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According to the Occupational Health & Safety Association (OSHA), 5 to 10 arc flash events occur per day in the U.S., with most of these resulting in injuries and some being deadly. Beyond being a severe hazard to personnel, arcing faults also result in significant economic losses due to electrical fire damage to equipment and interruption of processes.
The most common cause of arc faults is insulation failure. They can also be caused by loose connections, insufficient mechanical dimensioning, equipment malfunction, contamination or degradation of insulation, and animals. An arc flash is the light and heat produced as part of an arc fault. OSHA estimates that over two-thirds of all arc flash incidents are the result of worker error, for example due to entering a live panel or field, careless use of tools, or leaving a temporary earth connected.
In detail, an arc fault is a short circuit conducted through ionized air (plasma) between one live part and ground or between two live parts. High power arc-flash faults can be characterized as electrical explosions, releasing large amounts of energy in the form of radiant heat, intense light, and high-pressure waves. The temperature of the plasma can reach 20,000 Kelvin. The increase in temperature expands the volume of the air, causing a pressure wave. Because of the high temperature, circuit components can change physical state from solid to vapor. For example, vaporizing copper expands by a factor of 67,000, significantly increasing the pressure. In addition to danger caused by radiation, heat, and pressure wave, there may be shrapnel and toxic gases, causing additional hazards to personnel.
The consequences of an arcing fault depend on the incident energy. There are four factors that determine the energy: distance, voltage, current, and arcing time. What is certain, however, is that the result of an arc flash is equipment damage and hazards to personnel. Fortunately, there are protective steps that can be taken to optimize electrical safety. Let’s take a look at the advantages and limitations of each.
Protective clothing
Personal protective equipment (PPE), i.e. clothing, is typical required to protect against an arc fault. However, PPE should to be sized to the switchgear risk level, if it’s known. Though this sizing requirement is clearly defined by US electrical code it is not deployed in most geographies. PPE will protect personnel from some arc flash hazards, but it may not protect them against the blast of an arc flash event, which can break bones, puncture organs, and propel workers into walls or equipment. And, of course, PPE only protects people. Equipment is not protected and usually requires a costly refurbishment after the fault.
Arc flash containment
Switchgear can be type tested to IEC 60298. According to this standard it is possible to specify an internal arc rating with fault times of 100, 500 or 1,000 ms. The switchgear is designed to remain intact in the event of an internal arc, at least for the specified internal arc rating time. However, the arc-resistant design of such switchgear will not be effective if a switchgear door is open, which is often the situation during maintenance or commissioning.
High-impedance bus protection
Mostly used in MV/HV applications, high impedance bus protection will detect an arcing fault inside the protected circuits within 40 to 65 ms. The effectiveness of this protection method will be limited by the location of the installed CTs and monitor circuit. If a fault occurs in a cable zone, this protection method will not be totally efficient. Installation of many CTs will improve performance, but will have significant additional cost and be very hard to install in existing installations.
Optical arc flash protection
Based on optical detection of an arc flash ignition, active arc flash protection detects a fault very early, and then activates protection to extinguish it. This solution requires installation of optical sensors on surveilled zones. With some programs, it is possible to deploy selectivity to avoid a global shutdown. The protection system is active and responsive during operation or maintenance, even if the switchgear door is open. Depending on the hardware used, such a system will operate in less than 1 ms (without breaker operation time). For efficient arc flash fault protection, sensors should be deployed in all zones.
To choose the most appropriate arc flash protection system, it is necessary to evaluate the risk of arc flash occurrence. This can be determined by calculation and type testing for critical switchgears (for arc-flash containment).
For critical sites, all possible precautions are necessary; therefore active optical arc flash protection system should be deployed. This should include main/feeders control to avoid global shutdown. In all cases, protective clothing should be worn during maintenance. For non-critical sites, optical protection could be deployed to protect equipment, including main control.
For more information, read our electrical safety articles here on the SE Blog.
