Circuit protection devices, power distribution systems, and circuit protection device monitoring in a zone selective interlocking (ZSI) system are described. In one example, a circuit protection device for use in a ZSI power distribution system includes a trip mechanism configured to interrupt current flowing through the circuit protection device, and a trip unit operatively coupled to the trip mechanism. The trip unit is configured to monitor current flowing through the circuit protection device, output a first signal when the monitored current is less than a threshold value, and output a second signal when the monitored current is greater than or equal to the threshold value. The first and second signals are different from each other and different than an absence of a signal.