An arrangement for an overvoltage protection of a subsea electrical apparatus and a method for operating it. The arrangement includes a tank submersible below a water surface level, an electrical apparatus accommodated in the tank, and a surge arrester arrangement accommodated in the tank and coupled to a power supply of the electrical apparatus in the tank for providing the overvoltage protection of the electrical apparatus. The arrangement further includes a controllable grounding switch for connecting the surge arrester arrangement to a ground point in response to a control of the grounding switch to a closed state and for disconnecting the surge arrester arrangement from the ground point in response to a control of the grounding switch to an open state.