Arc flash labels, risk assessment and PPE awareness

Arc flash hazards are invisible until the moment they become catastrophic. Workers see arc flash labels on panels, switchboards and motor control centers every day, and qualified workers wear arc-rated clothing and face protection when they work on energized equipment. This lesson explains what those labels mean, how the arc flash risk assessment works at a high level, and what arc-rated protection does, so that unqualified workers can read the signs around them and understand why certain areas are off limits.

The arc flash risk assessment

NFPA 70E requires employers to perform an arc flash risk assessment for electrical equipment that could be worked on while energized. The assessment identifies arc flash hazards, estimates the likelihood that an injury could occur and how severe it could be, and decides whether additional protective measures, including PPE, are needed. It also sets the arc flash boundary. Engineers usually calculate the available incident energy at each piece of equipment using a standard method such as IEEE 1584, based on the available fault current, the time the protective devices take to clear the fault, the working distance and the equipment configuration. The assessment must be reviewed periodically, at intervals not exceeding five years, and whenever major changes occur.

Incident energy is measured in calories per square centimeter (cal/cm²) or joules per square centimeter (J/cm²). One calorie equals about 4.184 joules. The onset of a second-degree burn on bare skin is usually taken as 1.2 cal/cm², or 5 J/cm². The arc flash boundary is the distance from the equipment at which the incident energy falls to that value. Inside it, a person without protection could receive at least a second-degree burn if an arc occurs.

Reading an arc flash label

Equipment such as switchboards, panelboards, industrial control panels, meter socket enclosures and motor control centers that is likely to be examined, adjusted, serviced or maintained while energized must be field marked with a label. The label tells qualified workers what they are facing before they open the equipment, and it tells everyone else that the equipment contains serious hazards.

The workplace standards require the label to show at least the nominal system voltage, the arc flash boundary, and either the available incident energy with its working distance, or the arc flash PPE category, but not both in a way that conflicts. Many labels also show the minimum arc rating of clothing, the shock approach boundaries, the equipment identification and the date of the assessment. Labels often carry a signal word in capital letters, such as danger for higher hazards or warning for lower ones, following product safety sign conventions. The employer must keep labels accurate and update them when the assessment changes.

For unqualified workers, a label is a clear message: this equipment can produce a dangerous arc, do not open it, and keep your distance when qualified workers have it open. The arc flash boundary printed on the label tells you where to stand if you must be in the area while work is under way.

Arc flash PPE categories

When engineers have not calculated the incident energy for a task, the standards allow employers to use a table method, which assigns an arc flash PPE category based on the type of equipment, its voltage, the available fault current and the clearing time. Each category has a minimum arc rating and a list of required protection.

Arc-rated clothing is tested to find the incident energy it can block before the wearer would likely suffer a second-degree burn, or before the fabric breaks open. That value is the arc rating, shown on the garment label as the arc thermal performance value (ATPV) or the energy break-open threshold (EBT). Arc-rated clothing will not ignite and continue to burn, and it insulates the skin. It does not make the wearer immune to shock, blast pressure, flying debris or hearing damage, and it does not protect parts of the body it does not cover.

What to wear, and not wear, near electrical equipment

Even unqualified workers who never open equipment benefit from knowing a few clothing rules, because they may be near equipment that fails during normal operation. Synthetic fabrics such as polyester, nylon, acrylic and spandex can melt and stick to skin when exposed to an arc, turning a minor burn into a severe one. Under arc-rated clothing, workers must wear non-melting natural fibers such as cotton or wool. Conductive jewelry, watches, metal-framed glasses and key chains must be removed or made non-conductive before working near exposed energized parts.

Reducing the hazard through design and maintenance

PPE is the last line of defense. Employers can reduce arc flash hazards through engineering and maintenance: arc-resistant switchgear that directs blast energy away from the front, remote racking and switching devices that let workers operate equipment from a distance, infrared windows that allow thermal scanning without opening covers, maintenance switches that speed up breaker tripping during work, current-limiting fuses, and good maintenance of breakers and connections so that protective devices clear faults as quickly as the study assumed. Poorly maintained breakers that clear slowly can make the incident energy far higher than the label says.

The unqualified worker and arc flash

For unqualified workers, the rules are simple. Do not open labeled equipment. Do not operate equipment that shows signs of damage, overheating or arcing. Stay outside the arc flash boundary when qualified workers have equipment open, unless you are escorted and wearing appropriate protection for a specific reason. Report missing or unreadable labels, missing covers and unusual noises or smells. And never borrow arc-rated PPE to do work you are not qualified for: PPE does not make an unqualified person qualified.

Other protective equipment for electrical work

Arc-rated clothing is only part of the protection qualified workers use. Rubber insulating gloves, rated by voltage class and worn with leather protectors, protect against shock and must be air-tested before each use and electrically tested at regular intervals. Insulated and insulating tools are rated for use near energized parts. Insulating blankets, mats and line hoses provide temporary barriers. Safety glasses and hearing protection are worn under face shields and hoods because arc blasts can damage eyes and ears. Footwear with dielectric ratings provides some secondary protection but is never relied on alone.

All this equipment needs care. Arc-rated garments must be laundered according to the manufacturer's instructions, kept free of flammable contaminants such as oil and grease, and retired when damaged. Rubber goods must be stored away from sunlight, heat and ozone, and checked for cuts, holes and swelling. A worker in damaged or contaminated PPE may be far less protected than the label suggests.

For unqualified workers, the presence of this equipment is a signal. If you see someone suited up in a flash hood and rubber gloves, they are working on or near energized equipment with a significant hazard. Keep your distance, respect the barricade, and do not interrupt them.

Finally, the label is only as good as the study behind it. If an electrician notices that equipment has been changed, a transformer replaced or protective settings altered since the label was printed, the label may no longer be accurate. The change should be reported so that the risk assessment can be reviewed. Unqualified workers can help by reporting labels that are faded, damaged, missing or obviously out of date.