Overvoltage relays can be used to identify overvoltages and isolate equipment. These relays operate when the measured voltage exceeds a predetermined set-point. The voltage is usually measured using a Potential Transformers. The details of the ratio of the potential transformer are also entered into the relay. These relays are usually provided with a time delay. The time delay can be either instantaneous, fixed time or for IDMT (inverse definite minimum time) curves.
Generally, overvoltage relays are provided with sufficient time delay in order to avoid unwanted trippings due to transients (See article on Transients).
These relays can be used to isolate feeders and other equipment connected to the network. In the case of generators, these relay also switch off the excitation system to the generators thereby preventing voltage build-up.