I feel I need to interject that the magnitude of an
arc flash hazard is mainly dependent on two variables: the amount of available
fault current, and the fault clearing time.
I'm curious - How many licensed electricians out there calculate the
arc flash incident energy before doing live work? I do a lot of medium and high
voltage work in industrial environments (refineries and chemical plants). Almost all now have minimum
PPE requirements that are based on the
arc flash incident energy available at the
point of work.
Arc flash calculations result in the amount of
PPE required at a particular distance from the arc. For example, to close a medium
voltage switchgear
breaker (1200A) from directly in front of the switchgear cubicle, you would most likely have to wear at least a 45cal/cm^2 flash suit. That is even if you can close the
breaker when standing in front of it. A lot of times the incident energy is so high that there is no available
PPE that meets the requirements. In this case you have to remotely close the
breaker. This is at the medium and high
voltage level though. I can honestly say that I've never had to wear a flash suit to work at the 120/240V
level. Normally the source
impedance is high enough that there is not enough available
fault current to warrant anything more than standard FRC's, if that.
Of course, this is a generalization, and should not be taken in place of a proper engineering analysis of a particular situation.
Simply put - don't do hot work unless you are trained to do so, and are taking the proper precautions. Even then, if there is a way to de-energize, DO IT.
I would suggest there are ery few electricians who are capable of analysing the
arc flash energy for a given facility.
The
short circuit coordination study which is an essential prerequisite to perform an
arc flash analysis produce the
arc flash boundary and energy for each
breaker panel and the required
level of
PPE, this information must be attached to the panel and no one can open the panel without wearing the
PPE and no one may be within the boundary range who is not wearing the
PPE and who has not completed
arc flash training. A faulty
breaker,
fuse or piece of equipment can produce an arc flash.
I have just finally completed reviewing the
arc flash analysis and the
short circuit study performed by an electrical consulting firm for the industrial facility where I work. We use a large electrical contracting company to perform all our electrical installation and maintenance work - I retained a consulting engineering company to do the
arc flash and work because none of the two hundred plus engineers who work at this company - including myself - have the theoretical background or
practical exprience in this field. This work took six months to complete with all the recomendations.
This facility has two seperate high
voltage three
phase feeds with high
voltage transformers and multiple medium to low
voltage transformers and more than sixty panels. We have major air conditioning equipment on site a large machine shop, enironmental test capabilities and a large office facilities.
We had some surprising results from the
short circuit coordination study which required every panel
transformer,
breaker and load to be reviewed,
face plate data to be collected
etc. we found that the electrical contractors had installed the wrong fuses in some locations because they had attempted to work out the required fuses themselves using rules of thumb. Particularly problematic was they had not realised hat when you short out a
circuit which has motors whose armatures are turning that the motor behaves as a generator and also produces
current into the
short circuit which in the case of one of our circuits increased the
short circuit current by 25%. Unfortunately the time response and
current interrupt
rating of the fuses meant that the fuses nearest the load could not interrupt the
fault current so the
current continued to flow after the
fuse melted and the response time of the next
fuse was too slow producing a major
arc flash which while unlikely to happen - if it did would have extended seventy feet beyond the panel at an energy
level that would have killed anyone wearing even the highest
level of
PPE. The scary thing was that this was on a 208V three
phase. By the way when we got to the bottom of this the contractor had installed a
fuse/
breaker combination that was non compliant to he electrical code.
We had to change breakers, fuses and panels to correct this - sfterwards the
arc flash hazard was negligable.
The long explanation is that this type of analysis requires you collect a lot of data which can even mean talking to equipment manufacturers, requires a complete understanding of the complete electrical installation up to and including information of the supply transformers - which we had to get from the Hydro company, a lot of detailed mathematical analysis
etc. This is not something you do when you are looking at a panel - that is why there is supposed to be an
Arc Flash label on each panel.
The
master electrician from our electrical contractor who had done the work was shocked (pun intended by the results) and to this day he still does not understand what he did wrong after the engineers has attempted to walk him through everything multiple times. This is a very experienced and capable
master electrician who does excellent work but crossed over into the engineering domain.
Here in Canada only a Professional Engineer with the appropriate experience
etc. may perform this work.
