Surface Conductivity Investigations of Deposition Layers on VCB Ceramics

Vacuum circuit breakers (VCBs) operate for a long lifetime without service. They should still provide good making and breaking performance and dielectric strength even after many switching operations. During breaking operation, a metal vapor arc accompanies the transition to current zero. The condensation of this metal vapor occurs on the inner parts of the VCBs. For controlled condensation, VCBs are equipped with metal shields to protect the ceramic surfaces from being coated by metal vapor. However, some parts of the ceramic surface are still being coated and thus become semi-conductive, depending on the amount of the deposited coating. These regions may become weak points of the dielectric strength inside the VCB, especially if the deposition length on the ceramics reaches a critical value. An important factor to evaluate the performance and estimate the residual lifetime is the conductivity of the deposition layer on the VCB ceramic. In this work, a first approach shows how to correlate deposition layer thickness and resistivity. For this purpose, measurements of the coating thickness are related to the resistivity of the surface deposition layers on stressed VCB ceramics. In recent work, a deposition model for metal vapor arcs helps to calculate the deposition layer thickness on the ceramics. Now a structural model for the metal layer is proposed to estimate the resistivity of the deposition layer.