What Do You Need to Know About Arc Flash?

by Visitor Wattsup on ‎02-08-2012 09:56 AM

pp-homolka-banner2.jpg

Do you realize that copper expands 67,000 times when it changes state from solid to vapor in an arc flash incident? This pressure wave can cause eardrum damage, concussions or consequential falls, and it can also damage surrounding equipment or tear protective clothing. This blast damage is sometimes forgotten when discussing arc flash.

There are two primary types of physical hazards from arc flash when working around electrical equipment: shock and burns. Most people can relate to an electrical shock from personal experience. When I was a child, I would receive a shock if I unplugged the household vacuum and accidentally touched the plug prongs. I didn’t get hurt, but wow! I respected the power of electricity after that. Obviously, the electrical systems that we design and work around have much higher voltages and current levels than my childhood home, so the risk for injury is far greater.

In recent years, electrical burns have received greater attention and emphasis from OSHA, NEC, insurance companies and other groups. My father-in-law is an electrician at a large extrusion company, and over the years he has helped me better understand and appreciate electrical burn hazards.

Three factors affect the severity of arc flash burns:

  1. Incident energy level of the arc
  2. Distance from the arc
  3. Duration of exposure

 
Anyone who has been through arc flash training and seen videos or pictures of victims can testify to the severity of arc flash burns. It’s not a pretty sight, but it’s an important reminder of the hazards involved that motivate us to follow needed safety measures.

Facility managers and electrical technicians need to understand the risks of arc flash and properly quantify the electrical hazards. OSHA requires equipment owners to properly label all equipment in compliance with NFPA70E, and the National Electric Code (NEC) mandates that electricians mark the highest voltage contained within the enclosure on the cover or door. This enables qualified electrical workers to calculate the magnitude of the hazard and then select the personal protective equipment (PPE), tools and procedures rated for the nominal system voltages. This process also helps in developing the complete system design needed for short-circuit/coordination studies.

Some questions you need to ask yourself, because your job and the well-being of others depend on it:

  • Does your facility’s electrical distribution system have all the proper labeling?
  • Do you have a properly performed short-circuit/coordination study?
  • Do you have facility one-line diagrams to start the short-circuit/coordination process?
  • The key to arc flash mitigation is proper setting and verification of protective devices – namely relays and circuit breakers – which ties back to proper maintenance. Do you have an electrical maintenance program that includes these features?
  • If your staff is performing electrical maintenance, do you have proper PPE and training?
  • Does your facility have policies and procedures to address arc flash?

 
Please post below and share your experiences with arc flash, and what you do to address it.

Comments
by Visitor rwidup
on ‎02-09-2012 06:04 AM

Robb:

Great topic to discuss! Anyone that is exposed to the hazards of electricity should have a complete understanding of what it takes to comply with the rules under NFPA 70E (Standard for Electrical Safety in the Workplace). The 70E is great resource for understanding the minimum safe work practices for electrical work - hazards that we all are exposed to when working on and around generator power systems.

 

I was involved in an arc-flash incident back in the mid 80's. It was just a "little old 400-amp switch" that I was upgrading a portion of the controls on, and didn't think that a No. 14 wire would be that hazardous....until it accidentally contacted the 480-volt bus and caused an arc flash event that burned my arms and face. But hey - if you blow yourself up do it in a hospital across the hall from the emergency room (which is where I was). That event was one that created my motivation to understand and comply with the rules and work practices for safe electrical work. (A week and half in the hospital will do that to you). Interesting side note: the hospital couldn't turn it off - it was too critical of a circuit and would require too much coordination to de-energize (after all, it had an emergency generator on it for back up power!). But when the switch blew up and was laying in the floor that didn't seem to be the issue.

 

So for everyone out there that wants to take care of their employees or themselves - understand the hazards of electricity before you begin work on it. It is one of the fundamental requirements to being a qualified electrical worker. And get the 2012 edition of the 70E and follow its guidance.

 

One of the problems we face all the time is working around switchgear that has not had an arc flash study performed - and there are no labels to tell us what the [arc flash] hazard is for the gear. It's those situations that really require us to be on our game and understand what we are doing, because if you don't understand it, even "a little old switch" can cause you great harm.

 

And before you work on it - turn it off.

 

Thanks.....Ron

 

 

by Visitor mrmediumvoltage
on ‎02-09-2012 09:04 AM

 

Great comments Ron! You can't appreciate some of the aspects of the arc flash until you are up close and personal with one. Some still think that all of these rules and guidelines just slow them down. I was doing a site inspection of some MV gear at a remote pumping station for a large multinational pipeline company. The task was to be simple, remove power open the doors and inspect.

