Square D is an American manufacturer of equipment used to control and distribute electric power headquartered in Palatine, Illinois.
Square D was founded in 1903 in Detroit, Michigan by Bryson Dexter Horton who is credited with the invention of the safety switch which encased high voltage switches and started the company's main line of business of circuit breakers and encased control panels.[1] Horton served as Square D's president until 1928.[2]
In 1991, the company was acquired and became a subsidiary of Schneider Electric.[3]
Introduced in (either 1955 or) 1962/1963,[4] their "QO" line of 3/4-inch circuit breakers may be their best-known product line, used in the electrical distribution boards of many homes in North America.
A second product line is sold under the brand name Homeline. Different breaker connection design and lower list price is what distinguishes it from the QO series. Homeline devices are marked HOM while QO devices are marked QO.
Another well-known Square D product line is the Powerlink circuit breaker, created for lighting control, sold to bigger buildings/skyscrapers.