This European Standard specifies the requirements and tests for isolating spark gaps (ISG) for lightning protection systems. ISG's 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 AC 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. NOTE ISG.s may also be suitable for use in hazardous conditions such as fire and explosive atmosphere. Due regard should be taken of the extra requirements necessary for the components to be installed in such conditions. This does not cover applications where follow currents occur. Protective devices according to EN 50122-1 and EN 50123-5 are not part of this European Standard.