For high-voltage transmission systems, arresters are classified by "Line Discharge Classes" (Class 1 through 5).
Revised guidelines on diagnostics and end-of-life management (Clause 8 and Annex D). 3. Essential Selection & Application Content iec 600995 pdf
Substation deployment prioritizes the protection of high-value equipment assets, such as power transformers, breakers, and gas-insulated switchgear (GIS). Arresters are placed as physically close to the transformer bushings as possible to minimize separation distance effects. These installations typically require low residual voltages and high charge capabilities to damp fast-front switching surges. Overhead Line Protection (EGLA) IEC 60099-5:2018 For high-voltage transmission systems
The standard is not just about numbers; it includes practical engineering advice: such as power transformers