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Old 08-24-2019, 07:48 AM   #1
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Default Arc Flash in an MCC

If your operating a breaker in an MCC with the door of the bucket closed I understand that the door is designed to contain the arc flash.
My company wants us to wear 65 calorie suits to operate breakers for arc flash potential but I believe there is no potential with the doors closed and locked. Is this correct?
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Old 08-24-2019, 08:08 AM   #2
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Check out this video. I am sure the likelihood of an arc flash is small when turning the breaker off but do you want to take that chance.




Here is something else that states what I said but...

Quote:
Do you need PPE for Operating Breakers
“(Are) operating breakers – opened/closed considered an arc flash exposure even if the hinged cover is on?”
NFPA’s 70E addresses that question in the “Informational Note No. 1 under the definition of “Arc Flash Hazard”:
An arc flash hazard may exist when energized electrical conductors or circuit parts are exposed or when they are within equipment in a guarded or enclosed condition, provided a person is interacting with the equipment in such a manner that could cause an electric arc. (Underlining is mine) Under normal operating conditions, enclosed energized equipment that has been properly installed and maintained is not likely to pose an arc flash hazard.
The two key parts of this statement are:
1. Whenever a person interacts with electrical equipment, such as an opening/closing operation, even though the door is closed or the cover is on, an arc flash hazard may exist.
2. If the equipment has been properly installed, according to the National Electric Code, and properly maintained, using such resources as NFPA’s Standard 70B and the International Electrical Testing Association’s Standard for Maintenance Testing Specifications, an arc flash hazard is not likely to occur.
A person needs to take into account the age of the equipment, the design of the equipment, as well as its installed and maintained condition in order to determine if there is an arc flash hazard when open or closing a circuit breaker.

A similar response was given in an OSHA letter of interpretation:

02/29/2008 – Whether employees who are verifying that an electrical system is de-energized or are turning off circuit
breakers are required to use personal protective equipment.
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Old 08-24-2019, 08:26 AM   #3
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Quote:
Originally Posted by Jmoney View Post
If your operating a breaker in an MCC with the door of the bucket closed I understand that the door is designed to contain the arc flash.
My company wants us to wear 65 calorie suits to operate breakers for arc flash potential but I believe there is no potential with the doors closed and locked. Is this correct?
hi Jmoney
i have seen the consequences of a 11kv breaker exploding when it was operated by n person in front of it and the person and the breaker went through the substation wall so the operating suit didn't help BUT this was a different situation
the lesson i learned - take the punishment of the suit because you can not believe the power of a short circuit current, and rather be safe than sorry
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Old 08-24-2019, 09:09 AM   #4
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Hi all,

So I'm under the impression nobody is sure if doors work or not. That's good to know at least cause I was taught the doors do withstand the blast. I thought manufacturers were required to build the cabinets to withstand the arc blast but I guess they are not.
My plant is 8 years old so an arc flash is less likely right now but no I don't want to take a chance unless Im sure.
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Old 08-24-2019, 09:17 AM   #5
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In that video he’s testing a disconnect and not a typical MCC bucket. An MCC cabinet has locking pins on it so it’s a different animal. I’m still not 100% sure but I guess it’s best to err on the side of caution.
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Old 08-24-2019, 11:25 AM   #6
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hi Jmoney
i have seen the consequences of a 11kv breaker exploding when it was operated by n person in front of it and the person and the breaker went through the substation wall so the operating suit didn't help BUT this was a different situation
the lesson i learned - take the punishment of the suit because you can not believe the power of a short circuit current, and rather be safe than sorry

If there is enough force to send a human thru a wall of a substation , can somebody explain to me how a space suit is going to make this person survive the trip?
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Old 08-24-2019, 01:14 PM   #7
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If you honestly have a 65 calorie potential even with the door open that's got to be a scary place to work. What voltage is the MCC.
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Old 08-24-2019, 01:20 PM   #8
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If there is enough force to send a human thru a wall of a substation , can somebody explain to me how a space suit is going to make this person survive the trip?
The arc flash was not the problem and the suit could handle that.

