IEC 60099-5:2000 pdf download-Surge arresters – Part 5: Selection and application recommendations.
The rate of occurrence of back-flashovers is controlled by selection of a proper insulation level, by keeping the structure ground resistance to an acceptably low value by providing adequate clearance from conductor to structure, conductor to shield wire and conductor to conductor and by optimizing tower geometry. Procedures analogous to those used for shielding lines may also be used for shielding stations. Shielding methods include overhead ground wires, metallic masts, and lightning rods supported from the station structure. These methods may be used in many combinations. In installations not shielded against direct lightning strokes, for example at distribution transformers or cable junctions on wood-pole lines, both the insulation and the arrester can be subjected to direct strokes producing extremely high discharge currents. In general the lightning current and a representative discharge current shall be based on their probability of occurrence. The purpose of shielding in station applications is to reduce the risk of insulation failure to an acceptable level. In certain applications this may be achieved by shielding the station alone. In other cases it may be necessary to shield all incoming lines to the station. Shielding of the lines for a relatively short distance from the station affords the same benefits for station protection as complete line shielding. With well-designed shielding, insulation, and grounding systems, the probability of direct strokes to phase conductors is reduced to a low level and the voltages across insulation in the event of strokes to the shielding system are reduced below flashover levels. As a result, arrester discharge currents are reduced, thereby permitting the arrester to provide better protection to equipment insulation and reducing arrester duty.
In installations where long lines or cables or high capacitances are connected, the arrester must be capable of withstanding the discharge current and energy due to switching overvoltages. For heavy duty 1 0 kA arresters, therefore, test data have been specified for various ranges of system voltages (see 8.5.3 of IEC 60099-1 ), which represent typical energy dissipation through arresters in the system concerned. Heavy duty arresters are generally used for equipment of range II. For equipment of range I, heavy duty arresters are used in case of important stations, in case of required low protective levels or in case of long overhead lines, cables or capacitor banks connected to the busbar. For other cases, e.g. where incoming lines are relatively short (less than 1 00 km), 1 0 kA light duty arresters or 5 kA arresters are generally used.