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What is meant by Creep Distance in Insulator Design?
By Jon Rennie
You might have heard the term creepage or creep distance when referring to insulator design and specifications. So what is it and why should you care?
Creepage or creep distance is pretty simple. It relates to the surface shape of the insulator. Creep distance is sometimes referred to as leakage distance. It represents the shortest distance along the surface of the insulator between the conductive ends of the insulator. In a typical insulator application, one end would be at an elevated voltage level and the other end would be at a lower voltage or grounded. IEEE Std 100 – 1992 defines leakage distance as “The sum of the shortest distances measured along the insulating surfaces between the conductive parts.”
The objective in insulator design is to create an insulator shape that will prevent current from “leaking” along the surface of the insulator from live electrical circuits to ground. Current leakage is bad and will cause damage and eventually failure of the insulator.
Insulator geometry is critical to maintaining good creep distance. Engineers add “sheds” to the insulator to maximize the creep distance. This helps in two ways. First, the geometry of a shed increases the surface distance on the insulator from end to end. This increases the creep distance. Second, the sheds are engineered in way to remove environmental contaminants (salt, pollution, dust, etc.) from the insulator in rainy weather.
Environmental contaminants have a strong effect on insulator performance. The more surface contaminants on the insulator, the more likely you will have leakage current since many of the contaminants are conductive. In general, areas of high contaminants require more than 31mm/kV of creep distance whereas clean areas need only 16mm/kV.
Creep distance is a critical aspect of insulator design. The higher the voltage level and the higher the contaminant level, the more creep distance you will need.
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