These three types of relays have been specifically designed for the protection of electrical equipment against zero sequence (earth or ground) faults, and are used for rotating machines, transformers, or throughout the distribution network.
These relays must be supplied from a specific type of CEE ring transformer, either opening or non-opening type, and whose internal diameter may be in the range from 25 to 245 mm.

Their modular draw-out case, type R1, may be mounted either as a separate relay, flush or projection mounting, or as an alement in a standard 19" rack cradle.