Control circuit competence, control relays, time delay relays, latched relays. This topic covers control relays, time delay relays, and latched relays. A control relay or auxiliary relay is an electrically operated switch with no manual control. It operates basically on the principle of an electromagnetic coil and plunger. The plunger will actuate the contacts just similar to the plate in a push button. When the coil is energized, the contact configurations change, that is from meet to break and vise versa, and remain in that condition as long as coil remains energized. When the coil is de-energized, contacts return to the original state. Control relays are extensively used in electrical logic circuits. It can have any number of contact configurations as per the design of the manufacturer. The status of contact changes instantaneously with the coil energization or de-energization. Timer relays are similar to control relays, but a preset delay occurs while contact status changes after coil energization or de-energization. If the delay occurs after the energization of the coil, such relays are called ON delay timer or simply timer. Let us study the operation of an ON delay timer by means of a graph. The x-axis represents the time in seconds, y-axis represents energized or de-energized status of timer coil. Also corresponding on or off status of the timer's contacts are represented on the y-axis. As you can see, initially, the coil is de-energized and corresponding contacts are also off. Now, the timer coil is getting energized. As you can see, the timer coil is energized, but the timer contact still remains in off status. The timer contact shall switch on only after the preset time delay is completed. In this case, our set time delay is a five seconds. Preset on time delay of five seconds has elapsed and timer contact has switched on. Now the timer coil is getting de-energized. Contact status change is instantaneous during de-energization of ON delay timers. If the delay occurs after de-energization, such timers are called OFF delay timers. The operation of an OFF delay timer shall be explained now. As you can see, initially, the coil is de-energized and corresponding contacts are also off. Now, the timer coil is getting energized. As you can see, the timer coil is energized. Also, the timer contact has turned on without any delay. In an OFF delay timer, contact status changes instantaneously during energization of timer coil. Let us see the switching off operation. At this instant, the timer coil is getting de-energized. Preset of time delay of 10 seconds has elapsed and timer contact has switched off. In control relays, de-energization of the coil takes place when control supply is removed or in the absence of control supply. Any failure of the incoming signal, such as stuck contact of another relay, failure of power supply, and cable disconnection can de-energize or reset the coil. Unlike control relays, instead of one electromagnetic coil, there are two coils in a latched type of relay. Contacts change status during energization of the first coil. If the power supply is removed, the first coil is de-energized, contact status do not undergo any change. Status of the contacts reverts to the original position only if the second coil is energized. All energization can be just for a momentary period. The second coil is called as a reset coil. Latched type of relays is utilized in sensitive circuits to ensure contact status change occurs only on receipt of positive signal. Let us turn to the operation of a latched relay by means of a graph. The x-axis represents the time in seconds, y-axis represents energized or de-energized status of relay coils. Also corresponding on or off status of the relay contacts are also represented on the y-axis. As you can see, initially, the coil is de-energized and corresponding contacts are also off. Now, the relay coil A1 is getting energized. As you can see, the relay coil A1 is energized. Also, the relay contacts are turned on. The relay coil A1 is getting de-energized. Notice the relay contacts remain in on position. Relay contacts turn off only when the reset coil A2 is energized. In some latched relays, hand reset option is available. In this case, reset operation can be done manually as well. Sometimes the reset coil function may be replaced by manual resetting also. Application, circuit breaker contact multiplication is preferable to achieve through latched relays. Lock-out relays, ANSI designation-86 are used to trip the circuit breakers on getting trip signals from protective relays. Use of such relays prevent accidental re-closing CBs without removing the cause of fault. Lock-out relays may be off hand reset type or both electrical and manual reset type. Here's a quick recap of what you've learned thus far. Control relay or auxiliary relays are electrically operated switches with no manual control. Timer relays are similar to control relays, but a preset delay occurs while contact status changes after coil energization or de-energization. On delay timer and off delay timer are two types of timer relays. Control relays are prone to de-energization in case of control supply failure due to wire disconnection or control supply failure, etc. Two coils in a latched type of relays as compared to one in control relays, ensure contact status change occurs only on receipt of positive signal. Latched type of relays are utilized insensitive circuits like lock-out relay and circuit breaker contact multiplication
