A method and apparatus for earth fault in power high and/or medium voltage system may enhance a protection system for special technical task, which is to measure residual current, the invention is, that for earth fault protection in power system or a part of power system, for example, but not limited to, cable connectors, wherein residual current is determined indirectly by the calculation from the phase current values, obtained from the output signals of Rogowski-coil based current sensors.