S420 Insulation Tester is a fully automatic 5kV insulation test set designed to provide a comprehensive assessment of the conditions of electrical insulations of high voltage apparatus such as the insulation resistance of transformers, bushing, circuit breakers, and other machinery. S420 makes measurement by using the ohm’s law with the 3 terminal method, which is much more accurate than the 2 terminal method. An additional Guard Terminal provides a return circuit that bypasses the meter, providing users a more precise result. The instrument is also able to automatically calculate the absorption ratio (DAR) and polarization index (PI), giving users a comprehensive diagnostic of the insulation result.