- With the current focus on arc flash safety and the availability of means to upgrade obsolete and poorly maintained equipment, the path to safety and improved reliability is easier to navigate.
- The first step toward electrical safety is to be aware of the implications of poor maintenance and make a decision to improve it.
- Today there is a significantly heightened awareness of arc flash hazards, and consulting engineers who design power systems for new facilities are striving to design the safest possible system for their clients.
Electrical arc flash accidents that result in a serious injury or fatality occur five to 10 times every day in the United States. Approximately one fatality per day results from an electrical arc flash accident. The human consequences of these accidents are devastating to the victims and their families. If the victim survives a serious electrical burn injury, the injury is often so disabling that the victim is never able to recover sufficiently to return to work. The financial consequences are also very damaging to the employers and their insurers.
Electrical accidents have been occurring for more than 100 years, and the accident statistics have been fairly consistent until recently. The electrification of facilities that began during the early industrial revolution has continued with facility growth and process demands constantly requiring more and more electrical power density to sustain operations. At the same time, the manufacturers of electrical equipment improved on their products, developing improved designs to more safely control the electrical energy, protect workers and others from injury, and provide high reliability, reduced maintenance requirements, and long equipment service life.
Figure 1. With the current focus on arc flash safety and the availability of means to upgrade equipment, the path to safety and improved reliability is easier to navigate. (Source: Fluke)
With the current focus on arc flash safety and the availability of means to upgrade obsolete and poorly maintained equipment, the path to safety and improved reliability is easier to navigate, but the first step is to be aware of the implications of poor maintenance and make a decision to improve it (Figure 1).
Development of the electrical safety standards
The hazards of electric shock have been fairly well understood by most qualified electricians, and they have been reasonably well trained in the techniques of working safely to avoid shock and electrocution hazards. Arc flash and blast hazards were not really well understood until the late 1990s.The reason for the lack of awareness stems from the fact that, until 1999, consistently accurate methods to calculate potential arc flash hazards did not exist.
In spite of the statistical frequency of arc flash accidents prior to 1999, the amount of arc flash incident energy (thermal energy) wasn’t easily predictable, so means of protecting workers (personal protective equipment) wasn’t readily available for arc flash hazards. IEEE Standard 1584, IEEE Guide for Performing Arc Flash Hazard Calculations, was published in 2002. This document contained accurate formulas to calculate prospective incident energy levels based on empirical data from repeatable and accurate laboratory tests that were performed.
The first mention of arc flash hazard in NFPA 70E (Standard for Electrical Safety in the Workplace) appeared in the 1995 edition of that consensus standard. Subsequent editions of NFPA 70E have documented improved safety practices through the use of personal protective equipment (PPE) and other safety equipment and safer work procedures. Since then, considerable work has been done to improve arc flash calculation methods, improvements on PPE design to protect workers, training, and increased awareness of arc flash hazards (Figure 2).
Arc flash — causes and effects
Figure 2. Considerable work has been done to improve arc flash calculation methods, improvements on PPE design to protect workers, training, and increased awareness of arc flash hazards. (Source: Salisbury by Honeywell)
An arc flash results from some condition that compromises the insulation distance between two phase conductors or a phase conductor and a grounded conductor or surface. The cause is rarely the result of equipment failure. Evidence suggests that a very high percentage (more than 95%) of these events that cause injury or fatality to a worker were caused by some unsafe act by the worker.
A tool or other conductive metallic object dropped into energized electrical equipment or a tool that slips and shorts between components is a very common cause of arc flash accidents. Once the arc forms, it heats the air around it very rapidly and the space around the arc becomes more conductive because of ionized metallic particles within the arc plasma. This exacerbates the condition and the arc will continue to develop as long as there’s sufficient voltage and current feeding the arc.
The arc plasma temperature can in some cases exceed 35,000 °F, so the primary mechanism of injury is the thermal release of incident energy causing tissue burns and clothing ignition. A secondary arc hazard is called arc blast. Under certain arc conditions a pressure wave develops reaching a pressure of 2,500 lb/sq ft. This blast pressure wave can cause significant injury, as well.