The RXIDG 21H/RAIDG relay is used as a selective and sensitive earth-fault protection in solidly earthed transmission networks. It is a single-phase time-overcurrent relay with a combined definite and logarithmic inverse time relay function.
The logarithmic inverse time characteristic has been choosen to produce selective tripping when used as earth-fault protection in solidly earthed transmission networks. The selectivity is ensured when the largest infeed is less than 80 % of current to the fault. All relay locations are set with the same primary sensitivity in order to provide selectivity, which is then achieved without having to use a directional measurement criteria.
RXIDG 21H has replaced the obsolete RXIDG 2H relay.