Fog Type Insulators Creepage And Pollution Levels

What Creepage Actually Measures

Leakage distance is the length of the path a current would have to travel across the surface of the insulator from the cap to the pin. Adding sheds and underribs lengthens that path without making the unit taller. A standard U70BL runs 320 mm. A fog U70BLP1 runs 400 mm and a U70BLP2 runs 450 mm, all on the same 146 mm spacing.

Why Pollution Turns That Path Into A Problem

Dry deposits are not conductive. Wet deposits are. The dangerous condition is light wetting, dew, fog or the first drizzle after a dry spell, which dissolves the deposit without washing it away. Leakage current rises, the surface dries unevenly, dry bands form, and arcing across those bands can bridge the whole unit. This is why flashovers cluster after dry weather rather than during heavy rain.

The Two Profiles And What They Are For

P1 gives 400 mm of creepage on a 255 mm shed. P2 gives 450 mm on a 280 mm shed. Both are available at 70 kN, 100 kN and 120 kN mechanical failing load, all on a 16 mm coupling. P1 suits moderate pollution. P2 suits heavy pollution or a section with a documented flashover record.

Doing The Calculation

Your standard will express the requirement as a specific creepage in mm per kV for the pollution severity level at the site. Multiply that by the system voltage to get the string requirement, then divide by the unit leakage distance to get the unit count. The reason a fog profile is worth specifying is that the same string requirement is met with fewer units, or with the same unit count and more margin.

What Does Not Change

Spacing stays at 146 mm across the whole fog family, identical to a standard U70BL or U120B. The coupling stays at 16 mm. String length for a given unit count is therefore unchanged, tower clearance is unchanged, and the existing hardware still mates. Only the shed geometry and the unit weight differ, 5.2 to 6.0 kg against 3.8 to 4.2 kg.

Treat The Section As One Unit

The lowest creepage link governs the string, and along a corridor with uniform pollution exposure the same logic applies from tower to tower. A partial upgrade leaves the untreated spans as the flashover sites, which makes the improvement look smaller than it is.