Investigating Effects of Composite Dielectric Material Components and Temperature on Breakdown Strength

Breakdown measurements under applied AC voltage stress at a constant temperature are presented for pure epoxy, epoxy containing mica tape and epoxy containing glass mat tape. These form the individual components of the composite insulation system as used in the main dipole bending magnets at the ISIS neutron and muon source. Results are presented as Weibull plots from which characteristic breakdown fields (scale), and shape parameters are calculated. Subsequently the measurements are repeated over temperature on epoxy samples containing mica. A slight decrease in breakdown strength occurred when glass fibre mat was included, whereas the presence of mica caused a significant increase. The difference in breakdown strength between the different sample types are discussed in terms of the theories of electrical breakdown, and ultimately were related to the tensile strength of the corresponding materials.