Insulators used for high-voltage power transmission are made from glass,porcelain or composite polymer materials. Glass has a higher dielectric strength, but it attracts condensation and the thick irregular shapes needed for insulators are difficult to cast without internal strains.
Dirt, pollution, salt, and particularly water on the surface of a high voltage insulator can create a conductive path across it, causing leakage currents and flashovers. The flashover voltage can be reduced by more than 50% when the insulator is wet. High voltage insulators for outdoor use are shaped to maximize the length of the leakage path along the surface from one end to the other, called the creepage length, to minimize these leakage currents. To accomplish this,the surface is moulded into a series of corrugations or concentric disc shapes. These usually include one or more sheds; downward facing cup-shaped surfaces that act as umbrellas to ensure that the part of the surface leakage path under the 'cup' stays dry in wet weather. Minimum creepage distances are 20–25 mm/kV, but must be increased in high pollution or airborne sea-salt areas.

Model	U70B/140
Minimum mechanical failing load, kN	70
Minimum mechanical residual strength, kN	35
Diameter of the insulating part, D, mm	290
Spacing, H, mm	140
Nominal creepage distance, mm	320
Connection	16
Puncture voltage in insulating medium, KV	130
50 Hz withstand voltage (dry), KV	100
50 Hz withstand voltage (wet), KV	40
Radio interference voltage,μV	50
Weight, KG	3.6