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.