Multi-pole and single-pole circuit breakers include a housing and a reset lockout mechanism disposed within the housing. The reset lockout mechanism disables electrical communication between line and load terminals of the circuit breaker if a predefined condition exists. Some circuit breakers include a single actuator, transition between ON and OFF states, and are capable of performing test functions. The test functions may involve testing AFCI and/or GFCI functions of the circuit breakers. The test functions may be performed when the circuit breaker transitions from an OFF state to an ON state. Some circuit breakers including a reset lockout mechanism may be powered only on its line side. Some circuit breakers provide an electrical indication when they are in the OFF state.