No suit is designed to handle being hit by a motivated lump of MCC gear and my best guess would be that something like the back plate was not secured to the manufactures instructions for this to happen.
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Old 08-24-2019, 01:29 PM   #9
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In that video he’s testing a disconnect and not a typical MCC bucket. An MCC cabinet has locking pins on it so it’s a different animal. I’m still not 100% sure but I guess it’s best to err on the side of caution.
The difference is not the door pins its more to do with the rapidly expanding air having a way to expand before the door fails. In a MCC the air has plenty of room to expand which also means if you do have a massive flash that didn't blow the door off you are now looking at a few days work cleaning every bucket in the bank.
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Old 08-24-2019, 01:30 PM   #10
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If there is enough force to send a human thru a wall of a substation , can somebody explain to me how a space suit is going to make this person survive the trip?
The space suit aids in identifying the body.
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Old 08-24-2019, 02:07 PM   #11
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If you honestly have a 65 calorie potential even with the door open that's got to be a scary place to work. What voltage is the MCC.
The breakers are 480v to 69kv.
I’m dealing with 480 in this case. They want to go to a standard suit for all work which is 65 calorie. I think it’s wrong cause a standard 480 MCC should contain an arc flash if the doors are closed but I can’t find the proof. I still think the suit is only needed on live front work but I don’t know for sure.
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Old 08-24-2019, 02:09 PM   #12
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hi Jmoney
i have seen the consequences of a 11kv breaker exploding when it was operated by n person in front of it and the person and the breaker went through the substation wall so the operating suit didn't help BUT this was a different situation
the lesson i learned - take the punishment of the suit because you can not believe the power of a short circuit current, and rather be safe than sorry

If there is enough force to send a human thru a wall of a substation , can somebody explain to me how a space suit is going to make this person survive the trip?

The suit is designed to absorb high heat not force
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Old 08-24-2019, 02:48 PM   #13
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Some doors are designed to help contain the arc flash and arc blast, others are not. We refer to gear designed to contain arc incidents as dead front gear, its very beefy stuff. Not your typical thin metal with flimsy doors that bend and flex. My guess is that your gear is not designed to contain arc incidents to the degree needed to not wear a flash suit.

The gear maybe designed to contain an arc incident, but I would double and triple check before deciding that it is.

Unfortunately for the most part as others have said, the suit is for the flash, not the blast.

Near and above 40 cal, the suit is the difference between an open casket or a closed casket funeral, that's it.
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Old 08-24-2019, 03:20 PM   #14
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The breakers are 480v to 69kv.
I’m dealing with 480 in this case. They want to go to a standard suit for all work which is 65 calorie. I think it’s wrong cause a standard 480 MCC should contain an arc flash if the doors are closed but I can’t find the proof. I still think the suit is only needed on live front work but I don’t know for sure.
Ok here's the deal. Doors do not matter. The pressure created by heating up the air in an arc is so high it blows the doors off if it takes longer than 1-2 cycles to trip. Only fuses are that fast not breakers. Unless it's arc resistant gear in which case there is an escape vent...A "blast door" that blows out first. This happens in the first cycle or 2 so 0.015-0.030 seconds. No gear except arc resistant types can handle it. Plus if you're working on the equipment, what position is the door in? Not closed. So the arc resistant stuff does nothing for electricians. It only matters operating breakers which i will get back to. There no recorded cases of arc blast injuries from the air blast. Hit by pieces and parts but arc blast is over once it gets out of the panel. Highest pressure recorded in the lab is 8-10 PSI before panel ruptures. Fatalities from military explosions need 20+ PSI minimum. So with a size 2U bucket it's a 16" x 16" door. So 10 PSI is 2560 lbs. of pressure. No tiny 1/8" door pin is holding that. There are no valid lab pressure measurements outside the panel until recently (under 1 PSI). Arc blast is largely mythical. Video of arc flashes shows victims are "pushed" or jump back after 0.2 to 0.5 seconds. It's not arc blast, it's the flight mechanism kicking in. There was a stupid myth about 40 cal suits a few years back but it was just another outright lie.

Second you might have heard of AIC. This is the short circuit rating. It's a magnetic force thing...make sure the equipment doesn't blow apart from the force of a short including keeping the doors on. Shorts are kind of boring from a "blows up" point of view even if it fails. Kind of a bang/rip and it's don't with very little smoke and heat. Because the energy travels through the conductors. In an arcing fault electricity is trying to travel through air...not a great conductor to say the least so the air gets wicked hot really fast.

Also progress has been made on better equipment. Square D has some Masterpact NW draw out gear with arc chutes on the bus that are truly arc flash free. Very cool if you are putting in new. I believe some GIS gear is similar. Hard for an arc in an inert atmosphere you can't touch.

