A monitoring arrangement for monitoring a surge arrester which is connected to a live wire of a power network at a connection point on the live wire in order to protect an electrical apparatus of the power network against incoming surges, the monitoring arrangement comprising: - a conductor element, which is configured to be electrically connected to ground and to the surge arrester so that electric current impulses passing through the surge arrester will pass through this conductor element; - an analogue measuring circuit for sensing current impulses passing through the conductor element and producing analogue measuring signals representing such current impulses; - an analog-to-digital converter connected to the analogue measuring circuit and configured to sample the analogue measuring signals from the analogue measuring circuit in order to convert these analogue measuring signals into digital signals; and - an electronic processing device for receiving said digital signals from the analog-to-digital converter and producing monitoring data based on these digital signals; characterized in: - that the electronic processing device is configured to establish, based on digital signals produced by the analog-to-digital converter for a current impulse through the surge arrester caused by an incoming surge in the live wire, a voltage peak value (U T) representing the highest voltage at said connection point during the incoming surge; and - that the electronic processing device is configured to establish, based on said voltage peak value (U T) and by means of a calculation model or look-up table, a maximum-voltage value (U max) representing an estimated maximum voltage across the electrical apparatus during the incoming surge in question. - a conductor element, which is configured to be electrically connected to ground and to the surge arrester so that electric current impulses passing through the surge arrester will pass through this conductor element; - an analogue measuring circuit for sensing current impulses passing through the conductor element and producing analogue measuring signals representing such current impulses; - an analog-to-digital converter connected to the analogue measuring circuit and configured to sample the analogue measuring signals from the analogue measuring circuit in order to convert these analogue measuring signals into digital signals; and - an electronic processing device for receiving said digital signals from the analog-to-digital converter and producing monitoring data based on these digital signals; - that the electronic processing device is configured to establish, based on digital signals produced by the analog-to-digital converter for a current impulse through the surge arrester caused by an incoming surge in the live wire, a voltage peak value (U T) representing the highest voltage at said connection point during the incoming surge; and - that the electronic processing device is configured to establish, based on said voltage peak value (U T) and by means of a calculation model or look-up table, a maximum-voltage value (U max) representing an estimated maximum voltage across the electrical apparatus during the incoming surge in question.