CR4 - The Engineer's Place for News and Discussion®

Previous in Forum: Cable Size   Next in Forum: Determination of required capacitor rating from Power Company's Invoice.
Close

Comments Format:






Close

Subscribe to Discussion:

CR4 allows you to "subscribe" to a discussion
so that you can be notified of new comments to
the discussion via email.

Close

Rating Vote:







20 comments
Active Contributor

Join Date: Jul 2008
Location: pittsburgh
Posts: 12

480 volt arch flash from fused disconnect box

07/21/2008 3:07 PM

Have you ever witnessed it?

I have taken voltage and amperage readings in fused disconnect boxes dozens of times while circuits are energized, without rubber gloves or rubber mats. i typically secure the panel door and i avoid contacting metal ground. my only protection is typically Timberland Pro Shoe with electrical hazard protection.

i want to know what the likelihood is of an arch flash from 480v in a typical fused disconnect box that meets NEC. 480v is as high as i come close to.

__________________
-- tony
Register to Reply
Pathfinder Tags: 480v arch flash fused disconnect box
Interested in this topic? By joining CR4 you can "subscribe" to
this discussion and receive notification when new comments are added.

Comments rated to be "almost" Good Answers:

Check out these comments that don't yet have enough votes to be "official" good answers and, if you agree with them, rate them!
Guru
New Zealand - Member - Kiwi Popular Science - Weaponology - New Member Engineering Fields - Power Engineering - New Member Engineering Fields - Electrical Engineering - New Member

Join Date: Sep 2006
Location: Auckland, New Zealand
Posts: 6384
Good Answers: 254
#1

Re: 480 volt arch flash from fused disconnect box

07/21/2008 4:05 PM

Ignoring the fact that the disconnect box meets NEC regulations, I would say extremely high. Any careless, distracted or inexperienced electrical worker knows that a dropped spanner or misplaced hand can result in disaster, and any 480V circuit could have the potential of producing an arc.

Be warned, the current path doesn't have to be between your hand and the ground under your well-insulated foot, it can also be between your left hand and your right which is conveniently holding the earthed metal enclosure. And of course the fuses are not going to protect the incoming side of those fuses if you accidental apply a short there (unless proper guards are present and in place).

Always take full precautions. Short-cuts lead eventually lead to accidents, no matter how careful you are. I always wear a full arc face mask and light arc suit when operating an enclosed SF6 33kV switch and safety glasses even when working on the 230V test bench (ever been hit in the eye by the can of an exploding dry electrolytic capacitor?).

God help you if you are wearing synthetic fibre clothing during an arc flash event on a main distribution board.

I am sure there are others here who know of a colleague who has had an accident they could describe in detail. Otherwise you could refer to your local electrical accident register (or what ever it is called over there).

More information and videos of actual electrical accidents and arc flashes can be found on the internet and U-Tube.

__________________
jack of all trades
Register to Reply
Power-User
United States - Member - New Member

Join Date: Mar 2007
Location: Wisconsin USA
Posts: 136
Good Answers: 7
#2

Re: 480 volt arch flash from fused disconnect box

07/21/2008 5:11 PM

About ten years ago I was on a job-site where an electrician dropped a screwdriver while making a hot connection in a 480 V panel. He was released from the hospital a couple of days later with no permanent injuries, but the flash was incredible!! It gave me a whole new respect for dealing with 480V.

__________________
Troy
Register to Reply
Power-User
Hobbies - HAM Radio - New Member United States - Member - New Member Hobbies - Hunting - New Member

Join Date: Jun 2008
Posts: 442
Good Answers: 7
#3
In reply to #2

Re: 480 volt arch flash from fused disconnect box

07/21/2008 10:33 PM

JoAT's-Be warned, the current path doesn't have to be between your hand and the ground under your well-insulated foot, it can also be between your left hand and your right which is conveniently holding the earthed metal enclosure.

I think that is why I have been told that it is a good idea to keep your left hand in your pocket--Path to ground/through your foot is kindest to the heart through the right arm(Not practical, but worth mentioning).

Scary when you think about it--IT is invisible and will kill you if give it a chance. 120v is as deadly as 480v, or more so, as there is more of it and we think we can handle it differently. One thing that is worth considering--Your 480V flash is most likely going to only be 277v. Or to rephrase--if you are concerned about being electrocuted by 480v, it's most likely going to be 277v maximum potential. You would have to be above ground and be common with one leg and accidentally short yourself between it and another leg---There's 480v and it would be hard to do. Most arcs in 480v systems seem to fault to ground (277v)from conductors being damaged.

