Dynamic ac surface discharge characteristics of PMMA and LDPE

It is necessary to develop an understanding of the surface discharge behaviour of insulating materials under the influence of high electrical stress. A more recent development is a measurement system that is capable of visualising the dynamic movement of surface discharge. The measurement system is based on the use of a birefringent crystal as the sensing element. The method utilises the electro-optic effect (Pockels effect) of the crystal in order to detect the electric stress within it. A test material is bonded to the backface of the crystal and any surface charge upon it will alter the local field within the crystal. The field will affect the polarisation of light passing through the crystal and the amount of reflected light for a given angle of polarisation can be easily measured. Determining the change in polarisation of the reflected light allows the local charge density to be calculated.