This monitoring unit (1) for monitoring an electrical circuit breaker (D) includes: a central body including: an interconnection device (120) capable of receiving primary voltages (V1) from the circuit breaker, and comprising an electrical power circuit (1220) for converting the primary voltages to secondary voltages (V2); a control device (110), for measuring the secondary voltages (V2) delivered by the power circuit (1220), a removable electrical power supply module (20), comprising a power converter (203), configured to transform the collected primary voltages (V1) into an additional secondary voltage (V2′) and to supply electrical power to a shared electrical power supply bus (1102) of the control device (110).