiPo.  The amount of energy you can pull from a shorted LiPo is insane.

There are EVs which use LiPos.  For instance the Nissan Leaf and Chevy Bolt.  My ID.3 uses Prismatic cells which are kind of in-between the two technologies.

It really makes no difference. The cylindrical cells that Tesla use have a short circuit current of around 80A each.  In the 84P arrangement that's a prospective short circuit current per module bank of about 6700A, though the resistance of contactors, bond wires, etc. reduces it to around 2000A in the real world.

[Siwastaja]

There are EVs which use LiPos.  For instance the Nissan Leaf and Chevy Bolt.

Yeah. The correct term is "pouch cell" or sometimes "polymer pouch cell" signifiying the fact there is plastic in the vacuum packaged wrapping (it's plastic-aluminum sandwich).

"LiPo" refers to lithium polymer which refers to a technology using solid state polymerized electrolyte. None is commercially available as I know of, and never was. It was supposed to a be a big thing in 1990's but never realized. The name stuck.

[SeanB]

Yes, got some pouch cells that are puffy, going to discharge them and then recycle them.

[Shock]

240v mains supplies enough current to vaporize/instant molten copper in a dead short. That's pretty spectacular and undesirable in your face. Arc/stick wielding is an example of current, I'm sure if you find the right circumstances you can be badly burnt. Hand and internal burns can be quite nasty as well.

No rings or metal watch, wear safety glasses and use safe probing and handling techniques for mains on the bench. Even if you are super careful you should still use safety glasses due to shorts and exploding caps etc.

[Gyro]

Quite timely. Our street is just being rewired with new 3 phase main cabling to replace the old failing paper insulated one. This had been bent too much during original installation, 40 years ago, and has failed in many places over the years. I remember on one occasion, just outside our house, the ground was still steaming when they dug it up and there was a roughly 8 inch stalactite of melted Aluminum down the outside, where the insulation between one phase and the Neutral outer had burned away. This would undoubtedly have been an arc flash event if the whole thing hadn't been embedded in clay soil under the tarmac pavement.

I know these guys are are used to working live in trenches, but when the next upstream fuses are the big ones at the local substation (UK system, no pole pigs), the danger from an accidental arc flash is still significant. It's interesting to watch them work.

[bill_c]

Troubleshooting a 480V transfer switch, I found the problem, but it sure was bright for .03 seconds. Melted all the corners off the copper lugs and many of the  smaller control wires were just gone.  So that is what plasma tastes like...