Air circuit breakers are used on dc and ac circuits for the protection of general lighting, power, and motor circuits. Distinction is made between various protection classes and different service and ambient conditions.

For selection of a breaker, type and rating, operating speed, selectivity with fuses, and high voltage must be taken into account. Further consideration has to be given to severe or hazardous service conditions like tropical climate or marine- or explosion-proof installations. Reference is made to IEEE C37.1313, C37.1414, and C37.1715; UL-48916; and ANSI C37.1617.

Standard electrically and manually operated breakers are listed in ratings up to and including 5000 A ac and 12,000 A dc. Electrically operated breakers are available in higher current ratings for special applications.

Standard breakers are rated on the basis of a temperature rise on the contacts and terminals not to exceed 50 deg C above an ambient of 40#C (class 90 insulation). Voltage ratings are 254 to 635 V ac and 250 to 3200 V dc.

The short-time current ratings are based on 3-phase symmetric short-circuit currents; the singlephase short-circuit current ratings are 87% of these values. For details, refer to the latest revisions of ANSI C37.1617.

Assembly Variations.
The breakers are usually installed in a metal-enclosed cubicle for dead-front or drawout type of construction. Metal barriers between breakers and busbars provide increased safety in service.

Hand operation by means of a lever is common, even on large breakers. Electric operation by means of a solenoid or motor mechanisms for 48, 125, or 250 V dc, or 120 or 240 V ac is obtainable on all but the smallest sizes of breakers.

Breakers are supplied with an overcurrent trip mechanism which may be of the instantaneous or the time-delay type, or a combination of both. Trip devices are adjustable over a wide range of ratings. Other trip devices and arrangements may be used, for example, undervoltage trips, shunt trips connected to overvoltage, reverse current, or overcurrent relays.

Multiple-pole circuit breakers are commonly used in practically all capacities, one pole being used for each ungrounded line of a circuit, that is, a 2-pole breaker for a 3-wire grounded circuit or a single-pole breaker for a 2-wire grounded circuit.

Breakers can usually be equipped with auxiliary contacts, alarm contacts, push-button control, position indicator, and key interlock. The widely used drawout type of breaker may be moved into and locked in the connected, test, and disconnected positions and/or completely withdrawn. Refer to the latest revisions of IEEE C37.1313, C37.1414, and C37.1715, and ANSI C37.1617.

No comments:

Post a Comment