Defective insulators in live working on a 550 kV compact steel lattice tower

Research was conducted to determine the minimum number of healthy insulator units needed to ensure worker safety in live working on a compact 550 kV steel lattice tower. A large variety of worksite conditions was investigated. Test results confirm the conservative nature of the CIGRE formula for defective insulator units, and modifications to the formula are suggested, The recommended number of healthy units is 12 in a string of 24 units for the case of live work on a compact 550 kV tower, when the portable protective gap (PPG) is used. Adjustment for altitude should be made above 900 m (3000 ft.).