Circuit breaker operation

Air-blast circuit breakers operate on the low-resistance principle the arc length is minimized and a blast of air is directed across the arc to cool and remove ionized gas. Air break circuit breakers are now used extensively at medium voltages while air blast circuit breakers have been employed up to the highest voltages. 

A preventive maintenance plan should be conducted for aU plant switchgear. Normal maintenance includes checking and tightening cable connections, testing air circuit-breaker operations, and checking relay and meter caUbrations. 

Copper and silver combined with refractory metals, such as tungsten, tungsten carbide, and molybdenum, are the principal materials for electrical contacts. A mixture of the powders is pressed and sintered, or a previously pressed and sintered refractory matrix is infiltrated with molten copper or silver in a separate heating operation. The composition is controlled by the porosity of the refractory matrix. Copper—tungsten contacts are used primarily in power-circuit breakers and transformer-tap charges. They are confined to an oil bath because of the rapid oxidation of copper in air. Copper—tungsten carbide compositions are used where greater mechanical wear resistance is necessary. 

Contactors are designed lor Ireqiient operation, (Circuit breakers are designed lor lar lower operations and therelore are never used as motor starters when repetitive operation is required, A typical example ol Ireqiient operation is mine-hoist senice, in which the motor must be reversed at the end ol every hoisting or lowering operation contactors would be used. 

For oil-filled control equipment such as autotransformer starters or oil circuit breakers (BOCBs or MOCBs, Chapter 19), insulating oil should be checked periodically for its insulating properties. Leading manufacturers of this equipment indicate the number of switching operations under different conditions of load and fault, after which the oil must be replaced and these must be followed. 

To achieve the above the characteristic of the fuses should lie well above the characteristic of the overcurrent and short-circuit releases of the breaker for the lower region of currents, such that only the breaker operates. However, it should lie well below the eharacteristic of the breaker in the higher region of currents to ensure that the fuses operate sufficiently quickly and long before the in-built releases. The breaker is thus prevented from operating at currents that are in excess of its breaking capacity. Figure 12.22 illustrates such a coordination. 

The two comparatively new type of breakers, vacuum and SFg are exceptions and have gained favour in view of their reliability and durability. For details on these breakers, see Sections 19.5.5 and 19.5.6, which also deal with their switching behaviour and phenomenon of arc reignition. Figures 12.44 and 12.45 illustrate typical power and control circuit diagrams respectively, for a 6.6 kV breaker-operated motor starter. 

Figure 12.44 A simple power circuit diagram for a breaker-operated HT motor starter...

Figure 19.16(a) An SFg circuit breaker 123-145 kV, 31.5 kA. It can also be pneumatic or spring operated, depending upon the arc quenching technique adopted and energy required to extinguish 2 the arc (Courtesy BHEL) 3... 

Principle of operation of a ground leakage circuit breaker (application of a core-balanced CT) 21/680... 

More commonly known as earth leakage circuit breakers (ELCBs) or residual current circuit breakers (RCCBs). They operate on the principle of residual current. 

Residual current operated circuit breakers, without integral overcurrent protection. General rules ... 

Fig. 10-5 Protection measure by separation of electrical operational equipment that is connected to the cathodically protected object via the housing, with an FI protection circuit leakage current circuit breaker (see Ref. 14) Tj and isolating transformers (see Ref. 15).

In the CCPS taxonomy, where the equipment boundary for a motor-operated pump includes the motor, shaft, seals, casing, impeller and the circuit breaker, then the demand-related failure is attributed to the pump. 

In addition to circuit breakers, there are other classes of automatic switches that can be controlled or operated remotely, but with current-interrupting capability. These include circuit switchers, reclosers, and sectionalizers. 

Cable sheath. Additional overhead line conductor Protective multiple earthing fP.M.E.) Buried Strip/rod/plate Earth-leakage circuit breaker, Voltage-operated Earth-leakage circuit breaker, Current-operated... 

The selection of operating mechanism will depend upon the control regime required for the circuit breaker. If the circuit breaker is to be operated locally (i.e. at the switchgear itself) then the manually charged spring option will be acceptable. If it is to be operated from a remote location then either motor-wound spring or solenoid operation will be required. 

This choice will depend upon circuit breaker availability required, since a motor-wound spring has a finite spring-wind time. A solenoid operation will impose a significant load upon the D.C. supplies while a motor-wound spring will require a supply to the spring-wind motor. 

In laboratories in operation for some time, there are often not enough electrical outlets or enough circuits to handle the load. Retrofitting for more power is very expensive. One laboratory received several bids for wiring their new facility. The accepted one was much lower than the others because money was saved by installing the bare minimum number of circuits and no spare circuit breakers. Every piece of conduit was jammed full of... 

If the potential of the Daniell cell is to be determined accurately, we have already seen that the measurement has to be made at zero current. In order to ensure zero current, the internal resistance of the voltmeter shown in Figure 3.1 must be vast, as discussed in the previous chapter. The voltmeter operates in much the same way as a switch or circuit breaker does reaction (in this case, cell discharge) would occur but for the incorporation of the voltmeter in the circuit. 

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