Why Strike Distance Is Important for Insulators

Why is Strike Distance So Important in Insulator Design?

By Jon Rennie

When designing and specifying an insulator, one of the key electrical characteristics is its strike distance. So what is strike distance and why is it so important?

Strike distance is different than creep distance.  Whereas creep distance deals with the surface of the insulator, strike distance deals with the medium surrounding the insulator, in most cases this is air.  Strike distance is simply the shortest distance between the end fittings of the insulator in the surrounding medium.  It is also sometimes referred to as the dry arc distance, flashover distance, or tight string distance.

A tight string gives the best visualization of strike distance.  Drawing a string from the current-carrying component on one terminal to the other terminal in open-air would represent the strike distance.

IEEE Std 100 – 1992 defines the dry arc distance as “the shortest distance through the surrounding medium between terminal electrodes….” The reason this is so important is that typically each end of the insulator is operating at different voltage levels.  Strike distance is the shortest route in which voltage can “flash-over” or arc from the higher voltage to the lower voltage (or ground).

Strike distance is a fixed characteristic of an insulator based on its specific geometry.  Strike distance will be higher in insulators with a greater distance between its end fittings. When selecting an insulator, you need to consider the operating voltage where the insulator will be used.  The higher the operating voltage, the greater strike distance you will need.

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