A monitoring device for monitoring a surge arrester connected to a power network, the monitoring device comprising: - a grounding line, which is configured to be electrically connected to ground and to the surge arrester so that an electric current flowing through the surge arrester will pass through this grounding line; - a measuring unit comprising one or more analogue measuring circuits (25, 26) for measuring the electric current passing through the grounding line and producing measuring signals related to the measured electric current; - an electronic processing unit for receiving said measuring signals from the measuring circuits (25, 26) of the measuring unit and producing monitoring data based on these measuring signals; and - a data storage medium for storing monitoring data produced by the electronic processing unit, characterized in: - that the monitoring device comprises a base module and an auxiliary module detachably mountable to the base module; - that the grounding line and the measuring unit are arranged in the base module; - that the electronic processing unit and the data storage medium are arranged in the auxiliary module; - that the base module comprises a first connector member, which is connected to the measuring unit; and - that the auxiliary module comprises a second connector member, which is connected to the electronic processing unit, the second connector member being configured to be engaged with the first connector member when the auxiliary module is mounted to the base module to thereby allow the electronic processing unit to receive said measuring signals from the measuring unit via the first and second connector members (29, 49). - a grounding line, which is configured to be electrically connected to ground and to the surge arrester so that an electric current flowing through the surge arrester will pass through this grounding line; - a measuring unit comprising one or more analogue measuring circuits (25, 26) for measuring the electric current passing through the grounding line and producing measuring signals related to the measured electric current; - an electronic processing unit for receiving said measuring signals from the measuring circuits (25, 26) of the measuring unit and producing monitoring data based on these measuring signals; and - a data storage medium for storing monitoring data produced by the electronic processing unit, - that the monitoring device comprises a base module and an auxiliary module detachably mountable to the base module; - that the grounding line and the measuring unit are arranged in the base module; - that the electronic processing unit and the data storage medium are arranged in the auxiliary module; - that the base module comprises a first connector member, which is connected to the measuring unit; and - that the auxiliary module comprises a second connector member, which is connected to the electronic processing unit, the second connector member being configured to be engaged with the first connector member when the auxiliary module is mounted to the base module to thereby allow the electronic processing unit to receive said measuring signals from the measuring unit via the first and second connector members (29, 49).