a relay designed to create a required time lag in the transmission of an action between individual units of automatic devices or from one device to another. Three characteristic parts may be identified in a time-delay relay: the sensing part, which causes the relay to operate when a control signal is received; the delay part, which produces the specified time lag; and the actuating part, which applies the action to the controlled system.
Several classifications of time-delay relays are possible. With respect to the method of triggering the relay, we speak of time-delay relays with electric, hydraulic, pneumatic, or manual control. According to the type of output signal, electric, pneumatic, and hydraulic time-delay relays are distinguished. The most widely used classification depends on the method of delay. Thus, we speak of time-delay relays with electric, pneumatic, magnetic, mechanical, electromechanical, thermal, or hydraulic delay.
The type of time-delay relay chosen for a particular use is based on a number of factors. One factor is the operating principle of the automatic devices in question; for example, the device may be electric, pneumatic, or hydraulic or may be analog or digital in nature. Another factor is the operating conditions, which include operating temperatures, frequency of use, presence of vibrations, and level of humidity. Finally, the reliability and cost of the time-delay relay are also important factors.