Time-limit relays

In power stations and sub-stations feeding large alternating-and direct-current distribution systems, relays form a necessary part of the station equipment for the protection of apparatus and feeders, and for the purpose of rendering the operation of a system automatic. In connection with the operation of oil-switches on high-pressure alternating-current systems the function of the relay is to close the control-circuit to the switch, thereby opening the main circuit when the current exceeds the value at which the relay is adjusted to operate. Not taking into consideration the time-element of the switch, its operation would be practically instantaneous if controlled by a simple relay. Therefore, since heavy currents due to swings of the load may often flow for such short periods that it would not be desirable to open the circuit, relays are provided with some form of time-limiting device by means of which the length of time a given current may flow without operating the switch can be adjusted to meet existing conditions.