If you are taking chances while working on 480v systems while energized, Here's an idea and I have thought about doing this when I start pushing that envelope--Take a conductor and wrap it around your elbow or wrist and ground it so while you are working on the energized system, if you do come in contact with a live conductor it will just blow part of your hand or arm off and not cook your heart or organs, but you will get to go home eventually.

It is hard to get the job done--The best thing is to be able to judge situations and not have your face/hands or any other parts to where they can answer the ARC FLASH QUESTION intelligently. I will try hard to be safe and I hope you will too.

__________________
Those who believe in telekinetics, raise my hand --Kurt Vonnegut Jr.
Register to Reply
Commentator

Join Date: Mar 2007
Location: Chicago suburbs, USA
Posts: 60
Good Answers: 14
#4

Re: 480 volt arch flash from fused disconnect box

07/21/2008 10:43 PM

Be afraid - be very afraid, to do work on a live 480 volt circuit without wearing appropriate protective gear. I have a short video clip of an example of a 480 volt arc flash on my web site. This was done in a power lab on the West Coast as part of an employee safety program. It shows the results of a brief short circuit that lasted for less than one second. The average current was 17 kA (with current peaks of 30 kA). Read the text below the video for more details. The risk is far more than electrical shock.

http://www.capturedlightning.com/frames/longarc.htm#480_volt_arc_flash

Bert

__________________
Stoneridge Engineering - Wreaking Havoc with Electrons for over 40 Years!
Register to Reply
Commentator

Join Date: Mar 2008
Posts: 57
Good Answers: 3
#5

Re: 480 volt arch flash from fused disconnect box

07/22/2008 9:11 AM

A few years ago an electrician who worked for me was attempting to change a circuit breaker "HOT" in a 480/277volt 3 phase panel-board, some how dropped the buss screw, it caught between two phases, blew up the panel, took out the primary fuses up stream of the utility company transformer on the utility pole.

Some how the arc and pending explosion barely missed him, left his ears ringing and flash blinded for a few minuntes.

Also OSHA E90 requirements are that you are to be suited up with a fire proof suit suitable for the use, gloved, head cover, and face shield.

You are tempting faith my friend.

I also have two other stories about 480volt switchgear explosions, one where three people were seriously burned.

Register to Reply
Anonymous Poster
#6
In reply to #5

Re: 480 volt arch flash from fused disconnect box

07/22/2008 9:31 AM

wow - changing a hot circuit breaker - isn't that suicidal?

i saw a friend get hit by 277 volts while measuring power at a light fixture in a school. he was taking a voltage reading, but he touched a wire (either hot or neutral) with his finger. it jolted him pretty good for a second. the ground was his shoulder touching something metal. he later found a red spot on his shoulder. he finished work that night despite the injury, but his shoulder was sore the next day.

Register to Reply
Guru
Popular Science - Cosmology - New Member Technical Fields - Technical Writing - New Member Engineering Fields - Energy Engineering - New Member Engineering Fields - Electrical Engineering - New Member Engineering Fields - Control Engineering - New Member Engineering Fields - Electromechanical Engineering - Old Member, New Association

Join Date: Apr 2008
Location: Lexington, KY
Posts: 1038
Good Answers: 56
#7

Re: 480 volt arch flash from fused disconnect box

07/22/2008 9:48 AM

Now that everybody has gotten your attention, note that disconnect boxes are designed with arc suppressors. They are intended to stop an arc in the event that interrupting the circuit starts it. That being said, you should not be interrupting the circuit with the door open.

I watched a live (automatically actuated) test of what happens when a disconnect box interrupted a heavily inductive overloaded circuit with the arc suppressors removed. It blew the front door across the room and there was not much left of the metal inside. Moral of the story is don't modify any disconnect equipment.

__________________
A great troubleshooting tip...."When you eliminate the impossible, whatever remains, however improbable, must be the truth." Sir Arthur Conan Doyle
Register to Reply
Active Contributor

Join Date: Jul 2008
Location: pittsburgh
Posts: 12
#8

Re: 480 volt arch flash from fused disconnect box

07/22/2008 9:55 AM

great stuff - so i guess the better question is, for those of you who measure voltage and amps on energized "low voltage" circuits inside buildings, what kind of protective equipment do you use when taking measurements at fused disconnect boxes?

__________________
-- tony
Register to Reply
Guru

Join Date: Jun 2006
Posts: 503
Good Answers: 8
#9

Re: 480 volt arch flash from fused disconnect box

07/22/2008 11:13 AM

I too have been guilty of this unsafe behavior - but go to http://www.westexinc.com/flash_video/westex.html

and you will find that you need full flash gear when working in a hot 480V box!

A person can tell you but watch the flash videos at this site.

Cotton may not catch fire as readily as synthetic materials but it burn hotter longer.

Enjoy!

