Circuit-breakers

Circuit breakers Circuit fabrication Circuits B. circulans... 

Sulfur hexafluoride [2551-62-4] 6 molecular weight 146.07, is a colorless, odorless, tasteless gas. It is not flammable and not particularly reactive. Its high chemical stabiUty and excellent electrical characteristics have led to widespread use in various kinds of electrical and electronic equipment such as circuit breakers, capacitors, transformers, microwave components, etc (see Electronic materials). Other properties of the gas have led to limited usage in a variety of unique appHcations ranging from medical appHcations to space research. 

Mini-Subst tlons. Development of SF -iasulated lines and circuit breakers made possible development of a compact electric substation that requires one-tenth the land area of conventional designs (64,68). Other advantages of the ministation are freedom from effects of weather and pollution, and reduced risk of vandaUsm (69). Substation units ia which SF iasulates 400 and 500 kV d-c equipment have been developed (70). 

Transformers. Units iasulated with SF are used mainly ia circuit breakers. Some SF power transformers have been designed, but the... 

Because of the extreme toxicity of this material and the possibiUty it could be present in failed circuit breakers containing decomposed SF, several studies have been carried out to develop analytical methods and investigate possible ways to minimize environmental exposure. One method capable of determining S2F2Q in the ppb range has been reviewed (123). 

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. 

Polysulfones also offer desirable properties for cookware appHcations, eg, microwave transparency and environmental resistance to most common detergents. Resistance to various sterilizing media (eg, steam, disinfectants, and gamma radiation) makes polysulfones the resin family of choice for many medical devices. Uses in the electrical and electronic industry include printed circuit boards, circuit breaker components, connectors, sockets, and business machine parts, to mention a few. The good clarity of PSF makes it attractive for food service and food processing uses. Examples of appHcations in this area include coffee decanters and automated dairy processing components. 

Amino molding compounds can be compression, injection, or transfer molded. Urea molding compound has found wide use and acceptance in the electrical surface wiring device industry. Typical appHcations are circuit breakers, switches, wall plates, and duplex outlets. Urea is also used in closures, stove hardware, buttons, and small housings. Melamine molding compound is used primarily in dinnerware appHcations for both domestic and institutional use. It is also used in electrical wiring devices, ashtrays, buttons, and housings. 

In addition to the circuit breaker, there have been a number of other SMA appHcations for various functions in electric power generation (qv), distribution, and transmission systems. One such device is a thermal indicator that provides a signal visible from the ground of a hot junction or connector in a distribution yard. Such hot spots occur as a result of the loosening of bus bar connectors owing to cycHc temperature as the electric load varies. In addition to the use of SMA flags as a hot-spot indicators, actuators that automatically maintain the contact force in a bus bar connection have been demonstrated. Based on a BeUeviHe washer fabricated from a Cu—Al—Ni SMA trained to exhibit two-way memory, these washers, when heated by a hot joint, increase their force output and correct the condition. A 30 mm diameter washer 3 mm thick can produce a force of over 4000 N. Similar in purpose... 

Line Starters and Combination Starters A line starter consists of a contactor (motor-starting switch) and motor-overload relays, (Contactors are capable of canving and interrupting normal motor-starting and -running currents they are not, hovv ever, normally capable of interrupting short-circuit currents. They must be backed up by fuses or a circuit breaker for this function,... 

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. 

IEEE, Results of an investigation on the over voltages due to a vacuum circuit breaker when switching an H. V. motor, IEEE 85 SM 370-2(1985). 

MCCB (moulded case circuit breaker) or MCB (miniature circuit breaker) and contactor] and the overload relay. These recommendations permit damage of components on fault to varying degrees as noted below ... 

With the availability of 3.3 and 6.6 kV vacuum contactors the control of HT motors up to 6.6 kV systems has now become easier and economical, compact and even more reliable. For 11 kV. systems, vacuum as well as SF (Sulphur hexafluoride) breakers can be used. The HT motor s switching and protection through a vacuum contactor provides a replica of an LT system. The earlier practice of using an HT OCB, MOCB, or an air blast circuit breaker for the interruption of an HT circuit is now a concept of the past. 

Figure 12.54 Comparison of arc energy produced by medium-voltage circuit breakers...

Moulded case circuit breaker (MCCB) (Courtesy GE Power Controls)... 

Moulded case circuit breakers and moulded case switches up to 1000 V a.c. Rating more than 5000 A Moulded ease circuit breakers and their application, up to 1000 V a.c. Rating 5000 A and more Low-voliage cartridge fuses... 

Ramaswamy, R., Vacuum circuit breakers, Siemens Circuit, XXIII, April (1988). 

Note With the availability of modern circuit breakers with higher short-time ratings of 65 kA/80 kA/100 kA, it is now possible to use even larger transformers up to 2500 kVA, depending upon their other merits. 

Wherever a circuit breaker is to be housed, this should be in a separate compartment. 

Withdrawal of a draw-out circuit breaker or a withdrawable switch or contactor will not be possible unless they are in an OFF position. 

Metal enclosed low voltage power circuit breaker switchgear Metal clad and station type cubicle switchgear Metal enclosed interrupter switchgear Guide for calculating losses in isolated phase bus... 

Preferred ratings, related requirements and application recommendations for LV power circuit breakers and a.c. power circuit protectors... 

Short circuit generator Circuit breaker H.T. transformer Master breaker... 

Electrical accessories circuit breakers for over current protection for household and similar installations 8828/1996 BSEN 60898/1991... 

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