The break capacitance and dielectric loss factor tanδ of 500kV circuit breaker are important parameters reflecting operation condition of the equipment.Routine tests shall be carried out periodically

but the test data is susceptible to on-site conditions.At a test site

after breaking the designated switch

the breakcapacitance and dielectric loss measured by the positive connection of the unshielded wire is well beyond the limit range which

according to preliminary judgment of electrostatic noise

is the inductive voltage influence of adjacent bay and

at the same time

the influence of paint layer resistance is also considered.Such two factors are simulated and modeled by the use of ATP-EMTP software.It is shown by the result that the capacitance and dielectric loss across open contacts shows linear relationship with inductive voltage and oscillates up and down with the variation of the inductive voltage.Under normally condition

the influence of the contact resistance generated by the paint layer and the oxidization layer is very less on the measurement result of capacitance and can be neglected.The study result in this paper has definite reference significance for the test of electrical equipment at substation site.