This part of IEC 62561 specifies the requirements and tests for isolating spark gaps (ISG) for lightning protection systems. ISGs can be used to indirectly bond a lightning protection system to other nearby metalwork where a direct bond is not permissible for functional reasons. Typical applications include the connection to earth-termination systems of power installations@ earth-termination systems of telecommunication systems@ auxiliary earth electrodes of voltage-operated@ earth fault circuit breakers@ rail earth electrode of power and DC railways@ measuring earth electrodes for laboratories@ installations with cathodic protection and stray current systems@ service entry masts for low-voltage overhead cables@ bypassing insulated flanges and insulated couplings of pipelines. This does not cover applications where follow currents occur.