An arc which lasts for about 500ms can cause severe damage to people and equipment. An arc lasting 100ms will cause lesser damage. An arc lasting just 35 ms will cause far lesser damage.
Normal protection schemes involving over current and earth fault relays will not be fast enough to isolate these faults. Besides, making these relays more sensitive can cause undesirable false trippings.
Arc Flash Relays work by sensing the light intensity caused during the flash over. An arc causes a tremendous increase in the light intensity, often of the order of several thousand lux. The light intensity in a normal office lighting will be around 200 or 300 lux. The light sensed by the sensors is compared with a preset reference level. The relay activates the trip signal if this reference is exceeded.
Modern Arc Flash Relays can operate within 2.5 ms of a fault + the operating time of the circuit breaker. The operating time of the breaker needs to be checked periodically. Circuit breakers should open within 50ms of the command being given. Delayed opening will seriously compromise protection.