The dangers to workers from arc flash are so high that a new CSA arc flash standard is scheduled to be released at the end of 2008.
BY: SIMON FRIDLYAND
Every single day, it is estimated that between five and 10 arc flash explosions occur in electrical equipment in the U. S. A., according to statistics compiled by Cap- Schell Inc. of Chicago, IL. That means arcing faults could send more than 2,000 workers to burn centres in the next year (Canadian statistics represent roughly 10% of these numbers). Here are four examples of actual arc flash incidents:
Example #1: A journeyman electrician was working on an electrical panel when an arc flash/blast occurred. He was pushed back by the force of the blast and his shirt caught fire. He sustained burns to 20% of his body, including deep burns to his wrists and hands.
Example #2: An electrical foreman with over 20 years experience was working on a high-voltage circuit that he thought was de-energized. Unfortunately, he had de-energized the wrong circuit. While working on the circuit, he was thrown back by an arc flash/blast and received burns to his arm, neck and face.
Example #3: A journeyman lineman was holding an energized 2,200-volt wire when it grounded out through his leg. He sustained electric shock burns to his trunk and leg and associated flash burns to his hands.
Example #4: A journeyman electrician was installing a high-voltage panel when an arc blast occurred for unknown reasons. The explosion caused the worker to lose consciousness. He sustained burns to his hands, wrists and face.
In a recent arc flash test by an electrical equipment supplier, the amount of heat measured at 18 in. from the test box was capable of causing third-degree burns on exposed flesh and igniting conventional clothing; impulse sound levels were above 140 dB, meaning that without protection, permanent hearing impairment would be expected; light levels were intense enough to cause immediate vision impairment and increased chance of future cataract development; toxic gases such as copper oxide dust were forced out of the enclosure; shrapnel was ejected at high velocities; and molten metal projectiles contained enough heat to ignite conventional clothing.
Obviously, this is a dangerous hazard. So what is arc flash and what causes it?
An arc flash (also known as an arc blast) is a voltage breakdown of the resistance of air, resulting in an arc. With arcing faults, current actually flows through ionized air, causing an arc and releasing energy into the surrounding environment.
This can occur where there is sufficient voltage in an electrical system and a path to ground or to a lower voltage. An arc flash where there is a high level of current — in the range 1,000 amps or more — can cause substantial damage, fire or injury. Arcs can produce radiant energy four times hotter than the temperature on the surface of the sun.
Whether caused by a dropped tool, an accidental contact with a live circuit or build-up of dust, dirt, corrosion or particles that can act as a conductor, arcing faults release dangerous levels of radiant energy.
The massive energy released in the fault instantly vaporizes the metal conductors involved, blasting molten metal and expanding plasma outward with extreme force. A typical arc flash incident can be inconsequential but could conceivably easily produce a more severe explosion. The result of the violent event can cause destruction of equipment involved, fire, and injury not only to workers but also to people nearby.
In addition to the explosive blast of such a fault, destruction also arises from the intense radiant heat produced by the arc. The metal plasma arc produces tremendous amounts of light energy from far-infrared to ultraviolet. Surfaces of nearby people and objects absorb this energy and are instantly heated to vaporizing temperatures. The effects of this can be seen on adjacent walls and equipment — they are often ablated and eroded from the radiant effects.
The Canadian Standards Association is working on developing the CSA Z462 Arc Flash Standard. This document is Canada’s version of the U. S. NFPA 70E standard and is scheduled to be released at the end of 2008. This document provides guidance on implementing appropriate work practices that are required to safeguard workers from injury while working on or near exposed electrical conductors or circuit parts that could become energized.
The standard provides a method on how to evaluate exposure to potential arc flash/blast hazard. This includes determining Incident Energy, PPE Requirements, Flash Hazard Boundary, Shock Hazard and the Limited, Restricted and Prohibited Approach Boundaries for Shock Hazard.
Three key factors determine the intensity of an arc flash event on personnel. These factors are the quantity of fault current available in a system, the time fault until an arc flash is cleared, and the distance an individual is from an arc. Various design and equipment configuration choices can be made to affect these factors and in turn reduce the arc flash hazard.
Fault current: Fault current can be limited by using current-limiting devices such as grounding resistors or fuses. If the fault current is limited to five amps or less, then many ground faults self-extinguish and do not propagate into phase-to-phase faults.
Arcing time: Arcing time can be reduced by temporarily setting upstream protective devices to lower set points during maintenance periods or by employing zone interlocking.
Distance: Remote operators or robots can be used to perform activities that are of high risk for arc flash incidents, such as racking breakers on a live electrical bus.
Choosing the right PPE
Selection of appropriate PPE is normally handled by one of two possible ways. The first method is to consult a hazard category classification table, as can be found in the description of the Standard. The table lists a number of typical electrical tasks and various voltage levels, and recommends the category of PPE that should be worn.
For example, when working on 600-volt switchgear and performing a removal of bolted covers to expose bare, energized parts, the table recommends a Category 3 Protective Clothing System. The Category 3 system corresponds to an ensemble of PPE that together offers protection up to 25 cal/cm2. The arc rating is normally expressed in cal/cm2 (or calories of heat energy per square centimetre).
The minimum rating of PPE necessary for any category is the maximum available energy for that category.
The second method of selecting PPE is to perform an arc flash hazard calculation to determine the available incident arc energy. The Standard provides a guide to perform this calculation, given that the bolted fault current, duration of faults and other general equipment information is known. Once the incident energy is calculated, the appropriate ensemble of PPE that offers protection greater than the energy available can be selected.
PPE provides protection after an arc flash incident has occurred and should be viewed as the last line of protection. Reducing the frequency and severity of incidents should be the first option. This can be achieved through a complete arc flash hazard assessment and through the application of technology such as high-resistance grounding, which has been proven to reduce the frequency and severity of incidents.
The standard also requires that each piece of equipment (switchboard, motor control centre, etc.), where arc flash may take place, must have a label identifying the worst-case conditions. Figure 1 shows an example of such label.
The Occupational Safety & Health Administration (OSHA) of the U. S. Department of Labor states that workers shall use rubber gloves, mats, shields or other protective equipment and procedures that are adequate to ensure protection from electrical shock and burns while performing the work. As well, a general duty clause makes it clear that the employer has an obligation to protect workers from known hazards. The arc flash hazard is real and requires proper evaluation in all workplaces.
Simon Fridlyand, P. Eng., is president of S. A. F. E. Engineering Inc., a Toronto-based company specializing in industrial health and safety issues and PSR compliance. For more information, visit www.safeengineering.ca.