Third as mentioned in 70E Working On and Operating are two totally different activities. Working on live equipment is dangerous. If you drop a tool into energized bus, guess what can and often does happen? What about blind fishing or blind reaching into an energized space? How about drilling out a live bus in a 480 panel to accept a breaker from a different manufacturer? I'm not even going to comment on the Code violations here. All I will say is I know someone who used to do this and that it has been documented multiple times by OSHA. I can go on but you get the idea. The worst issues are that some equipment by design has big issues inserting it into a live bus. Some bus plugs, MCC buckets, and even draw out buckets are notorious for this. Some designs are better than others. Some Square D MCCs have a separate mechanism that plugs and unplugs the bucket from the bus once it is inserted and bolted in place. Very different from say some old Allis breakers where the bus arms were longer than the space between them so that one arm can and has folded into the other one if they are misaligned.

There has been once that I know of in places I've worked and once documented by OSHA where just operating a breaker caused an arc flash. First one was an operator repeatedly reset a breaker that kept tripping in an MCC that fed a pump. Breaker was 25+ years old and never inspected or tested. Electrician comes and resets it several times too before FINALLY meggering motor...dead shorted. Still no breaker inspection. Motor replaced. Electrician reset breaker and boom, takes a trip to the burn unit. Second one was at a parking deck. Electrician worked on panel and just kind of jammed some sheet metal strips in between the outer and inner cover to fill a hole. Later attendants throw breakers to shut off lights at end of shift. One of the metal strips falls into the bus and panel blows up sending attendant to burn unit.

I know of no cases of operating a breaker that was properly maintained spontaneously exploding. Not in the 5 years of OSHA data going through every documented case I did not in the 10 years if data Caulesly did, not in the 55 cases of arc flash Neal documented from DuPont data. The issue with molded case breakers at least is that they are only built to trip a limited number of times, usually 50-100. When I'm testing them and I get a marginal one it often literally smokes before it fails.

The breaker instructions tell you to visually inspect after EVERY trip and then once a year for good measure. It takes all of 30 seconds and if it tripped you're already working on it anyways so why not. Greasing, testing, etc., is for main and critical breakers, especially switch boards and draw out gear. But the simple tests for molded case breakers take so little time you cn easily properly maintain everything.

Also there is another gotcha to this. 70E nd IEEE 1584 state that the calculated arc flash is absolutely meaningless if you don't properly maintain your equipment. It is based on that concept. You can't calculate what a breaker does if it isn't working properly. It might trip too soon, too late, or never. It's all based on the "upstream" device. So if I'm working on an MCC bucket the breaker feeding that bucket is what protects me, not the one I'm in. It can be 100% defective as long as everythimy upstream works properly. This is also how arc flash works. The calculated value is based on the upstream wiring, transformer, breaker, etc.

65 cal seems excessive. If you go over 12 ATPV there is a severe likelihood of heat exhaustion. Also your dexterity is nil and you can't see what you are doing with something like 75% blocked light. IEEE has documented cases of refineries, paper mills, and similar operations making some equipment changes to get to 8-10 cal maximum and have proven it can be easily done. As an example of one of the major issues is a plant had an I-Line panel fed directly from a 1500 kVA transformer. Arc flash was like 200 cal there. So we just used the 4160 disconnect on the transformer primary where it was like 3 cal. Sounds like your engineers just plugged numbers in a program and got numbers out without bothering to check if they were right, realistic, or not.

Keep this in mind. The arc flash on a 4160 V, 1500 kVA transformer is about 10 times lower than on a 489 V, 1500 kVA transformer. On the 69 kv bus it will be almost a nonhazard. On the 69 kv bus though watch out for the type of model. If it says Lee or 1584, it was done incorrectly according to OSHA. It must be done with ArcPro. This is true of anything over 15 kV. The Lee model has some serious errors in it. At 15 kv it is at least off by 300% and gets worse as voltage goes up.


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Old 08-24-2019, 03:25 PM   #15
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Some doors are designed to help contain the arc flash and arc blast, others are not. We refer to gear designed to contain arc incidents as dead front gear, its very beefy stuff. Not your typical thin metal with flimsy doors that bend and flex. My guess is that your gear is not designed to contain arc incidents to the degree needed to not wear a flash suit.

The gear maybe designed to contain an arc incident, but I would double and triple check before deciding that it is.

