scope:

This method describes a procedure for determining the ability of rigid insulating material to resist breakdown parallel to laminations (or in the plane of the material) when subjected to extremely high voltages at standard AC power frequencies of 50-60Hz.

As for most electrical properties, values obtained on most materials are highly dependent on the moisture content and tests using different conditioning cannot be compared. Tests in other mediums, e.g. air are generally impractical due to its relatively low breakdown.

This method is based on the test technique described as ASTM D229.