Register to Reply
Commentator

Join Date: Apr 2008
Posts: 73
Good Answers: 10
#10

Re: 480 volt arch flash from fused disconnect box

07/22/2008 2:05 PM

It doesn't matter how many times you may have done this before and been just fine. That is the main comment of people involved in industrial accidents, "But we've always done it that way," or "I've done that hundreds of times before without any problems." At one of the places I worked we had an electrician who did as you apparently do. A circuit flashed on him. He lived but was horribly burned and his one arm was blown completely open from the middle of his hand nearly to his shoulder. He was in the hospital a long time and had several sureguries to repair his hand, his arm and do skin grafts on hands, arms, face, chest etc. He never came back to work that I am aware, but he very likely was fired anyway because he had ignored the company safety policy and procedures for that kind of work. If he had followed it, he would not have been injured. I'm sure that he believed as you seem to that it would never happen to him.

Absolutely wear the proper protective gear! Absolutely follow all of the safety procedures! It only takes once and taking short cuts is not worth your life or your quality of life!!!

By the way, if you are securing the grounded door with one hand (or bump it with one hand) while the other hand is probing a 480V circuit and it arcs on you, the current will pass directly through your heart and you are dead. Period. If I have scared you, GOOD!

Register to Reply
Guru

Join Date: Jun 2006
Posts: 503
Good Answers: 8
#11

Re: 480 volt arch flash from fused disconnect box

07/22/2008 2:29 PM

Just because you are working on a fused box and are working on the load side of the fuses does not protect you. I have seen fuses explode when presented with a dead short. The fuse box exploded right along with the fuses. If someone had been in the line of fire they would have died!

Register to Reply
Active Contributor

Join Date: Jul 2008
Location: pittsburgh
Posts: 12
#12

Re: 480 volt arch flash from fused disconnect box

07/23/2008 4:45 PM

Thank you everyone for the graphic stories. i appreciate it.

Just to clarify for a couple of you since there seems to be some confusion, when i say "secure the door"...that means to secure the door in the open position so as NOT to make any contact with it.

Now back to question # 2 (and this question is only directed to those of you who actually take amps and volts measurements at 480v fused disconnect boxes inside buildings), what PPE have you used in the past other than appropriately insulated tools? FP clothing? full protective mask? rubber gloves? rubber mat? electrically insulated boots? all of the above? consider this a "confession" if you will for those of you who have actually taken measurements inside buildings at 480 v disconnect panels...have you always worn the full protective body gear?

thanks for sharing :)

__________________
-- tony
Register to Reply
Power-User
Hobbies - HAM Radio - New Member United States - Member - New Member Hobbies - Hunting - New Member

Join Date: Jun 2008
Posts: 442
Good Answers: 7
#13
In reply to #12

Re: 480 volt arch flash from fused disconnect box

07/23/2008 7:40 PM

I couldn't count the the times without any, but there can never be enough caution even with.--You sound knowledgeable and qualified, but in defense of the sharp responses to your post--You sounded reckless and maybe you aren't--The criticism some gave you will not hurt, but not heeding it may.

Using the proper PPE with energized circuits versus disconnecting the power and working on them without it, is an unloaded gun, PERIOD. I have caught dozens of welded contacts in disconnects(fuse-able or not), breakers, contactors and motor starters. By rights that PPE should be implemented even after the power is discharged until every conductive path is checked--Not just between phases, but to earth ground. It may not be code, but we all should, for the sake of anyone who doesn't want you dead, treat any conductors as energized no matter the voltage or the state of any disconnecting means.

"Unloaded" guns are so dangerous--False sense of security unless you thoroughly check.

__________________
Those who believe in telekinetics, raise my hand --Kurt Vonnegut Jr.
Register to Reply
Anonymous Poster
#20
In reply to #12

Re: 480 volt arch flash from fused disconnect box

01/22/2009 8:23 PM

Not bragging by no means here, but I have done electrical work from 90 vdc to high voltage distribution work as an apprentice lineman.

I have been hit by all of it except the high voltage( Thank God)...oh ya lets get one thing straight right now for all you people who think that anything over 50 volts ac is hight voltage, if you do you do not work with it you study it from a book.

I have arc busses and nevere had my arm blown off( like the one guy said happened)

and if it is going to blow your arm off I really don't believe a suit is going to help with that, maybe flash burn but not explosion.

The suit are for working in distribution panel and switch gears not to check amps on a motor disconnect...

If you "Electricians" are that scared of your job...oh ya we are reckless i forgot... Then maybe you should of been plumbers.

I am so tired of hearing about this arch flash stuff, people are getting it all worked up.

It is a good thing but only for the right applications...please pretty soon OSHA will have us wearing this gear to change a light bulb.

