Square D was founded in 1903 in Detroit, Michigan as Detroit Fuse and Manufacturing 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. Horton served as Square D's president until 1928.
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. The 1960s Square D introduced the Safety Line distribution centers for large industrial electrical power loads. This design had solid or tubular busing with each large fused switch clamping onto the busing. The company also sold the B line 277-480 volt panels generally used for lighting in the 1970s and 1980s. The QO and QOB are by far their most popular commercial products produced. The QO standing for "quick open", and QOB for "quick open bolted" breaker. The QOB was usually specified for buildings with lots of vibration, or in industrial settings where a higher degree of reliability was desired. A QO breaker could easily be clipped into a QOB panel, but the chance it could be popped out accidentally when an adjacent breaker was being changed made bolting the breaker in a nice feature.
Another well-known Square D product line is the Powerlink circuit breaker, created for lighting control, sold to bigger buildings/skyscrapers. B Line and D Line were the breaker panels in which power line breakers were used in. Eventually the company branded Powerline and Powerlink panels and breakers. Powerlink was a heavy duty 277 volt lighting breaker used in large buildings with 277 volt lighting loads. These were also favored for their robust design, and functional reliability. Using Ohm's Law, a 277 volt lighting circuit can run over 2x the wattage as a 120 volt circuit. This means less panels space, and less wire. This is why high rises favor higher voltage lighting for general task lighting.
The I-Line series of distribution panelboards is favored by many electricians for its ease and safety of adding new breakers. The panel uses a stacked bus system that protects the energized bus from accidental contact and the breaker can be easily installed using a flat head screwdriver contacting only de-energized parts of the backplane of the panel. The design of the breakers and panel require that extra attention be paid to phase rotation as the right (even numbered) side of the panel will have the phases in a C-B-A configuration whereas the left (odd numbered) side of the panel will have a more standard A-B-C arrangement.
Square D also made disconnect switches, both fused, and unfused, as well as HACR (Heating Air Conditioning and Refrigeration) rated switch boxes that held an HACR breaker for use as a disconnect. The QO type was the breaker that fit this disconnect box. These were popular with food service, grocers, and other cold storage users.
Other companies used terms like "Quick Lag" and similar brand name letter coding on their circuit breakers e.g.: QL, QLRB, etc. No one knows who started the brand coding for sure. QO is an industry recognized standard like "Kleenex" is used to refer to a tissue regardless of brand. These brand specific letter codes had nothing to do with NEMA specifications or coding though it looks like they were intended to emulate NEMA codes.
- The city of Detroit, Michigan, 1701-1922