The Evolution of the Circuit Breaker


By Paul Grein, Electrical Engineer, Circuit Breaker Sales Co., Inc.

After World War II ended in 1945, Westinghouse introduced its first medium-voltage air-magnetic circuit breaker, and the first American standard for AC power circuit breakers was published. Circuit breakers of the day were heavy, expensive, and unreliable devices that required regular upkeep to ensure they performed as expected.

The Oil-Breaker Years:

• Before and immediately following WWII, the most common medium-voltage circuit breaker was the oil tank–type circuit breaker. Maintenance consisted of testing, cleaning, and changing the oil, and then physically inspecting the operating mechanism, contact condition, and connections, as well as looking for signs of carbonization.

• The 1940s saw the beginning of a 20-year transition from oil-tank to air-magnetic circuit breakers. But oil tank technology remained the workhorse of American utilities and factories.

The Air-Breaker Years:

• By the 1970s, stored-energy spring mechanisms had phased out the solenoid operator; cast mechanisms were replaced with machine mechanisms; and monolithic pole units were phased out by the postinsulator pole unit — the same single/isolated pole designs employed today. These and other advancements to circuit breaker design improved reliability so much that it is common to find equipment commissioned in the 1960s still in use today.

• Safety was not a priority in industrial environments until the Occupational Safety and Health Administration (OSHA) formed in 1970 when the Williams-Steiger Act was signed into law. Electrical safety would not be emphasized until 1979, when the National Fire Protection Association (NFPA) published NFPA 70E: Standard for Electrical Safety in the Workplace, at OSHA’s request.

• In 1972 the InterNational Electrical Testing Association (NETA) was founded with the goal of establishing uniform testing procedures for electrical equipment. Five years later, NETA published its Acceptance Testing Specifications for Electrical Power Equipment and Systems (ATS-1977). CBS was formed shortly after in 1981. The Vacuum-Breaker Years: • In 1978, Westinghouse manufactured its first vacuum circuit breaker model, and competitors quickly followed suit.

• In 1989, vacuum-generation safety practices were enhanced by OSHA’s institution of lockout/tagout procedures, the recognition of arc-flash hazards by the NFPA in 1994, and subsequent efforts to reduce exposure to arc flash.

The Future of Circuit Breakers

Factors such as globalization, advances in manufacturing technologies, and a move toward modular circuit breaker designs will continue to drive costs down. The trend began with plug-and-play parts such as coils and motors and now includes modular mechanisms and interrupting assemblies that can be replaced in the field as well as easily tested and maintained, further reducing costs and increasing reliability.