For low-voltage circuit protection in the U.S., circuit
breaker designs and tests are based on the requirements of three standards
organizations; the American National Standards Institute (ANSI), Underwriters
Laboratories (UL), and the National Electrical Manufacturers Association
(NEMA).
They have short-time ratings, but they are not fast enough
in interruption to qualify as current limiting. LVPCBs are designed to be
maintainable in the held. The ANSI C37 series of standards and UL 489-1991 were
jointly developed by IEEE and NEMA and apply to LVPCBs and ICCBs/MCCBs,
respectively.
The two classifications of circuit breakers these organizations
defined are as follows:
- Molded-case circuit breaker class
- Low-voltage power circuit breaker class
Three types of circuit breakers are based on the two classifications
above. The classifications themselves lend their names to the first two of the
three types, while the third type, derived from the molded-case circuit breaker
class, is known as an insulated-case circuit breaker.
The three types of circuit breakers are as follows:
Molded-case circuit breakers (MCCBs)
Low-voltage power circuit breakers (LVPCBs)
Insulated-case circuit breakers (ICCBs)
The following are some of the salient features of these
types of circuit breakers. MCCBs, as a class, are those tested and rated
according to UL 489-1991 and whose current carrying parts, mechanisms, and trip
devices are completely contained within a molded case of insulating material.
MCCBs are available in the widest range of sizes, from the smallest (15 A or
less) to the largest (6000 A), and with various interrupting ratings for each
frame size.
They are characterized generally by fast interruption
short-circuit elements. With electronic trip units they can have limited
short-delay and ground-fault sensing capability.
Virtually all MCCBs interrupt fast enough to limit the
amount of prospective fault current let through and some are fast enough and
limiting enough to be identified as current-limiting circuit breakers.
MCCBs are not
designed to be held maintainable. ICCBs are also rated and tested according to
UL 489-1991. However, they utilize characteristics of design from both the
power and molded-case classes. They are of the larger frame sizes, fast in
interruption, but normally not fast enough to qualify as current-limiting
circuit breakers.
ICCBs also utilize electronic trip units and can have short-time
ratings and ground fault current sensing. They utilize stored energy operating
mechanisms similar to those designed for LVPCBs and their design is such that
they are partially held maintainable. LVPCBs are rated and tested to satisfy
ANSI C37 standard requirements and are used primarily in drawout switchgear.
They are generally characterized as being the largest in physical size.
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