SEL Arc-Flash Detection (AFD)

Arc-Flash Basics What is an arc-flash hazard? According to the National Fire Protection Association (NFPA) 70E: Standard for Electrical Safety in the Workplace, an arc-lash hazard is “a source of possible injury or damage to health associated with the release of energy caused by an electrical arc.” In an arc-lash event, a lashover of electric current leaves its intended path and travels through the air from one conductor to another or to ground. During such an event, the air gets suficiently heated to become ionized; the air becomes the conductor. Arc-lash events can cause dangerous and potentially fatal heat, ultraviolet radiation, blast pressure, and lying shrapnel along with deafening sound waves. The resulting release of arc-lash energy, referred to as “incident energy,” is a measure of thermal energy at a working distance from an arc fault. What causes an arc flash? Arc-lash events can be caused by numerous factors, such as the shorting of two phases, which creates a phase-to-phase fault. Common causes of phase-to-phase faults include dropped tools, accidental contact by maintenance personnel, the buildup of corrosion or conductive dust on conductors, and the presence of pests, like mice, rats, or snakes, that come in contact with energized conductors. What elements must be present for an arc-flash event to occur? 1. 2. Air gap from using air as an insulator or from a damaged insulator. Suficient energy (fault current available from the power source) to initiate and sustain the arc-lash event. This is normally satisied in low-voltage (480 V and above) and medium-voltage switchgear environments. 3. Something to trigger the arc lash. This could be a rodent, acting as a conductor across the phase conductors, or one of the causes listed above. How does a protective relay help mitigate incident energy from an arc-flash event? The faster the extinction of the arc-lash event, the smaller the amount of incident energy that is released. Since arc-lash energy is proportional to voltage multiplied by current multiplied by time (V•I•t), the protective relay acts to reduce the duration of the arc-lash event, or “t.” In an arc-lash event, this equals the time elapsed before the relay trips plus the breaker clearing time. Relays don’t have any control over the voltage and current. The voltage across an arc is determined by the conductor gap; the arcing current is limited by the available fault current. What the relay can impact is time. By limiting the duration of the arc-lash event, the relay can dramatically reduce the amount of incident energy produced by an arc-lash event. What role do air gaps play in an arc flash? While air is a good electrical insulator under normal conditions, if the air gets heated up and becomes ionized, that air will become a conductor, resulting in an arc-lash event. In environments with air gaps, such as electrical switchgear, arc-lash hazards exist with exposed copper or aluminum conductors; e.g., a busbar, contacts on the motor contactor, or breaker contacts. 4 Are there other dangers in an arc-flash event besides intense heat and light? High-voltage arcs can also produce considerable pressure waves by rapidly heating the air and creating a blast. This pressure blast can hit a worker with great force and send molten metal droplets from melted copper and aluminum electrical components great distances at extremely high velocities. How much light is produced in an arc flash? In a very small (2,000 A) arc-lash event, the measured light intensity can be 2,000,000 lux, or twenty times brighter than bright sunlight, at a distance of 1 meter from the arc-lash source. For more realistic arcing currents (e.g., 17,000 A), we have recorded arc-lash light intensities of 12,000,000 lux. This is equivalent to the amount of light generated by 120 suns in the sky at