If an arcing fault occurs on either a medium or low voltage switchgear bus, the amount of incident energy released is related to how long of a delay occurs between the initiation of the fault and the time the main circuit breaker is signaled by the protective device to open.
One solution to this problem is to install an optical detection relaying system that detects the intense light given off by the arc-flash and instantaneously trips the main circuit breaker. This type of detection system is available through at least one manufacturer and consists of a light-detecting relay that is connected by a fiber optic cable to the light sensor(s).
In order to prevent false trips due to flash bulbs, arcs from circuit breakers performing switching, strong sunlight, etc., phase and ground peak over current detectors are used to supervise the optical sensors. Solid-state tripping outputs provide a typical arc flash sensing time less than 2.5 milliseconds. Total clearing time is reduced to the breaker operating time plus 2.5 milliseconds. This speed advantage over more conventional protective schemes can significantly reduce the hazard risk category, allowing maintenance personnel to wear a more manageable level of PPE
