Table 1. Incident energy distribution.
In my last column, I discussed the concept of keeping your distance as the best way to avoid the shock hazards of exposed energized electrical equipment. However, staying outside of the prescribed limited approach boundary for the appropriate voltage may not always ensure your safety. In some cases, the hazardous effects of electric arcing can reach beyond this distance.

An arc is produced when electric current flows through an ionized air path instead of a metal conductor. During an equipment fault, high current arcs may develop between conductors of different phases or between conductors and the metal enclosure of the equipment. These arcs produce heat, light, and sound just like the low power arcs used in welding, and that is where the hazards exist for workers who are exposed to them.

Arc Hazards

Temperatures within an arc may reach 20,000°C (compare this to 5000°C for the surface of the sun) and power dissipation in a 480V arc may exceed 10 MW. The arc is a big radiant heater, transferring thermal energy directly and instantaneously to any surface exposed to it. Hold your hand 18 in. in front of a 115V household radiant heater, which typically produces 1500W; extrapolate that effect to 1,000 or 10,000 times the energy input, and it's easy to see why high-current arcs produce second- and third-degree burns to exposed skin in fractions of a second, long before a fuse or circuit breaker can interrupt the current flow. The intense light will usually temporarily blind the worker and may cause permanent damage.

The crackling sound associated with a welding arc is the result of rapid expansion of vaporized metal and heated air. Vaporized copper expands in volume 67,000 times, and heated air expands roughly proportionally to the temperature. In fault arcs, this expansion produces an explosive effect and pressures that can be high enough to propel a worker across the room. Even at lower pressures, the blast may "knock the wind out" of the worker who, struggling to recover, is then likely to inhale a cloud of vaporized metal and superheated air, severely burning the trachea and lungs.

Protective Measures

The incident energy threshold for second degree burns on exposed skin has been established at 1.2 calories(cal)/cm2. The distance required to stay below this limit can be calculated from the amount of available fault current at the equipment and the operating time of the fuse or circuit breaker that will interrupt the fault, and is called the "flash protection boundary." Work inside this boundary requires the use of personal protective equipment including natural fiber undergarments, shirts and pants, flash protection outer garments, and face, head, and hand protection. Flash protection equipment must be rated to withstand the expected level of incident energy.

A study of 33 industrial plants published by the Institute of Electrical and Electronic Engineers (IEEE) determined energy levels at typical working distances (Table 1). Flash protection equipment is available rated up to 100 cal/cm2, but even with thermal protection, some experts question the safety of worker exposure to levels over 40 cal/cm2 due to blast effects.

Distance Is Still Your Friend

In my experience, mechanical failures initiate most faults, and the most likely time for mechanical failure to occur is when operating a device. The probability of a fault is high at the same time a worker is present at the equipment, a dangerous combination. Most equipment enclosures are not designed to contain the effects of an internal arcing fault. If at all possible, don't stand in proximity to equipment when it is being switched or energized. If it's your job to throw the switch, using your left hand may allow you to position your face and body away from the enclosure instead of in front of it.

As an "unqualified" person not provided with training or protective equipment, your safest course is to stay well away from exposed energized parts. Don't rely on the limited approach boundary as a safety criteria unless you know for a fact that the flash protection boundary at the equipment is less than that. Faults are also frequently caused by dropped tools or other accidental contact, so avoid the temptation to "hold the flashlight" or supervise troubleshooting or other energized work by qualified persons. ES