Effect of electrical and thermal ageing on the breakdown strength of silicone oil

The electrical and thermal ageing behavior of silicone oil was considered in these investigations. The two ageing methods had very different physical effects; electrical ageing resulted in darkening of the oils but with no gelling (increased viscosity), whereas thermal ageing did not produce darkening but could produce gelling. Whilst ageing tended to reduce the AC breakdown strength and increased the electrical conductivity, one set of thermally aged oils, whose viscosity increased after thermal ageing, exhibited increased breakdown strength. The unexpected effects of oil viscosity on AC breakdown strength were confirmed in separate experiments on an oil of a higher viscosity.