Molded case circuit breakers

Capacitors as bulk units can be connected to the supply busbar via a fuse switch, molded-case circuit breaker of air circuit breaker. In this type of installation control is purely manual, and in cases of a reasonably constant load and where the amount of power factor correction is limited such a manually controlled system is perfectly adequate. The supply authority may, however, require to be informed that a capacitor bank is permanently connected to the supply. Capacitors are more generally connected either in banks controlled from a VAR sensitive relay or across individual loads (e.g. motors). 

Molding Compounds. Melamine molding compounds can be produced by compression or injection molding and are used to make dinnerware for the home, in food service and for institutional settings. They also have some applications in circuit breakers, mixing bowls, small appliance housings and cosmetic cases. 

Service entrance equipment should be installed in an indoor electrical room whenever possible. The equipment should include one to six main service entrance power circuit breakers or load break disconnect switches and feeder circuit breakers for loads within the plant. Circuit breakers and disconnect switches for 480/277 V systems that are rated at 1000 A or more must include groimd fault protection. Feeder circuit breakers will typically be molded case. Integrally fused circuit breakers are available with very high short circuit lAC ratings for both power and molded case circuit breakers. Fuses and current limiters for integrally fused circuit breakers must be stored within the electrical room. 

A maintenance program should document all tests, inspections, and faults cleared by a main and feeder circuit breaker. Lack of such a program will reduce the reliability of the electrical equipment.Molded case circuit breakers require no internal maintenance. They should be inspected for broken casing and loose connections after they operate to clear a fault and should be replaced after they have operated to clear two faults. Feeder circuit breakers, especially older circuit breakers, should be exercised (repeatedly opened and closed) whenever possible.f" Part of the initial commissioning of an electrical power system must include testing of the ground fault protection. ... 

Switching devices for the power circuits that need to be operated frequently are usually circuit breakers and contactors. Manually operated load break switches, fuse-switch combinations and molded case circuit breakers are used for feeder circuits that are infrequently operated. 

Circuit Breakers. Examine the enclosure interior and the circuit breaker for evidence of possible damage. If evidence of damage is not present, the breaker may be reset and turned on. If it is suspected that the circuit breaker has opened several short-circuit faults or if signs of possible deterioration appear within the enclosure, the test described in paragraph AB 1-2.38 of the NEMA standards publication, Molded Case Circuit Breakers, Publication AB 1, should be performed before restoring the breaker to service. 

The common enclosure design (Fig. 5.141) and the use of combination starters offer both economy and ease of installation in multiple motor control installations. In addition, motor control centers (MCCs) provide proper coordination between short-circuit protective devices and the controller. Since MCCs are engineered systems, the components are closely coordinated to work together, and the unit is rated for a particular value of short-circuit interrupting duty at the point of its installation. MCCs may contain a molded-case circuit breaker and starter, or a fused switch and a starter. 

Molded-case circuit breaker of the quick-make, quick-break, trip-free, heavy duty,... 

Branch Protective Devices. All molded-case circuit breakers, fusible switches, and/or motor starter nnits nsed as a protective device in a branch circuit should meet the requirements of the appropriate paragraph below. 

Two types of circuit breakers are commonly used for applications at 600 V and less power circuit breakers and molded-case circuit breakers. 

Power circuit breakers are open-construction assemblies on metal frames for service in switchgear compartments and similar enclosures. Molded-case circuit breakers also are used in switchgear. While their protective value is high, they offer few control options. 

Low-voltage power circuit breaker, either drawout or stationary (molded-case breakers... 

Low-Voltage Power Circuit Breaker Switchgear. This is metal-enclosed and generally consists of low-voltage power circuit breakers bare or insulated bus and connections control instruments, meters, and relays and control wiring and accessories, together with cable and busway termination facihties. Circuit breakers may be drawout type, stationary air circuit breakers, or molded-case circuit breakers, usually of the solid-state, adjustable trip variety. 

Protection. The electrical protection of the generator system can vary from simple molded-case drcuit breakers to insulated-case breakers and to metal-frame air circuit breakers. Both insulated-case and air breakers are available with either fixed mounting or drawout provisions. For optimum flexibility in long-term use, the drawout breaker is superior and recommended. Since engine generators have a limited short-circuit capability, care should be taken in breaker selection for both generator and distribution to achieve proper selective trip coordination. Where selective trip coordination is required, the generator must be supplied with three per unit short-circuit sustain capability for 10 s. 

USNRC Information Notice 92-51, Misapplication and inadequate testing of molded case circuit breakers, July 9,1992. 

Varistor installation for auxiliary relays in reclosure. Independent molded case circuit breakers (MCCB) installation for dc circuits in reclosure. 

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