Conclusion The fundamental electrical properties of the HV arm of a typical PD test circuit used in laboratory-based tests on small insulation samples were investigated. It was confirmed that the circuit behaves like an underdamped series RLC network in which oscillations are induced ignition of electrical dis- by charges. Consequently, a spark-gap transmitter model has been proposed to represent the test circuit. oscillation frequencies in the range of several megahertz to several tens of megahertz are characteristic of such a circuit. The overall circuit inductance and capacitance obtained from measurement of the oscillation frequency agreed well with the inductance values L predicted from the circuit dimensions, and the overall circuit capacitance calculated from the capacitances of its fixed components. The overall circuit resistance estimated from the measurements of the signal decay rate was higher than the resistance of the signal acquisition circuit. This discrepancy has been attributed to the skin effect in conductors and to additional damping of the signal caused by dielectric losses in the samples under test.