10-21-2008 10:56 AM
An arc flash is a voltage breakdown of the resistance of air resulting in an arc which can occur where there is sufficient voltage in an electrical system and a path to ground or lower voltage. An arc flash with 1000 amps or more can cause substantial damage, fire or injury. The massive energy released in the fault instantly vaporizes the metal conductors involved, blasting molten metal and expanding plasma outward with extreme force. A typical arc flash incident can be inconsequential but could conceivably easily produce a more severe explosion. The result of the violent event can cause destruction of equipment involved, fire, and injury not only to the worker but also to nearby people.
In addition to the explosive blast of such a fault, destruction also arises from the intense radiant heat produced by the arc. The metal plasma arc produces tremendous amounts of light energy from far infrared to ultraviolet. Surfaces of nearby people and objects absorb this energy and are instantly heated to vaporizing temperatures. The effects of this can be seen on adjacent walls and equipment - they are often ablated and eroded from the radiant effects.
10-21-2008 11:43 AM
11-25-2008 06:58 AM
For Short circuit calculation on Alternator one should aware of the sub transient reactance value of the alter nator ( Xd") . Itis available on Data sheet of Alternator .
Isc= Alternator Full load Current * 100/ Xd" -- It is in pu
12-22-2008 10:28 AM
01-17-2009 04:56 PM
Sorry for being a little slow in getting back. But thanks for the calc. That is what I was looking for!
Yes, Plantpro, I agree an engineer has to end up doing the arc-flash study. I don't think 70E demands it, however, who else is going to do the short circuit studies, arc-flash calcs and, most importantly stamp the drawings that are produced. I have seen some DIY arc-analysis items on the web but have wondered. I played with the demos of Easypower and SKM tools. Both extremely powerful.
07-12-2009 01:21 PM
02-22-2010 11:45 AM
A few points on the arc flash analysis.
Yes, a lot of people are still hearing it for the first time. It was only in 2002 that NFPA & IEEE put the standards together to define how an arc flash should be done. OSHA has indirectly adopted the standards by stating that workers must be warned about electrical hazards and the proper PPE to wear and referencing NFPA. The only way to properly warn the workers of the threat level and required PPE is to do an arc flash analysis with the exception of using the NFPA 70 E tables. The problem with the NFPA 70E tables is that in order to use them, you must already know the short circuit current and clearning time of your protective devices and the results of that must fit within the parameters of the chart (read the footnotes). We have never found anybody who can actually use the charts as they were designed, although they are a good reference point for PPE safety if you haven't got an arc flash analysis done yet and need to approach an energized part.
Getting an arc flash analysis done is not cheap internally or externally, but it's a heck of a lot cheaper than the damages if there is an accident and the analysis wasn't done. OSHA has been laying down heavy fines in these cases, not to mention the legal and insurance costs.
No, you don't need to be a PE or outsource the arc flash analysis, however, there are a lot of challenges in doing an analysis internally, the start of which is putting money up front for the software and learning to use it which can cost $10K - $30K and several weeks for that process alone. The bigger problem, however, is the simple inexperience of doing an arc flash analysis. It takes literally a dozen jobs under the direction of a PE with a lot of arc flash experience to to do a job right. The biggest problems we see with self- analysis are; a) data collected isn't accurate b) analysis is only done at top voltage loads, not the lowest, which can be more dangerous c) lack of ability to provide recommendations on how to make minor changes to something like the setting of a breaker to take it from Cat 4 down to Cat 2. We see companies buying sets of expensive Cat 4 PPE based on the outcome of the analysis when really, a few minor adjustments or a change in a breaker might eliminate both the need for the Cat 4 equipment and more importantly, reduces the hazard for their employees.
And here is the million dollar question (or $9 million question as the estimated cost of an arc flash accident involving serious injury). If you do the analysis yourself and there is an accident and a forensic engineer comes in and finds out that the analysis was done wrong, who do you think is going to be held liable?
03-08-2010 12:26 PM
The history of arc flash started with the electric utility industry in the 1980's. By 1994 OSHA had put in place 1910.269 requiring utilities to not allow clothing which "could increase the extent of the injury" in the event of an arc flash. Coupled with the 1910.335 requirements for face protection in the event of an electrical explosion, many of us in the utility industry began researching options for arc flash clothing and PPE. In the early 1990's ASTM's F18 committee for electrical protective equipment developed test standards which were later incorporated into NFPA 70E. In early 1999, some of us formed IEEE 1584 and relied on limited donations of a very few companies (ours included) to do some arc flash testing to add to the data we had amassed for about 15 years through the ASTM committee. I do most of the arc flash testing at www.arcwear.com on clothing through the Kinectrics lab in Toronto but some is done through our IEC 61482 method at labs in Russia, Switzerland and in Canada.
Actually NFPA put arc flash calculations in the 1995 version and clothing entered in 2000. The 2004 and 2009 versions have added much clearer guidance and we have many improvements coming for the 2012 version in process right now. There was no NFPA 2002 version but IEEE 1584's latest version is 2002 but a large study is going on right now to update this substantially. We use IEEE 1584 (embedded in almost all software) for calculations up to 480V and we also use ArcPro software beyond 480V as a double check since IEEE 1584 tends to over predict the energy the higher the voltage goes.
The NFPA 70E tables are very limited and rarely can officially be used BUT they have saved many lives. Dr. Tom Neal, Dan Doan and I have a paper presented at the IEEE-ESW which explores 40 incidents in which the tables fully protected the workers in almost every case. Don't put off training or PPE waiting for arc flash studies BUT the studies can help reduce injuries and exposures.
I recommend having a PE review all flash studies because we've seen many companies enter this in the last few years who use software to do the studies but don't really understand what is needed to do a real fault current & coordination study properly and they end up with outlandishly high calorie ratings or extremely low ratings. It is often a function of not taking all factors into consideration. Many flash studies are very simple to do. The most costly portion is data gathering and most of our customers do these themselves with a local contractor and have a PE review the information after the model is build. We recommend doing flash studies in-house if you can since the NFPA standard and good engineering practice requires review of the study on a 5 year basis and keeping it up to date. Doing a study has many advantages.
For more info see my article on American Chronicle.
Sr. Managing Partner