Arc flash blast, HOLY ****!

[andyvh1959]

Going through our annual training, which this year includes awareness training about Arc flash/blast. Now I don't work on electrical other than some home or garage wiring, but I work in many factory locations (both my employer and customer sites) so I understand the rationale of being aware of the potentials for Arc flash/blast injuries. Unless you know, how can you reduce the risks. Just like being aware of fluid injection injuries if you work on anything with pressurized fluid like hydraulics.

I know we have persons on the forums with work experience in high voltage applications, three phase power and more. Is an Arc flash/blast possible or probable only in high voltage or three phase wiring? Certainly an arc fault is possible in 120v home or garage wiring, which is why the arc fault breakers exist. But can 120v wiring create and arc flash/blast condition? Just curious.

 

[zmotorsports]

Yes, there is enough energy in a 120/240 panel to create an arc flash incident.

Our goal with our panels was to get everything down to a Category 0 in our Arc Flash Analysis. It took a few months and luckily corporate was generous with funding but even with replacing various fuses and other components we still have a few of higher voltage panels that are Category 1's and a panel that is a 2 in which we have to make sure to don full electrical PPE when working on. We have one old panel that is a Category 4 that we are not allowed to open so if that is needed to be worked on we are to call in our outside electrical contractor.

I was involved in an arc blast about 15 years ago and it is some scary ****. Our electrical contractor was working on a 480 3ph panel installing some monitoring CT (Current Transformers). Our management team wouldn't allow us to take the entire panel down so we shut the half down that we were working on. It was an outside panel and was just past dusk so I was standing behind the contractor holding the light for him to see. He was tightening the last lug after installing the last remaining CT and dropped his allen wrench. It dropped down onto a lug block feeding the other half of the panel and blew the wrench in half. The heat and flash that came from the panel was tremendous. I had a severe sun burn on half my face and one arm while my contractor friend had some burns to his arms and ear as he turned his head just in time and closed his eyes as he dropped the wrench. Evidently he was somewhat expecting it and I wasn't.

The EMT's and fire department were called. They transported my buddy to the hospital to be checked out and I was checked out on site and found to be alright other than some "sun burns" as they referred to them.

It was enough to wake my *** up around electrical as I may have gotten a bit complacent from my younger years. My buddy and I still joke around with each other about that night but it was no joking matter and scared the hell out of both of us. Shortly after that our company implemented the Arc Flash Analysis and training.

 

[rlitman]

...But can 120v wiring create and arc flash/blast condition? Just curious.

Not in a residential system, except perhaps within the meter enclosure (and even there, it's more of a theoretical risk than a real one). There just isn't enough current or voltage available to sustain an AC arc on a residential service. The zero-crossings that happen 120 times per second on a 60hz grid extinguish the arc too quickly for it to output dangerous energy levels. Even in commercial systems, 120/208 panels most often are found downstream of relatively small transformers, so the energy just isn't there.

Now on the 277/480V stuff I see at work, arc flash is a very real risk, and the panels are labeled where required. It's especially risky in certain panels fed by computer controlled breakers that may not clear a fault for 5 cycles or more. And the 540VDC battery system we have requires a healthy dose of respect, since DC arcs are far more stable.

 

[MacMcMacmac]

We had an unfortunate squirrel ground a 13.8Kv lug to a transformer case a few years ago. It was like a bomb going off. Video of the incident was impressive, as there was secondary arcing on the pole, and a large crack could be heard from the substation up the road.

 

[cmandp]

My wife does arc flash studies as part of her job. Its basically surveying a buildings electrical system and building a model of it in a special modeling software.

These are industrial/commercial buildings.

The NFPA70/NEC now requires the incident energy for panels down to 120V/208V to be calculated if they are fed by a 30KVA or larger transformer.

As far as residential panels go my wife says there is just not enough energy and copper to be able to create an arc flash.

