Arc flash hazard raises many questions

Electrical Apparatus, Apr 2007 by Nailen, Richard L

And here are the answers to ten of them

SURE, WE WANT TO BE SAFE -but how?" Many electrical contractors, system designers, maintenance electricians, and facility operators aren't certain just what safety measures are required to deal with the dangers of arc exposure-a risk seldom thought about just a few years ago, particularly in systems below 600 volts.

Readers of the applicable standards (IEEE 1584 and NFPA 70E) find themselves with many unanswered questions, some of which will be discussed here. Choosing between apparently conflicting options may be necessary.

1. As long as we don't open any cubicle doors to expose live parts, isn't it okay to work around the outside of apparatus without PPE [Personal Protective Equipment]?

Opinions differ here. One electrician has said: "If you have arc-resistant switchgear and the doors are closed, PPE is not required." Countered another: "I have seen arcs started in closed enclosures by moisture, dust, small animals, coil burnouts . . . and mechanical failures. I don't accept your basic argument." In other words, arcs do not result only from some worker's action in an open compartment. Added a third: "The notion that having a closed door reduces arcflash risk . . . is controversial at best."

Erring on the side of safety, however uncomfortable, is always wise. The exposure limits are based on a worker's distance from the arc, whether or not it's behind a closed door or panel. Some switchgear is built with ventilating louvers to allow convection cooling for internal components. An internal arc flash may readily erupt through such an opening, endangering anyone nearby. Sealing off the vent isn't the answer, because something may overheat as a result.

Furthermore, blast pressure developed by an arc inside a closed compartment-a concern not presently dealt with by any industry standardcan cause injury or death at distances well beyond standardized flash protection boundaries, and in other directions.

Incidents have been reported in which an internal arc burned large holes through a compartment door or cover, not only exposing workers to the arc flash but adding more shrapnel to the blast.

2. What exactly are the safe working distances measured from? Is it the outside of a panel or enclosure, or some other point?

One published article describes the safe distance as measured "from the equipment." But again: the most important distance is what separates you from the arc itself, not from the outer wall of any enclosure. Section 4.8 of IEEE Standard 1584 tries to make that clear in these words: "Typical working distance is the sum of the distance between the worker standing in front of the equipment, and from the front of the equipment to the potential arc inside the equipment."

That's not as simple as it appears, however. In NFPA 70E, Article 130, we find this: "The incident energy exposure level shall be based on the working distance of the employee's face and chest areas from a prospective arc source. . . ."

Why that "face and chest" limitation? When performing some task on energized equipment, what's probably going to be closest to the arc are the hands and arms-why aren't they mentioned? The reason is that burns on those extremities are considered more "curable" than on the body itself. Furthermore, what about the worker who is not in front of the equipment, but alongside it (or even above)? For example, according to a 2004 IEEE Conference Paper, for ". . . an electrical worker working at the end of a long run of terminating bus bars and directly looking at the [ends] of the bus bars, the incident energy may have been much larger."

Consider Figure 1. Arc energy calculations may result in a safe distance "A" for worker No. 1 in front of the enclosure. Worker No. 2, at the end of the enclosure, may be further away, at the distance "B," yet be in harm's way because magnetic effects will drive the arc along the conductors in that direction, pushing a superheated plasma cloud ahead of it and possibly blowing off the end of the cubicle.

Finally, "prospective arc source" itself may not be easily defined. In 2003, a published illustration attributed to an earlier edition of NFPA 70E showed the various safe boundaries as spherical surfaces centered at a point, described as "any point on an exposed energized electrical conductor or circuit part."

That illustration does not appear in the 2004 version of 70E. Nevertheless, in a cubicle containing horizontal and vertical buswork as well as various device terminals, a host of such "points" have to be considered. Each one will create its own set of boundary spheres, which may overlap in complex patterns.

3. Calculating arc energy requires a value of available fault current, which has to come from the supplier-the electric utility-serving the site. What if the utility says they can't give us an accurate number because the supply doesn 't always come through the same feeders or substations?

This is a common quandary. Utilities are often reluctant to supply a firm figure, not only because it can change from day to day or from time to time, but because they don't want to be held liable for any injury resulting from someone else's arc energy calculation based on a stated value of current that proves to be too low. Whoever makes the calculation must use reasonable professional judgment based on whatever information is available, and make sure it's fully documented. All you can do is find the best data you can, and if confronted with a range of possibilities, adopt a conservative approach.