QO™ (plug-on) and QOB (bolt-on) 1, 2 and 3-pole thermal-magnetic circuit breakers provide over current protection and switching on ac and dc systems. Plug-on QO™ circuit breakers are for use in QO™ load centers, NQ and NQOD panel boards, OEM mounting bases and Speed D™ switchboard distribution panels. Bolt-on QOB circuit breakers are for use in NQO and NQOD panel boards. QO™ and QOB circuit breakers have an over center, trip free toggle mechanism with quick make, quick break action and positive handle indication. The tripping mechanisms in 2 and 3-pole circuit breakers operate such that an over current on any pole of the circuit breaker will cause all poles of the circuit breaker to open simultaneously. Each pole has an individual thermal-magnetic trip element calibrated for 40 deg C ambient temperature.