- Recently, in an effort to improve workplace safety, the industry has begun to focus on protecting workers from the arc-flash and arc-blast hazards that are present when they must perform work on energized electrical equipment.
- NFPA 70E is an industry consensus standard that defines the specific requirements for safely working on or around electrical equipment.
- One of the most fundamental safety requirements for electrical workers is to turn off the power to the equipment before they attempt to work on it.
Serious workplace injuries and fatalities from electrical arc flash incidents have been occurring ever since electrical energy was first generated and distributed for productive applications. Arc flash accidents that result in a serious injury or fatality occur five to 10 times a day in the United States. Approximately once per day a worker involved in an electrical accident does not survive. Recently, in an effort to improve workplace safety, the industry has begun to focus on protecting workers from the arc-flash and arc-blast hazards that are present when they must perform work on energized electrical equipment.
Figure 1. NFPA 70E defines specific safe work practices and personal protective equipment (PPE) for workers to help protect them from these hazards. (Source: Oberon Company)
One of the results of that focus has been the development and publication of NFPA 70E — Standard for Electrical Safety in the Workplace. NFPA 70E is an industry consensus standard that defines the specific requirements for safely working on or around electrical equipment. OSHA recognizes the NFPA 70E standard as a written, published standard, available to the industry, and it cites the requirements of this standard for employers. NFPA 70E is updated every four years. It defines specific safe work practices and personal protective equipment (PPE) for workers to help protect them from these hazards (Figure 1). OSHA recognizes this important document and will use these requirements to determine compliance for employers regarding electrical workplace safety.
An arc flash results from a short circuit or fault condition that occurs when the insulation between energized electrical phase conductors, or between a phase conductor and ground, is somehow compromised. The other type of major electrical fault is called a “bolted fault.” During a bolted fault, the fault current normally flows over a conductive path and is not usually released outside of that path. While such faults can be damaging, little energy is released into the surrounding environment during the fault, and an upstream overcurrent protective device will respond rapidly to open the faulted circuit.
During an arcing fault however, the fault current flows through the air rather than through a conductor or other conductive component, and a great deal of thermal energy is released into the environment. This sudden release of thermal energy, similar to that seen in an electrical arc furnace, is referred to as an arc flash event. As a result of the sudden release of energy, a pressure wave also develops and expands outward at a high velocity. This pressure wave is usually called arc blast when it is a result of an arc flash event. Arc blast is another human hazard that may cause serious injury, and workers must be protected from it, as well.
The degree of arc flash hazard is measured by the incident energy that is released, which is expressed in calories or joules per square centimeter. This incident energy defines the thermal exposure that a worker standing at a certain distance from the source of the arc (the “working distance”) would expect to receive on the head and torso. In addition to the thermal energy release, there are other hazards produced by these events, including arc blast (a high-pressure wave), sound levels that can lead to permanent hearing damage, and often a ballistic threat from high-speed flying particles and objects.
The incident energy level at a given location in an electrical system is dependent on many factors, such as system voltage and component impedance, available fault current, and the arcing fault duration. The faster an arcing fault is detected and cleared from the system, the less energy it releases into the air, so the action of the overcurrent protective device — specifically, how quickly it can detect and clear the fault — is a critical parameter in determining the level of arc flash hazard in a given system. In fact, in most cases the fault clearing time is the only variable in the equation that can reasonably be controlled in order to limit the incident energy that will be produced by an arcing fault.
The human damage and financial costs that result from arc flash accidents can be very significant. It is estimated that a serious accident from which the victim survives will on average cost more than $10 million, which is a combination of direct and indirect costs. Some very serious accidents have resulted in much higher costs. The victim often suffers permanent and disfiguring physical trauma that shortens the life span and prevents them from ever returning to work. This is a serious risk not only to the worker and the worker’s family, but to the employer and its insurers. For small employers, the company itself may not survive.