A power control device for controlling power supply to a downhole component, the device comprising: a communication device configured to be disposed in a borehole in an earth formation and configured to couple electrical power from a power source to a downhole component from a conductor disposed along a borehole string; a circuit breaker system including a first circuit breaker disposed at a connection between the conductor and the downhole component and configured to be closed to connect the downhole component to the conductor; and a controller configured to be disposed in the borehole, the controller configured to monitor at least one of a current level and a voltage level at the connection, the controller configured to control the circuit breaker system and autonomously perform: opening the first circuit breaker in response to detecting a deviation in the at least one of the current levels and voltage levels at the connection, to isolate the downhole component from the conductor and the power source; and characterised by the controller being configured to monitor a plurality of downhole components and the circuit breaker system including a plurality of circuit breakers (62; CBR7, CBR8, CBR9), each of the plurality of circuit breakers connected to the controller and to a respective downhole component, the controller configured to control each of the plurality of circuit breakers to individually connect or disconnect each of the plurality of downhole components. a communication device configured to be disposed in a borehole in an earth formation and configured to couple electrical power from a power source to a downhole component from a conductor disposed along a borehole string; a circuit breaker system including a first circuit breaker disposed at a connection between the conductor and the downhole component and configured to be closed to connect the downhole component to the conductor; and a controller configured to be disposed in the borehole, the controller configured to monitor at least one of a current level and a voltage level at the connection, the controller configured to control the circuit breaker system and autonomously perform: opening the first circuit breaker in response to detecting a deviation in the at least one of the current levels and voltage levels at the connection, to isolate the downhole component from the conductor and the power source; and characterised by the controller being configured to monitor a plurality of downhole components and the circuit breaker system including a plurality of circuit breakers (62; CBR7, CBR8, CBR9), each of the plurality of circuit breakers connected to the controller and to a respective downhole component, the controller configured to control each of the plurality of circuit breakers to individually connect or disconnect each of the plurality of downhole components. opening the first circuit breaker in response to detecting a deviation in the at least one of the current levels and voltage levels at the connection, to isolate the downhole component from the conductor and the power source; and characterised by the controller being configured to monitor a plurality of downhole components and the circuit breaker system including a plurality of circuit breakers (62; CBR7, CBR8, CBR9), each of the plurality of circuit breakers connected to the controller and to a respective downhole component, the controller configured to control each of the plurality of circuit breakers to individually connect or disconnect each of the plurality of downhole components.