 

The first step was to open a non-load break isolation switch feeding a VFD's isolation transformer. I was super impressed that the site had all of their arc flash labeling done and that they were fully compliant with NFPA-70E. In fact, I was VERY thankful! Even though I am very experienced with MV gear and working around MV, I was still unqualified related to this site; its practices and procedures. (I teach classes on arc flash safety and NFPA-70E compliance). I commented, as we were going through our pre-work meeting, how impressed I was with their practices! One of the points of the pre-work meeting was the discussions on the flash boundaries- which, at points of the system, >800 inches! So, even more reason to treat this site appropriately.

 

After I had signed the forms indicating that I was briefed on the hazards and understood my role as an unqualified guest, we then proceeded to execute the plan per our pre-work meeting. One of their qualified team members put on his arc flash suit (appropriately suited for the arc flash incident energy on the equipment containing the switch). He alone walked to the equipment to open the switch. I, along with two other people where outside the arc flash boundary, in an outer room protected by a metal door with a large pane of safety glass which permitted a safe observation point.

 

What still lingers in my head, that moment were time seems to stand still, is the sight of molten copper on the floor glowing through all of the smoke and arc gases and the form of a hulking guy in a flash suit bursting through the smoke tearing towards the door.

 

Moral here is - the rules and guidelines in NFPA-70E save lives!! A mere 10 years ago we would all have been standing in the electrical room chatting as the electrician, in his regular greasy work clothes, opened the switch. In this situation, under similar circumstances, we would have all been seriously injured or worse.

 

All you have to do is look at the history of automobile safety to know how far behind we are in relationship to working with electrical energies; air bags, energy absorbing bumpers, seat belts etc. !! I remember getting into my father’s old pickup truck where the starter button was on the floor. No interlocks- no safety features... this was state of the art then! Look where we have come. We need to continue to promote the ongoing efforts of safety standards like NFPA-70E to refine safety to its highest level.....my kids are counting on it! (They sure were glad that I made it home that day - could have been a totally different outcome if not for the NFPA-70E guidelines and a corporate body willing to stand behind it!)  Glad you survived to tell your story too!

by Visitor ii623
on ‎02-09-2012 09:51 AM

One big thing to remember is that 99% of the people working in the vicinity of electrical equipment know how dangerous it can be.  There's a bit of education necessary for everyone in every workplace (perhaps even in homes) to understand that something as simple as closing a breaker can result in an explosion.  Education and use of safety procedures religiously is important, and generally not well-enough done.

by
on ‎02-09-2012 09:57 AM

I am a PSSR for a Caterpillar Dealer, Power Systems Division.  When inspecting an Automatic Transfer Switch for the first time, I get all the information for the switch (Model, Serial Number, Volts, Amps...) and make a lable for the outside of the ATS cabinet.  The less we as service personnel need to open that cabinet, the better!  Some education to our customers about arch flash goes a long way also.  THINK SAFE, BE SAFE!

by Contributor see-deif
on ‎02-09-2012 11:09 AM

Very timely topic.

May I add that you, as the supplier of the genset and/or emergency power system, can do many things to help isolate and minimize the damage in the event of a arc flash.

 

For example, you have seen the "2-breakers-in-series" convention being used very often today by consulting engineers. You can do that or, much cheaper, a breaker in series with a no-load disconnect switch. If it is in swgr, you can add a "ground and test device" in series with the breaker. This device racks in like a breaker but has provisions for shorting line-to-load, line-to-ground, load-to-ground, or open. Contact your breaker vendor to find out more.

 

To limit the damage/reduce the incident energy level, you could add differential relaying zones. This is often used to protect generators, especially MV generators, but you can also put the entire switchgear into a zone. Differential relaying is fast, but still relies upon the clearing time of the breaker. So it is not perfect, but if you are close to the next level of incident energy, maybe differential relaying could help.

 

Be safe,

Steve

by Visitor Wattsup
on ‎02-10-2012 01:26 PM
Thanks to everyone for sharing your experiences and yes be safe!
by
on ‎02-13-2012 11:14 AM

Good topic.

 

I always wonder about ATS's.  They are much easier to open, and be exposed to, arc flash energy than any switchboard.

 

Add to that, some generators are classified as "unsafe at any distance" due to the generator's ability to sustain an arc without shutting down/tripping.  That is how I have seen them modeled by some engineers.

by Joe Hundley
on ‎08-07-2015 01:57 PM

What is the arc flash potential of a 80 Kw generator ,480 volt and 120 amps. Hourse Power of engine is 20% more than HP needed to run generator under full load.

Post a Comment
Be sure to enter a unique name. You can't reuse a name that's already in use.
Be sure to enter a unique email address. You can't reuse an email address that's already in use.
Type the characters you see in the picture above.Type the words you hear.
Announcements
Help us grow the Caterpillar Community: Invite a Friend

Meet Our Bloggers