RVT is used in sub-stations to sense unbalance of voltage in H.T. 3-phase system. Its H. V. winding is star connected. It has 2 L.V. windings. First one is star connected which is used for metering purpose. Second one is in open-delta. Under healthy conditions, output voltage of this winding is very low. But if there is earth fault on any one of the lines, output voltage of this open-delta winding goes up instantaneously & gives signal to earth fault relay. This, in turn trips the HT circuit breaker in substation.