The maximum discharge voltage of a modern lightning arrestor should be what percentage less than the BIL of protected equipment?

The maximum discharge voltage of a modern lightning arrestor should be 80% less than the BIL (Basic Insulation Level) of the protected equipment. This guideline ensures that the lightning arrestor can effectively protect electrical equipment from high voltage surges that can occur during lightning strikes.

The BIL represents the insulation strength of equipment, and the 80% reduction in discharge voltage is designed to maintain the integrity of the equipment's insulation under stress. By keeping the discharge voltage significantly lower than the BIL, the risk of breakdown or failure of the insulation material is minimized. This effectively shields the equipment from potentially damaging high voltage transients and enhances overall reliability and safety in electrical systems.

The other percentages mentioned are not appropriate for modern lightning arrestors, as they may not provide adequate protection according to current standards and practices in electrical insulation and surge protection. A lower percentage than 80% would increase the risk of damage to equipment by allowing surges that exceed the protective capabilities of the lightning arrestor.