In a simple way electrical relay is shown in the figure above. Here the fixed coil is fed from the circuit to be protected. When there is a current in fixed coil above the pick up value, the iron plunger is so attracted, it moves upward and makes NO contact closed. The function of this relay is very fast. The normally open (NO) contacts of the relay are closed as soon as current in the fixed coil crosses the pick up value. This is a simplest example of instantaneous relay. Because ideally there is no time delay between the instant when actuating current crosses the pick up level and the instant of closing the No contacts.
Definition of Instantaneous RelayAn instantaneous relay is one in which there is no time delay provided intentionally. More specifically ideally there is no time required to operate the relay. Although there is some time delay which can not be avoided.
As the current coil is an inductor, there would be a certain delay to reach the current in the coil to its maximum value. There is also some time required for mechanical movement plunger in the relay. These time delays are inherent in the instantaneous relay but no other time delay is intentionally added. These relays can be operated in less than 0.1 sec.
Examples of Instantaneous RelaysThere are various types of relay which can be considered as instantaneous relay. Such as, attracted armature relay where an iron plunger is attracted by an electromagnet to actuate the relay. When attractive force of the electromagnet crosses its pick up level, the iron plunger starts move towards the magnet and crosses the relay contacts. The magnetic strength of the electromagnet depends upon the current flows the coil conductors.
Another popular example of instantaneous relay, is solenoid type relay. When current in the solenoid crosses pick up value, the solenoid attracts an iron plunger which moves to close the relay contacts. Balance beam relay is also an well known example of instantaneous relay. Here balance of one horizontally placed beam is disturbed due to pick up current in the relay coil. Due to unequal torques at two ends of the beam, it starts rotating against the hinge and ultimately closes the contacts of the relay.