Evaluation and failure analysis of high-resistance collar coating of varistor

Zinc oxide (ZnO) based varistors are polycrystalline devices that are widely used as surge absorbers in electrical power systems, including appliances, due to their excellent I-V characteristics. The external flashover often occurs at discharge currents far below that the block is capable of carrying without damage. Thus various high insulation resistance (HR) collar coatings are applied on the peripheral surface of ZnO varistors blocks in order to prevent the external flashover. Conventional process produces collar coating of thickness about 90 μm. A new thinner collar coating of about 32 μm was developed in-house and dielectric strength was compared to that produced by conventional process. It was established from the results of switching surge operating duty test (SSOD), that this thinner coating was able to meet all necessary electrical parameters requirements as per IEC 60099-4. Further, failure analysis of the blocks revealed that viscous melting at the fracture surface was responsible for disintegration of the blocks. The failure mode of the collar coating during the electrical testing was attributed to flashover, due to non-uniformity of the collar coating on the side-surface of the blocks.