QO® (plug-on) and QOB (bolt-on)1,2 or 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. bolt-on QOB circuit breakers are for use in NQO and NQOD panelboards.