Selecting Creepage For The Pollution At Your Site

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Selecting Creepage For The Pollution At Your Site

Almost every repeat flashover we are asked about comes back to the same thing: creepage was chosen from the system voltage and nothing else. Voltage tells you the minimum. The air at the site tells you what it actually needs.

Every Stage And What Can Go Wrong At It

Site Assessment

Pollution severity is often described as light or heavy with nothing measurable behind it.

Describe the location concretely: distance from the coast, nearby cement, steel or chemical plants, agricultural spraying, desert dust, and how often washing is actually possible.

Total Versus Protected Creepage

Two insulators with the same total creepage can behave very differently in service.

We publish both figures where the data exists. The P-33-Y gives 900mm total with 450mm protected under the sheds, while the QS11405 gives 599mm total but only 200mm protected. Where rain washing is unreliable, the protected figure is the one that matters.

Envelope Constraints

The obvious fix, a bigger insulator, often will not fit the terminal board or the crossarm.

Several ranges offer more creepage inside a nearly unchanged envelope. The CD525 bushing adds 85mm of creepage over the 440mm version at almost the same body size.

Profile Selection

Dust that settles between washes bridges the leakage path regardless of how long it is.

Aerodynamic profile glass discs are shaped so wind carries deposits off the surface, which is why they are specified in desert and heavily polluted corridors.

Impulse Margin

Creepage solves pollution flashover but does nothing for lightning trips.

Impulse withstand is selected separately. The 24kV bushing at 150kV BIL, for example, sits above the 125kV usually quoted at that class, which suits overhead-fed networks.

Verification

A specification that cannot be checked on delivery is a specification in name only.

Creepage is verified against the drawing figure on routine inspection, and the tolerance band is published where one applies, such as 400mm plus or minus 10mm on the ShF-20G1.

Products This Applies To

High Voltage Transformer Bushings

DIN 24kV 630A Transformer Bushing CD525

Pin Type Insulators

Creepage Distance 525 mm
Net Weight 6.8 kg
Height 385 mm

ZS-770 Pin Post Porcelain Insulator 770mm Creepage

Glass Suspension Insulators

Creepage Distance 770 mm
Rated Electromechanical Failing Load 16 kN
Lightning Impulse Withstand Voltage 190 kV

Aerodynamic Toughened Glass Suspension Insulator XYAY Series

Rated Mechanical Failing Load 120 kN
Nominal Creepage Distance 365 mm
Power Frequency Puncture Voltage 130 kV

Who Uses This Route Most

Power Utilities And Substations

A line section or substation that keeps flashing over after every wet season

Overhead Line Contractors

Replacement insulators whose groove width or spindle bore does not match