While an arc flash is sometimes used interchangeably with “arc fault”, an arc flash is more accurately defined as the light produced during an arc fault. An arc fault is a type of electrical fault that results from the breakdown of an insulating medium between two conductors where the energy is sufficient to sustain an arc across the insulator (often air) and can cause extreme amounts of light (arc flash), immense heat upwards of 19,000 degrees (substitute the symbol) C, and a resulting explosive pressure wave (arc blast). These forces combine to create a hazardous condition that can vaporize metal, destroy equipment, and pose a significant hazard to anyone in the vicinity.
Arc-Flash relays are microprocessor-based devices that use optical sensors to detect the onset of a flash. The sensors are strategically placed in various cubicles or drawers inside the switchboard. Installing an Arc-Flash relay to rapidly detect developing arc flashes greatly reduces the total clearing time and the amount of energy released through an arcing fault. In turn, there is less damage to equipment and fewer and less severe injuries to nearby personnel.
How much energy is in a typical arc-flash incident?
A phase-to-phase fault on a 480-V system with 20,000 amperes of fault current provides 9,600,000 watts of power. Imagine that there is no arc protection and the fault lasts for 200 milliseconds before the overcurrent protection clears it. The released energy would be 2 MJ, which corresponds roughly to a stick of dynamite.
The energy formula is as follows:
Energy = voltage x current x time = 480 V x 20,000 A x 0.2 s = 1,920,000 J
For a given system voltage, two factors can be adjusted to reduce arc-flash energy: time and current
Time can be reduced by using a device such as the PGR-8800 or AF0500 to rapidly detect an arc flash, thus cause the connected circuit breaker to trip at its instantaneous speed, overriding any inverse-time delay. Current can be reduced by using current-limiting fuses or, in case of phase-to-ground faults, by using high-resistance grounding.
What is the electrical symbol of an arc-flash relay?
Although there is no formal symbol, Littelfuse is currently using the following:
How long has Littelfuse had an arc-flash relay in service?
Littelfuse acquired arc-flash technology from Selco, which had been manufacturing arc-flash relays for almost 20 years. The product was launched in North America in August of 2011. Littelfuse has numerous installations throughout the world and an estimated 18,000 arc-flash relays are installed world-wide.
What are the key considerations when choosing an arc-flash relay?
We have written a white paper on considerations for choosing an arc-flash relay that examines advantages and drawbacks. It is available to download on our website at Littelfuse.com/ArcFlash.
The Consequences of Arc Flash
Arcing and arc flashes are uncontrolled, intense, luminous discharges of electrical energy that occur when electric current flows across what is normally an insulating medium. The most common cause of arc faults is insulation failure. These failures may be caused by defective or aging insulation material, poor or incorrect maintenance, dust, moisture, vermin, and human error (touching a test probe to the wrong surface or a tool slipping and touching live conductors). Arc-Flash events are dangerous, and potentially fatal, to personnel. According to OSHA, industrial Arc-Flash events cause about 80% of electrically-related accidents and fatalities
among qualified electrical workers. Even if personnel injuries are avoided, Arc Flash can destroy equipment, resulting in costly replacement and downtime.