QO™ (plug-on) 1, 2 and 3-pole thermal-magnetic circuit breakers provide overcurrent protection and switching on AC and DC systems. Plug-on QO™ circuit breakers are for use in QO™ load centers, NQ and NQOD panelboards, OEM mounting bases and speed D™ switchboard distribution panels. QO™ circuit breakers have an overcenter, 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 overcurrent 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.