Arc fault protection is a relatively new technique employed for the fast clearance of arcing faults on BUS bars & within metal clad switchgear & associated cable boxes. The arc is detected using an optical sensor & the signal input to a protection device which also monitors the load current on the system. A trip signal can be achieved in less than 10ms using arc detection only or within 15ms when using the integrated instantaneous overcurrent check. This is considerably faster than a traditional IDMT overcurrent relay & provides additional protection from the onset of arcing faults with relatively low fault currents.
The 1S26 Arc Fault relay is designed for application on new or existing medium voltage BUS systems with up to two incoming lines. For new installation the high speed arc protection technique offers cost advantages over traditional high impedance differential schemes. For existing installations which may not have any BUS protection or using simple earth leakage protection it is relatively simple & cost effective to retrofit an arc protection scheme as dedicated protection CT’s are not required.
For the outgoing feeders arguably the greatest risk of arc fault damage exists at the CB cable termination & in the CB chamber itself due to the slow clearance times of the IDMT feeder protection. The CB cable termination is particularly at risk to ingress of moisture & rodent damage. Application of arc protection in the feeder circuits can be achieved using spare sensors & trip contacts on the 1S26 or alternatively additional 1S20 & 1S25 relays may be employed for this purpose.