This paper provides an overview of diagnostic methods for determining the condition of the insulation on power transformers, cable lines, switches, disconnectors, surge arresters, cable terminals and cable connectors. Diagnostic methods include determining the insulation resistance, temperature, moisture and air presence in insulation depending on the type of insulation material and they determine the detection of partial outbreaks in isolation. The basic diagnostic methods are electrical, mechanical, thermal, optical and chemical. Thermal imaging tests of electrical equipment have been elaborated. Diagnostic methods are important because they timely determine the status of electrical equipment in order to increase plant reliability, reduce maintenance and repair costs. Partial outbreaks are dealt with in detail. The partial discharge method is one of the basic methods used in order to determine the state of the isolation and estimate the remaining insulation life, which was proved in the paper that the method is reliable and enables monitoring of online measurements without excluding primary equipment from the system during operation.