Abstract: A testing arrangement is provided for an arc detecting system that includes at least one optical detector at a piece of electric equipment, where the testing arrangement includes at least one light source and a testing unit, where the testing unit is configured to control the light source to emit a test light to the optical detector with steps of increasing luminance starting from a minimum test level and continuing towards a maximum test level, investigate if the optical detector detects the emitted test light and determine that the arc detecting system is operational if the optical detector is able to detect the test light at any of the used test levels.