Putting out the spark

Associate Editor

Minimizing arc-flash hazards and improving workplace safety is the focus of the new National Fire Protection Association (NFPA) 70E guidelines. An estimated 5 to 10 arc-flash incidents happen daily in the U.S., often sending workers to hospitals and burn centers.

A new approach to circuit-breaker design aims to eliminate arc-flash hazards. The Masterpact Low-Arc Flash circuit breaker from Schneider Electric, Palatine, Ill. (www.schneider-electric.com), protects against both overcurrent and arc flash. The key is a unique blow-open design that clears faults quickly, providing better arc-flash protection at lower current levels.

An arc flash is a release of energy caused by an electric arc. Up to 1 MJ of concentrated radiant energy can explode outward from the fault. Temperatures exceeding 5,000°F can melt metal and cause severe burns. Copper wire vaporizes into plasma that conducts through the air. The associated pressure waves can damage hearing and the flash can also damage eyesight.

Low-voltage power circuit breakers, less than 600 V, and other main or feeder circuit breakers have traditionally used current-path geometry designed to withstand high circuit forces up to a short time delay of 30 electrical cycles. Magnetic forces in the circuit breaker keep the contacts closed, letting downstream overcurrent devices open to clear the fault. But as current flow increases, this configuration lets the force increase, keeping the contact assembly closed and current flowing.

The new blow-open design acts in reverse to interrupt current flow. The blow-open terminal in the breaker is shaped so there is a reverse current loop in the moving arm. This reverse current flow creates a magnetic force proportional to current and, when the current is sufficiently high, the force quickly pushes open the contacts without waiting for the mechanism to unlatch and the springs to pull the moving arm open. The result is an opening time of 9 msec, or roughly half of one 60-Hz cycle.

The breakers also house a filtered arc chute that contains an assembly of metallic grids and meshes that reduce the gases released during an interruption. These grids deionize and cool the emissions, reducing the volume of vented gas and absorbing up to 95% of the energy.

Until now, bolted fault current was the standard by which the effectiveness of electrical-protection equipment was measured. But extensive study and testing by the IEEE has quantified arc-flash hazards in different types of equipment and led to better understanding of the difference between arc-fault current and bolted-fault current. The latter is the measure of current flowing through bolted bus bars. An arc flash, on the other hand, is a current flowing through the air. Because air creates a restriction or impedance, the arc fault current is always lower than the bolted fault current, which flows unimpeded through the metal of a bus bar.

This is important because circuit breakers and fuses have traditionally been designed to disconnect current rapidly when it reaches a specific bolted current fault level, yet allow a time delay when the current magnitude is lower. Because an arc flash happens at a lower current level than the bolted fault, these protective devices may allow the release of greater arc-flash incident energy at lower current values than at higher ones.