This is coming second hand and from what I've learned from her so its worth what you paid for it

 

[MBfreak]

I doubt that a 120 or 240 V residential installation will be able to create an arc flash that results in serious damage to a person.
Slight burns and disturbed eyesight for several seconds, yes. And of course a fire hazard.

In high power 240 V circuits there can be 50 kA short circuit current for up to seconds. And SLOW fault disconnection. This will be very dangerous .

In medium voltage indoor panel switchgear it is quite common to install flash detectors that applies a VERY fast acting short/earth swith between all three phases and earth. Flash will be killed in less than a ms and damage to plant minimal. To personnel in the same room only the bang of the short crct device is heard .

In high voltage applicaions there is certainly more than enough short circuit power to create very serious sparks. Several meters long.
However, relay protection at 72 kV and all the way up too 800 kV is very fast and almost always with at least two independent channels, so distyrbances are cleared very fast, usually within 45 ms. Unless the circuit breaker fails, in that case the trip is sent to all adjacent breakers And if it open terminal plant, the live parts are many meters away from personnel and there are no walls that reflect the energy, at leasts 70 % goes upwards in free air.

Ola

 

[zmotorsports]

I guess I should have been more specific as well. The OP didn't specify so I just assumed he was talking about commercial panels and not residential. I was told the same thing that in a residential 120/240 panel there isn't enough stored energy available to sustain an arc. Not so in an industrial panel being fed by a larger source. We have our own sub-station on site and have 13.8kV fed directly from our provider then step it down around the site so a much larger potential.

 

[andyvh1959]

Yup, I was referring to commercial versus residential. Still alarming to see what a high voltage arc flash/blast can do.

Arcing in a 120v circuit can create issues. Decades ago I wired my 1st garage and used metal boxes and conduit. I also did some iffy wire connections in a light switch box that actually positioned a black wire (twisted connection covered with tape) that was close enough to the metal box that on humid summer days it would arc from the wire to the box. And yes, the box was hot. On a humid summer day I could hear buzzing, and my ear lead me to the box. After I turned off the power, I opened the box and found a spark/weld arc spot inside the box where the tip of the hot wire was arcing to the box. After I corrected that the buzzing was gone, the box was not hot anymore, and my monthly usage on the meter dropped a lot. Not sure it would have caused a fire issue, but it could shock someone. Dumb *** younger years.

 

[theoldwizard1]

Not in a residential system, except perhaps within the meter enclosure (and even there, it's more of a theoretical risk than a real one). There just isn't enough current or voltage available to sustain an AC arc on a residential service. The zero-crossings that happen 120 times per second on a 60hz grid extinguish the arc too quickly for it to output dangerous energy levels.

Not really trying to start a debate on this, but sure you can get an arc from a residential 15A circuit. You might even get a burn. Highly unlikely that it would require any type of medical treatment. Blast ? No way !

My gut says in the past 25 years the people who write the National Electric Code have been infiltrated by equipment manufacturers. Do we really need tamper resistant outlets ? I have 6 grandchildren. None have been interested in outlet but maybe it is because my wife has the common sense to use those plastic outlet plugs. Yet, we still allow "back stabs".

 

[u3b3rg33k]

Going through our annual training, which this year includes awareness training about Arc flash/blast. Now I don't work on electrical other than some home or garage wiring, but I work in many factory locations (both my employer and customer sites) so I understand the rationale of being aware of the potentials for Arc flash/blast injuries. Unless you know, how can you reduce the risks. Just like being aware of fluid injection injuries if you work on anything with pressurized fluid like hydraulics.

I know we have persons on the forums with work experience in high voltage applications, three phase power and more. Is an Arc flash/blast possible or probable only in high voltage or three phase wiring? Certainly an arc fault is possible in 120v home or garage wiring, which is why the arc fault breakers exist. But can 120v wiring create and arc flash/blast condition? Just curious.

to clarify, arc fault breakers aren't designed to protect YOU against arc flash in a panel, they're designed for things like "halogen torchiere lamp with arcing bulb/socket contacts", "nicked NM insulation from hanging a picture with a nail" or "super cheap extension cord plug connection sizzling" issues.

