December 2012 Electrical Apparatus

December 2012 Electrical Apparatus

This is a summary of the Electrical Apparatus December 2012 featured technical article,  by Richard L. Nailen, P.E.    

One of the most serious hazards faced by electrical equipment installers and operators is the high-power electric arc in a faulted circuit. The cause may be some equipment malfunction. More often, however, the arc results from unintentional human interaction with energized conductors. Although all authorities agree that complete safety requires workers to deal only with de-energized apparatus, circumstances sometimes dictate otherwise.

Even when energized conductors aren’t being worked on or exposed, an arc fault within apparatus can endanger anyone standing or working nearby. Hence, safety standards (such as NFPA 70E in the U.S.) mandate that anyone approaching a possibly energized circuit should be wearing appropriate PPE (Personal Protective Equipment).

Such equipment is designed to limit the severity of arc flash burns. Its selection is based on the thermal energy released by the arc. What PPE is required depends upon the arc’s possible energy content and its distance from the wearer. Procedures have been developed to calculate that relationship.

The explosive air pressure wave also created by the arc is more difficult to quantify. It received little attention until recently. That wave can move at hurricane speed, and impact an exposed person with as much as 25 kilo-newtons of force, sufficient to knock down or hurl the person across a room. The blast may also violently project metal fragments 20 or more meters–well beyond the distance considered “safe” thermally. No PPE, designed to insulate against thermal burns, protects against those hazards.

Although blast pressure, like the arc plasma cloud, intensifies with arc current and decreases with distance, it may vary widely with shape and volume of any enclosing structure. The more confined the space, the more explosive the pressure. Research into the blast hazard is far from complete.

Meanwhile, switchgear designs that can safely vent blast pressure to a safe area, without endangering nearby personnel, are being developed and tested to international standards such as IEC 62271.

To order a back issue with the full article, “Arc Blast: Still on a Learning Curve” call 312-321-9440 or visit our online webstore.

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