ELECTRIC POWER
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switchgear is usually painted a light gray, ANSI Std. 70, using lacquer or enamel. Outdoor switchgear is usually painted the same color inside, but the outside is painted a dark gray, ANSI Std. 24. Epoxy paints may be necessary where corrosive atmospheres exist and special protection is required. Outdoor switchgear is mounted on sill channels and the structure is undercoated with a heavy coat of an asphalt material to prevent rusting. (b) Metal-clad switchgear may be obtained for use indoors or in weatherproof structures for outdoor use. The same comments that apply to outdoor metal-enclosed switchgear (paragraph (a) above) apply to outdoor metal-clad switchgear. Outdoor switchgear may be used to serve indoor loads when indoor space is limited, when corrosive or explosive atmospheres are present inside the building, or when the indoor atmosphere is excessively dusty. Metal-clad switchgear structures differ from the standard low-voltage switchgear structures in several respects. By definition of metal-clad switchgear the circuit breakers must be of the removable type. Circuit breakers must be enclosed, as in lowvoltage switchgear, but buses, potential transformers, control power transformers and cable terminals must also be enclosed in separate metal compartments. All metal barriers must be grounded. Shutters must be provided, which close automatically when a breaker is withdrawn, to prevent operators from contacting the primary contacts or the bus which may be energized. Interlocks must also be provided to prevent moving the breaker into or out of the connected position when it is closed and to prevent closing the breaker when in an intermediate position. Instruments and relays may be mounted on the door through which the breaker is inserted into the cubicle, but when this is done, a barrier must be provided between the instruments and the breaker. Circuit breakers may be moved into the connected position in metal-clad switchgear by either the horizontal-draw-out or the vertical-lift method. When the horizontal-draw-out method is used, the breaker is moved horizontally into position in the cubicle; then a racking mechanism is provided to force the breaker into the operate position where the primary contacts are fully engaged. The secondary contacts for control of the breaker must also be in contact. A test position is also provided where the secondary contacts are separated from the primary contacts by a safe distance. Vertical-lift breakers are moved into the cubicle beneath the stationary primary contacts, then raised with either a manually or electrically operated hoist until the primary and secondary contacts are fully engaged. To connect a breaker located outside the cubicle, a plug-jumper (replacing a test position) is provided to control circuits in the switchgear for test breaker electrical operation. 3.5.6.4 Mechanism. The mechanism of a circuit breaker is the complete assembly of levers and other parts that actuates the moving contacts. The mechanism consists of two parts, the tripping mechanism and the closing mechanism. (a) The tripping mechanism is an electrically or mechanically operated device that releases the contacts with a mousetrap like spring-driven snap action. The tripping mechanism consists of an electromagnet (trip coil) acting as a trigger that releases a latch permitting the breaker to open. The opening energy is normally supplied by accelerating springs that are charged (compressed) when the breaker is closed. All circuit breakers are equipped with a manual trip device that is connected to the tripping linkage. Automatic tripping is normally performed by one of three methods: o Series-overcurrent tripping. o Shunt tripping. o Undervoltage tripping. Series-overcurrent tripping is the tripping of a circuit breaker from a trip coil in series with the circuit responding to an increase in the circuit current above a predetermined value. Series tripping is normally used on low-voltage circuit breakers. These breakers are complete with adjustable (longtime, short-time, and instantaneous) direct-acting series overload tripping devices. Most manufacturers offer static trip units on large low-voltage air circuit breakers as an alternate to magnetic overcurrent devices. These solid-state devices are more reliable, have time-current curves with narrower performance tolerance bands, are easier to coordinate with other protective devices, and are easier to calibrate and set. Shunt tripping of a circuit breaker involves a FOR MORE DETAILS VISIT US ON WWW.IMTSINSTITUTE.COM OR CALL ON +91-9999554621