Register to Reply
Active Contributor

Join Date: Jul 2008
Location: pittsburgh
Posts: 12
#14

Re: 480 volt arch flash from fused disconnect box

07/23/2008 8:29 PM

Understood - however do any of you actually take amps and volts in low voltage disconnects on a regular basis?

If so what PPE were you wearing the last time you measured at a 480v disconnect?

It doesn't have to be a confession. Let's call it a "survey."

I understand the safety codes. I am just curious what you were actually wearing the last time you took amps and volts at a 480v disconnect?

__________________
-- tony
Register to Reply
Active Contributor

Join Date: Jul 2008
Location: pittsburgh
Posts: 12
#15
In reply to #14

Re: 480 volt arch flash from fused disconnect box

07/24/2008 2:42 PM

No body takes amps and volts in low voltage disconnects on a regular basis?

__________________
-- tony
Register to Reply
Power-User
Hobbies - HAM Radio - New Member United States - Member - New Member Hobbies - Hunting - New Member

Join Date: Jun 2008
Posts: 442
Good Answers: 7
#19
In reply to #15

Re: 480 volt arch flash from fused disconnect box

10/11/2008 12:43 AM

I just got done with 70E arc flash training--Are you still there?

I am sorry.

As an admission of guilt I have(in the past only) taken those readings and on a regular basis and way more dangerous things than that. I didn't want to answer this way until now--There were considerable errors in those ways.

I have specific information that I would be happy to share--including what I used to do and now recommend not doing.

__________________
Those who believe in telekinetics, raise my hand --Kurt Vonnegut Jr.
Register to Reply
Guru
New Zealand - Member - Kiwi Popular Science - Weaponology - New Member Engineering Fields - Power Engineering - New Member Engineering Fields - Electrical Engineering - New Member

Join Date: Sep 2006
Location: Auckland, New Zealand
Posts: 6384
Good Answers: 254
#16
In reply to #14

Re: 480 volt arch flash from fused disconnect box

07/24/2008 4:02 PM

Safety glasses a full length cotton lab coat, using 1 hand and assuming that there could be an explosion whenever I close a circuit (it happens more often than you might think). I have a full arc face mask but think that is a bit excessive unless I am working with our 11kV power feed.

Hey, its a hell of lot more than what our site electrician was wearing - a shirtsleeve shirt and no PPE. There was a carbonised path fault on the bus bar and the board arced over when he closed the switch (located above the board) and was standing right in front of it. BANG. 10 minutes later he reset the circuit and closed the switch again. BANG.

He went and got his supervisor (he was working alone) after I came down to see what all the noise was about. I also gave his supervisor a pamphlet on the wide range of PPE and arc flash gear we sell. Don't come back on site unless you have the proper protection thanks. He was damn lucky the upstream breaker operated to limit the fault current.

Whenever I train anyone (or prospective electricians) the first thing I do when they open the equipment to make a measurement or change a fuse is point and say "look where your hand is, that is live metal there. Always assume that it is live unless you have personally disconnected and disabled the equipment AND taken a voltage measurement. You touch that you die, end of story. Never forget that.", also "always assume that anything you go near has the potential to explode. Violently. wear the protective gear assuming that it could when you are standing next to it or have your head in it".

__________________
jack of all trades
Register to Reply Score 1 for Good Answer
Anonymous Poster
#17

Re: 480 volt arch flash from fused disconnect box

07/26/2008 11:11 PM

I don't know if you are licensed or not, but most all states have gone 70e compliant. In the state of Va it is now law. All Employers will provide 70e protective equipment to all Electrical workers, Gloves, hoods, 70E clothing at minimum Category 2 along with setting ARCH Flash boundries. I would hope that you refer to NFPA 70E as a referance.

Register to Reply
Participant

Join Date: Jul 2008
Posts: 2
#18

Re: 480 volt arch flash from fused disconnect box

07/29/2008 6:09 AM

wow ! u just like superman , 1 off my friends has past away when one of the phase short to panel door . 90% burn . please be very-very careful to do like this .

Register to Reply
Register to Reply 20 comments
Interested in this topic? By joining CR4 you can "subscribe" to
this discussion and receive notification when new comments are added.

Comments rated to be "almost" Good Answers:

Check out these comments that don't yet have enough votes to be "official" good answers and, if you agree with them, rate them!
Copy to Clipboard

Users who posted comments:

Anonymous Poster (3); BertHickman (1); energymv (4); jack of all trades (2); jmart23 (2); kelevin (1); Kilgore Trout (3); NotUrOrdinaryJoe (1); PhysicsProf (1); They Know! What? Nah! (1); Troy36 (1)

Previous in Forum: Cable Size   Next in Forum: Determination of required capacitor rating from Power Company's Invoice.

Advertisement