High-voltage circuit breakers (including breakers rated at intermediate voltage) are used for service on circuits with voltage ratings higher than 600 volts. Standard voltage ratings for these circuit breakers are from 4,160 to 765,000 volts and three-phase interrupting ratings of 50,000 to 50,000,000 kVA.

In the early stages of electrical system development, the major portion of high-voltage circuit breakers were oil circuit breakers. However, magnetic and compressed-air type air circuit breakers have been developed and are in use today.

The magnetic air circuit breaker is rated up to 750,000 kVA at 13,800 volts. This type of circuit breaker interrupts in air between two separable contacts with the aid of magnetic blowout coils. As the current-carrying contacts separate during a fault condition, the arc is drawn out horizontally and transferred to a set of arcing contacts. Simultaneously, the blowout coil provides a magnetic field to draw the arc upward into the arc chutes. The arc, aided by the blowout coil magnetic field and thermal effects, accelerates upward into the arc chute, where it is elongated and divided into many small segments.

The construction of this type of circuit breaker is similar to that of a large air circuit breaker used for low-voltage applications, except that they are all electrically operated.

Compressed-air circuit breakers, or air-blast circuit breakers, depend on a stream of compressed air directed toward the separable contacts of the breaker to interrupt the arc formed when the breaker is opened. Air-blast circuit breakers have recently been developed for use in extra high-voltage applications with standard ratings up to 765,000 volts.

Oil circuit breakers (OCBs) are circuit breakers that have their contacts immersed in oil. Current interruption takes place in oil which cools the arc developed and thereby quenches the arc. The poles of small oil circuit breakers can be placed in one oil tank; however, the large high-voltage circuit breakers have each pole in a separate oil tank. The oil tanks in oil circuit breakers are normally sealed. The electrical connections between the contacts and external circuits are made through porcelain bushings.

Circuit Breaker Control

As we have discussed, circuit breakers may be remotely operated. In order to operate the breakers from a remote location, there must be an electrical control circuit incorporated. Figure 7 shows a simple control circuit for a remotely-operated breaker.

Control power is supplied by an AC source and then rectified to DC. The major components of a simple control circuit are: the rectifier unit, the closing relay, the closing coil, the tripping coil, the auxiliary contacts, and the circuit breaker control switch.

Figure 7 Simple Circuit Breaker Control Circuit - Breaker Open

To close the remotely-operated circuit breaker, turn the circuit breaker control switch to the close position. This provides a complete path through the closing relay (CR) and energizes the closing relay. The closing relay shuts an auxiliary contact, which energizes the closing coil (CC), which, in turn, shuts the circuit breaker, as shown in Figure 8. The breaker latches in the closed position. Once the breaker is shut, the "b" contact associated with the closing relay opens, de-energizing the closing relay and, thereby, the closing coil. When the breaker closes, the "a" contact also closes, which enables the trip circuit for manual or automatic trips of the breaker. The circuit breaker control switch may now be released and will automatically return to the neutral position.

To open the circuit breaker, turn the circuit breaker control switch to the trip position. This action energizes the trip coil (TC), which acts directly on the circuit breaker to release the latching mechanism that holds the circuit breaker closed.

When the latching mechanism is released, the circuit breaker will open, opening the "a" contact for the tripping coil and de-energizing the tripping coil. Also, when the circuit breaker opens, the "b" contact will close, thereby setting up the circuit breaker to be remotely closed using the closing relay, when desired. The circuit breaker control switch may now be released.

Figure 8 Simple Circuit Breaker Control Circuit - Breaker Closed

As you can see from Figure 7 or 8, the circuit breaker control circuit can be designed so that any one of a number of protective features may be incorporated. The three most commonly-used automatic trip features for a circuit breaker are overcurrent (as discussed previously), underfrequency, and undervoltage. If any one of the conditions exists while the circuit breaker is closed, it will close its associated contact and energize the tripping coil, which, in turn, will trip the circuit breaker.

Summary

The important information covered in this chapter is summarized below.

Circuit Breaker Summary

The purpose of a circuit breaker is to provide a means for connecting and disconnecting circuits of relatively high capacities without causing damage to them.

The three most commonly-used automatic trip features for a circuit breaker are overcurrent, underfrequency, and undervoltage.