Unfortunately for the most part as others have said, the suit is for the flash, not the blast.

Near and above 40 cal, the suit is the difference between an open casket or a closed casket funeral, that's it.
Dead front is very different. Back in the days of open switches and breakers and I'm talking 1930s or earlier that was live front. Everything is dead front today except outdoor overhead style gear like cutouts on open wire pole line or tubular bus circuits.

What you described is arc resistant. And it is NOT capable of withstanding an arc blast either. It just vents it.

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Old 08-24-2019, 03:53 PM   #16
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Neal from Dupont collected 'about 55 cases of bonafide arc flash incidents which is the largest known collection with pretty detailed data. The data has documented that about 15% of the time victims without FR PPE walk away. With arc rated PPE even if is under-rated it goes up to 50% at any incident energy. If it exceeds incident energy it's 100%. By this I mean no major injuries. Still minor burns. So you can basically "wing it" and still get some benefit.

I believe most of the improvement in electrical safety isn't in the PPE anyways...It's avoiding doing energized work and using better tools and techniques. You can do a lot more troubleshooting with an ohm meter with the power off than most people realize. If you put your mind to the idea of doing everything you can do testing while dead, you can test every fuse, megger the motor, do a lot of contactor inspections and testing, check breaker continuity, test for shorts and opens in wiring, test grounding, test coils, test transformers, test buttons, test aux contacts, test a lot of control circuit things, with no power and no risk. It also builds confidence if/when you do get to energized testing about what is and isn't working. Sure I might have to isolate a neutral or pop out the fuses but I can go a lot faster testing dead equipment because I'm not limited in what I can touch or do. It's more of a matter of changing your habits.

I did not say NO live testing. I can't measure voltage or current on a dead circuit. The last step in an electrical LOTO is ALWAYS test for voltage. This is especially true with controls. There are almost always some 120 V circuits coming from somewhere else to read dry contact feedback signals in most MCC buckets.

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Old 08-24-2019, 11:27 PM   #17
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Dead front is very different. Back in the days of open switches and breakers and I'm talking 1930s or earlier that was live front. Everything is dead front today except outdoor overhead style gear like cutouts on open wire pole line or tubular bus circuits.

What you described is arc resistant. And it is NOT capable of withstanding an arc blast either. It just vents it.

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Good to know!

The way it is talked about at work I thought dead front had to do with the rating of the door and enclosure, happy to learn the difference.
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Old 08-25-2019, 02:13 PM   #18
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No the plant engineers didn’t come up with arc flash rating. That came from corporate and they wanted to eliminate confusion on what to wear when opening and closing breakers so they went to 65cal suit to cover all bases. Of course in a power plant you have a large range of voltages as you could imagine. I’d say 95% of what we operate to LOTO equipment is 480v, 20 amp breakers. I’m not sure the bucket size but it’s the small ones. I just think they went overboard making us wear a 65cal suit to operate it since it was only rated at 4 cal flash rating. I’m not there now to confirm the exact rating.
As for working non-energized I agree I hate working on live equipment but it happens. In 1980 almost 80% of distribution utility work was done unenergized and now days 80% of work is done energized. There’s a big shift to keep power on to save money I think.
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Old 08-25-2019, 09:02 PM   #19
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Good to know!

The way it is talked about at work I thought dead front had to do with the rating of the door and enclosure, happy to learn the difference.
Here is live front:

http://dismuke.net/ftworthforum/brownwood/electric.jpg

Old knife switches.

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Old 08-25-2019, 09:06 PM   #20
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No the plant engineers didn’t come up with arc flash rating. That came from corporate and they wanted to eliminate confusion on what to wear when opening and closing breakers so they went to 65cal suit to cover all bases. Of course in a power plant you have a large range of voltages as you could imagine. I’d say 95% of what we operate to LOTO equipment is 480v, 20 amp breakers. I’m not sure the bucket size but it’s the small ones. I just think they went overboard making us wear a 65cal suit to operate it since it was only rated at 4 cal flash rating. I’m not there now to confirm the exact rating.
As for working non-energized I agree I hate working on live equipment but it happens. In 1980 almost 80% of distribution utility work was done unenergized and now days 80% of work is done energized. There’s a big shift to keep power on to save money I think.
I don't just imagine. I have power plant customers. The breaker size doesn't matter it's the upstream stuff that does. Power plants usually have extremes since everything has such low impedance tripping is mostly instantaneous or very, very slow.

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