Arc flash hazard raises many questions

Electrical Apparatus, Apr 2007 by Nailen, Richard L

The need for those added steps was emphasized by a recently publicized incident in which the main switch supplying a 4,000 ampere load was opened and locked out. After electricians began working on the downstream system, the switch suddenly closed by itself (opening the switch had automatically charged the closing spring).

One worker was badly burned before another could re-open the switch. An unsuspected design defect in the switch mechanism allowed slight vibration to shift a bar that had blocked the charging mechanism.

8. Has OSHA adopted NFPA 70E with all its arc flash requirements?

In Sections 29 CFR 1910.1 and 1910.2, OSHA's "general duty clause" empowers enforcement of any "national consensus standard" such as NFPA 70E. Failure to comply with its provisions can be considered by OSHA as a "presumptive failure" to adopt accepted safety requirements. Elsewhere in Subpart S of 29 CFR 1910, OSHA's mandate states that "safety related work practices shall be employed to prevent electric shock or other injuries resulting from either direct or indirect electrical contact. . . ." Simply put: as long as OSHA dictates safe practices, and NFPA 70E is the only national standard detailing those practices, then OSHA has effectively "adopted" NFPA 70E regardless of the specific wording.

9. If the arc energy calculation is right, and if I am wearing the right PPE, can I be sure I won't get burned?

No. Quoting from a 2004 consultant's statement, "Wearing the proper PPE for the risk hazard involved doesn't guarantee that a worker will remain free from injury or burns." The approach boundary limitations are based on protection from second-degree burns (again, on the face and torso). First-degree burns are still possible. The aim of the standard boundaries is to safeguard workers from burns that are considered serious.

Furthermore, the burns involved are presumed to result from radiated heat produced by the arc. The standards provide no practical way to deal with injury caused by flying fragments. Most of the PPE items prescribed are meant to resist heat only. As IEEE engineers pointed out in 2000, "While PPE can greatly reduce the chance of receiving a flash burn from the hot gases and molten metal, this . . . provides only minimal protection from the shrapnel that might be expelled."

Also, like any energy spreading outward from a centralized source, radiant arc energy continually decreases in intensity with distance from that source. It does not drop in discrete steps. If the safe approach distance is calculated at, say, 4 feet, we cannot say that a person at 4 feet 1 inch will be perfectly safe.

10. Now that we know so much about arc burns, how soon is somebody going to come up with the answers to the blast hazard?

Don't hold your breath. Three years ago the National Fire Protection Association announced a "consortium-based project" to measure the "explosive, thermal, and spectral effects of arcing electrical faults." Twelve months later, in January 2005, a second announcement stated that the NFPA and the IEEE had agreed to develop a plan by mid-year for a "collaborative initiative" to study the consequences of arcing, including "arcblast" effects of pressure and "acoustic energy." (As with any other kind of explosion, getting close enough to a high-power arc can damage one's hearing.)