This article expands on “Control the Risk” (ELECTRICAL CONTRACTOR, March 2015). It addresses six key areas in the risk-control hierarchy and how each can be used to reduce the risk associated with the arc flash hazard. The hierarchy goes from the most-effective to the least-effective control methods. Elimination is ranked first, and PPE is ranked last.
The term “hazard analysis” was an integral component of past editions of NFPA 70E, but it has been replaced in 2015 with “risk assessment.”
Here are some key definitions:
• Risk assessment—An overall process that identifies hazards, estimates the potential severity of injury or damage to health, estimates the likelihood of occurrence of injury or damage to health, and determines if protective measures are required
• Risk—A combination of injury likelihood and severity
Two key words in both definitions need to be considered as part of controlling the risk: “severity” and “likelihood.”
The hierarchy of risk-control methods is specified in ANSI/AIHA Z10, American National Standard for Occupation Health and Safety Management Systems. The hierarchy includes the following:
3. Engineering controls
5. Administrative controls
Eliminating the hazard is considered the most effective method for controlling the risk and should always be the first choice when possible. The Occupational Safety and Health Administration (OSHA) and NFPA 70E emphasize the need to work on electrical systems only when they are placed in an electrically safe working condition. Creating and verifying this condition requires more than just de-energizing.
According to NFPA 70E 120.1, it involves several steps to ensure the system is safe and no voltage is present. Although the entire process may sometimes seem unnecessarily complex, each step is designed to serve as a safety net, and skipping a step removes one of these nets. In several of my forensic investigations, steps were skipped, leading to tragic results.
This method involves substituting an alternative component or piece of equipment that would reduce the severity or likelihood of exposure to the hazard. Here are a few examples of using substitution as part of the risk control strategy:
• Arc-resistant equipment—Rather than specifying and installing equipment such as switchgear and motor-control centers with the traditional design, more arc-resistant equipment is being used. This substitution of equipment reduces the exposure to the arc flash hazard by directing the energy away from a worker that is performing a task, such as operating or racking a circuit breaker in or out with the doors closed.
• High-resistance grounding—The use of a high-resistance grounding (HRG) scheme can reduce the likelihood of an arc flash as a result of a line-to-ground fault. An HRG system uses a grounding resistor to limit the ground-fault current to a value typically around 10 amperes. Note that the HRG system will not eliminate a fault that begins as phase-to-phase or three-phase event.
• Current-limiting devices—The use of current-limiting devices, such as fuses and circuit breakers, can greatly reduce incident energy by interrupting the fault in less than one-half of a cycle when the device operates in its current-limiting region.
3. Engineering controls
If the hazard cannot be removed through elimination or reduced to an acceptable level with substitution, the next method in the hierarchy is to use engineering controls to reduce the exposure or severity.
• Temporary device settings—This broad category includes methods such as an arc-energy-reducing maintenance switch, protective relays with multiple setting groups, and similar devices and functions. This type of control method provides the capability to temporarily change the speed at which a protective device responds during an arc flash. By reducing the device’s fault-clearing time, the severity of the arc flash can be greatly reduced. If permanent device setting changes can be made, that is the simplest approach. However, it is not usually possible, since miscoordination between protective devices could lead to a more widespread outage during a fault.
• Remote operation—The farther a worker is from the arc flash source, the less energy he or she would be exposed to. Remote operation, such as remote racking and remote opening and closing of protective devices, permits the worker to increase the distance from the arc flash source.
The three D’s
Most methods that are available to reduce the severity of the arc flash hazard focus on three controllable variables, including duration, distance and direction.
Table 1 provides a sample list of methods that can be used to reduce the incident energy exposure by controlling one of the three variables. Many other methods are also available, and manufacturers continue to develop new ways to reduce arc flash hazards.
Duration—Controlling the duration of an arc flash is an effective method for reducing the severity of the hazard because the incident energy is directly proportional to the duration. Controlling the duration is normally accomplished by selecting or setting the upstream protective device for the circuit or enacting a more elaborate protection scheme, such as zone-selective interlocking, differential protection or using of arc flash detection relays.
As a general guideline, if a protective device takes longer than five or six electrical cycles to clear a fault, the device is not operating instantaneously. As an example, if a protective device had a time delay of 30 electrical cycles (0.5 seconds), reducing it to an instantaneous operation of five cycles could reduce the incident energy by the ratio of 5:30, or one-sixth of its original value.
Distance—The magnitude of the incident energy decreases exponentially with the inverse of the distance from the arc flash source. The more distance that can be placed between the worker and the arc flash, the less severe the exposure to the hazard. According to IEEE 1584, the type of equipment also has an effect on rate of decay with distance and should be considered.
Direction—Redirecting the energy released during an arc flash is the concept behind arc-resistant equipment such as switchgear and motor-control centers. By containing the energy and redirecting it away from the worker through ducts, the hazard is reduced by directing the arc flash away from the worker.
Arc flash warning labels provide the worker with awareness of the hazard as well as important information about the protection required. NFPA 70E 130.5(D) lists the minimum requirements for information to be listed on the label.
Most arc flash software has the capability to include a significant amount of additional data on the labels. However, a good philosophy to adopt is “keep it simple.” Too much information could confuse the end-user.
Barriers create awareness. A barrier, such as warning tape, may be used to cordon off an area and alert people to stay away while electrical hazards may be present. The area is defined by the arc flash boundary as well as the approach boundaries defined in Table 130.4(D)(a) and (D)(b) of NPFA 70E.
5. Administrative controls
Administrative controls rank near the bottom of the hierarchy. These include job briefings, visual inspection, energized electrical work permits, safe work practices and training.
PPE is ranked last in the risk-control hierarchy. PPE is an important part of the overall electrical-safety strategy; however, if a person actually has to rely on his or her PPE for protection, it is already too late. In that case, the first five risk-control methods have all failed. PPE is analogous to a parachute. If you are a test pilot, you want to have a good parachute, but it would be better if you never have to use it.
What is tolerable risk?
Simply accepting the arc flash hazard and selecting PPE accordingly is not all there is to an arc flash study. Factoring in the risk, exposure and severity of the hazard to develop methods for controlling the risk to an acceptable level is a more comprehensive approach and is a major focus of the 2015 edition of NFPA 70E.
Keep in mind that, just because a particular hazard may be considered unlikely, there is still a likelihood. The best risk-control method is to place the system into an electrically safe working condition when possible. In other words, eliminate the hazard, and you’ll be good to go.
The long-awaited arc flash study has been completed, and the results indicate the prospective incident energy at the main switchgear is 34 calories per centimeter squared (cal/cm2). That means the 40 cal/cm2 “moon suit” will be required if energized work is to be performed there. Sound familiar? An arc flash study should be more than just incident-energy calculations and recommendations for personal protective equipment (PPE) including protective clothing. The study should also include recommendations for reducing or eliminating the hazard and risk.