The working principle of hydraulic mechanism for high voltage circuit breaker is introduced

and the source of air contamination in hydraulic system is analyzed.The harms of the air contamination to the circuit breaker are emphatically discussed

including oil leakage

unstable action time

and cavitation.Moreover

the methods for preventing air contamination are proposed

such as avoiding the air retention through improvement of structure design and whole layout

and exhausting the air by vacuum pump or exhaust valve

for the purpose of controlling air contamination effectively.