What are QO Breakers and where are they used? QO (plug-on) and QOB (bolt-on) one-, two- and three-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. 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. What are the general characteristics of QO Breakers? QO and QOB circuit breakers have Visi-Trip trip indication, which provides a visual indication that the circuit breaker has tripped and interrupted the circuit. They feature an over center, trip-free toggle mechanism with quick-make, quick break action and positive handle indication. The tripping mechanisms in two- and three-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. What are the specific characteristics and benefits of this breaker? A 3-pole QOB branch-mounted sub-feed lug kit designed for use in QOB panelboard applications.