 

[cvairwerks]

Arc flash demo at 240 fps....

 

[TractorJeff]

I want to see Marky Sparky do that on a 30 amp breaker, then again on the 240 volt side. LOL!
The whole point is that he did get an Arc Flash type of occurrence.
If he was wiring overhead without safety glasses, he may very well end of with some of those burning bits of Copper in his eyes!

PLEASE!! Take the time to turn off the Breaker on what ever circuit you are working on!

FWIW; I have witnessed the damage of 12kv cable failure in a 1 megawatt generator. It arced until the Regulator shutdown the Generator. Which seemed like FOREVER!!!!

 

[rlitman]

Arc flash demo at 240 fps....

We're discussing two totally different things. From an OSHA and NFPA perspective, that is not an arc flash. Or, in the very least, it is not one that requires any special PPE beyond standard eyewear, or special labeling and precautionary measures. A real arc flash isn't about blinding a camera or spraying molten copper bits. It is about a flash of radiant energy that can set your exposed skin on fire before you even feel it, and that rivals the incident energy from a nuclear blast, at least within a small radius.

For that to happen, you need an extraordinary current to flow, for longer than just 1/120s (at 60hz, the voltage drops to zero 120 times per second), as energy is measured as power over time. 120VAC just doesn't have the ability to do this in real-life situations, as the plasma of the arc cools too much during the zero crossings to be stable. In laboratory conditions, it has been proven that as little as 200VAC can achieve this, given enough current supply (something way beyond the reach of a residential service), but by the time you reach 480VAC (a pretty common commercial voltage), even the current available to many subpanels may be enough.

 

[u3b3rg33k]

Arc flash demo at 240 fps....

hold my beer:

 

[Snipe]

While you may not get arc flash there is enough amperage in 6, 12 and 24 volt automotive systems to do serious damage. I always take off my wedding ring and watch when I work on my wiring. They are good conductors and can seriously burn you if they short out to ground.
The hospital I worked at was heavy into arc flash protection. Had to have special equipment. Worked with 3 phase motors and emergency generators.

 

[Bert_]

I can be a bit careless in 240v panels and equipment. The small stuff is hard to screw up bad enough to cause arc flash.

I've got six breakers to add to a 1200A 480v panel in a few weeks. You can bet I'm going to shut it down. Dropping a screw while bolting in a breaker could have very bad results.

 

[pcmeiners]

Caused a 240 arc flash in a panel, other than slight hand burn, near heart attack, a minute of temporary blindness, it was a learning experience

"We had an unfortunate squirrel ground a 13.8Kv lug to a transformer case a few years ago. It was like a bomb going off."

Was a foreman on a construction site where a backhoe went through a multi megawatt lead encased feeder line ( 3 megawatt if I remember correctly) in a Brooklyn street. The bucket had about 1/3 of the metal missing afterwards, guessing a few hundred pounds of metal. I felt the shock wave from around the corner of the building. Guy was not hurt as the explosion effected the lower right part of the bucket, basically shielding the operator from the blast, UV and melted metal. The explosion literally moved the ground I was standing on.

 

[rlitman]

...Was on a foreman on a construction site where a backhoe went through a multi megawatt lead encased feeder line ( 3 megawatt if I remember correctly) in a Brooklyn street. The bucket had about 1/3 of the metal missing afterwards, guessing a few hundred pounds of metal. I felt the shock wave from around the corner of the building. Guy was not hurt as the explosion effected the lower right part of the bucket, basically shielding the operator from the blast, UV and melted metal. The explosion literally moved the ground I was standing on.

So this was your fault?

 

[pcmeiners]

.

 

[pcmeiners]

I swear I was not in Queens that night, I have witnesses.

During Sandy Staten Island had a fireworks show for hours due to hundreds of transformers over-loading, looked somewhat like the Queens transformer. On my street alone I watched 3 transformers blow while sitting in my pool chair in 15" of water on my front stoop....kind of a fun night even with knee high water in my living room.