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Electromechanical Relays

The actuating forces are created by a combination of input signals, stored energy in springs, and dashpots. The plunger type relay consists of a moving plunger inside a stationary electromagnet. It is typically applied as an instantaneous level detector. The induction-type relay is similar to the operation of a single-phase ac motor in that it requires the interaction of two fluxes across a disc or cup. The fluxes can be produced by two separate inputs or by one input electrically separated into two components. Depending on the treatment of the inputs (i.e. one current separated into two fluxes, two currents, or a current and a voltage), this design can be used for a time-delay overcurrent relay, a directional relay, or a distance relay.

Solid-State Relays

All of the functions and characteristics of electromechanical relays can be performed by solid-state devices, either as discrete components or as integrated circuits. They use low-power components, either analog circuits for fault-sensing or measuring circuits as a digital logic circuit for operation. There are performance and economic advantages associated with the flexibility and reduced size of solid-state devices. Their settings are more repeatable and hold closer tolerances. Their characteristics can be shaped by adjusting logic elements as opposed to the fixed characteristics of induction discs or cups.


Individual types of electrical apparatus, of course, require protective schemes that are specifically applicable to the problem at hand. There are, however, common detection principles, relaying designs and devices that apply to all.

Transmission Line

Protection Transmission lines utilize the widest variety of schemes and equipment. In ascending order of cost and complexity they are fuses, instantaneous overcurrent relays, time delay overcurrent relays, directional overcurrent relays, distance relays, and pilot protection. Fuses are used primarily on distribution systems. Instantaneous overcurrent relays provide a first zone protection on low-voltage systems. Time delay overcurrent relays provide a backup protection on low-voltage systems. Directional overcurrent relays are required in loop systems where fault current can flow in either direction. Distance relays provide a blocking and tripping function for pilot relaying and first, second, and third zone backup protection on high-voltage and extra-high-voltage systems. Pilot protection provides primary protection for 100 percent of the line segment by transmitting information at each terminal to all other terminals. It requires a communication channel such as power line carrier, fiber optics, microwave, or wire pilot.

Rotating Apparatus

The dominant protection scheme for generators and motors is the differential relay. Access to all entry points of the protected zone is usually readily available, no coordination with the protection of other connected apparatus is required, and the faulted zone is quickly identified. Motor protection also includes instantaneous and time delay overcurrent relays for backup.

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