Increased energy absorption in ZnO arrester elements through control of electrode edge margin

ZnO nonlinear dielectric has a small temperature coefficient of conductivity up to about 550 C, and above 550 C the temperature coefficient of conductivity becomes substantially positive, which tends to result in thermal runaway through condensation of the current density into filaments which "melt" their way through the ceramic. Thus to obtain maximum energy absorption in a ZnO element, it is important to avoid hot spots so that the element will heat quite uniformly to its maximum stable operating temperature in the range of 550 C. The current density at the electrode edge is enhanced by spreading of the current into the ZnO at radii larger than the electrode. This causes an enhanced temperature at the electrode edge which results in the "melting punctures" commonly observed. Using transient, nonlinear finite element analysis with coupled thermal and electric fields, the authors have quantified the temperature enhancement as a function of edge margin and optimized the electrode topology.