To the uninitiated, the term “power breaker” would seem to apply to any circuit breaker. To those more familiar, the term focuses on a range of low voltage circuit breakers. Because breaker nomenclature can seem complex, this article reviews the terms that describe commonly used in low-voltage, backup power systems.

Molded Case Circuit Breakers

Some breaker types are named for the manner in which they are constructed. The term molded case circuit breaker identifies a breaker that is assembled as an integral unit within a supportive enclosure of insulating material … typically, a molded plastic case. Within this category, breakers are differentiated by their trip mechanisms. There are two principle types - thermal-magnetic and solid-state. Molded case breakers are tested according to requirements specified in UL 489 - Molded-Case Circuit Breakers, Molded-Case Switches, and Circuit-Breaker Enclosures. Those that test successfully are listed by UL. Notably, breakers are identified as “80%-rated” and “100%-rated” models. Those rated at 100% undergo thermal testing beyond those listed at the 80% threshold. Consequently, 80%-rated molded case breakers are sometimes called “standard breakers”. //

Insulated Case

Sometimes called a “power” breaker, an Insulated Case Circuit Breaker is similar to a molded case breaker, but Is built on a frame inside an “insulated” molded plastic case. They are tested to the same UL489 standard as molded case circuit breakers. Insulated Case Circuit Breakers can be fixed-mounted or draw-out types. Because of their molded case design, they offer limited serviceability.

ANSI-Rated Breakers

The term “power” breaker more often refers to circuit breakers listed to UL 1066 - Standard for Low-Voltage AC and DC Power Circuit Breakers Used in Enclosures, which in turn references additional standards promulgated by the American National Standards Institute (ANSI). Also known as Metal Frame or Air Frame breakers, these are built as an assembly of parts in a welded metal frame, which is mounted on a draw-out mechanism that enables it to slide out of its enclosure for inspection and service. This enables greater serviceability when compared to other types.

Common Power Breaker Design Elements

Insulated case and ANSI-rated Breakers consist of three principle components: (1) Element, (2) Cell, and (3) Trip Unit. These are explained below.