The utility model discloses a low pressure isolator's watertight fittings for the auxiliary contact's of mechanism's incasement is waterproof, including fixed connection in the buckler of transmission shaft periphery with set up in the drainage device of buckler below, drainage device is located the auxiliary contact top and is equipped with and communicates in the outside outlet of mechanism's case, buckler and drainage device all are located incasement portion of mechanism. This scheme is followed the waterproof position of equipment and is shifted downwards between transmission shaft and the mechanism's box to the inside of mechanism's box, with buckler and transmission shaft fixed connection, has avoided the influence of sliding friction to sealing up the effect. During the use, the water of outside is followed the transmission shaft and is flowed down, in buckler guide stream arrives the drainage device of below, after outlet discharge mechanism case. Reduce the fault rate that the low pressure isolator manually operation auxiliary contact of mechanism wet rainy season, avoided the direct current system ground connection defect because of rainwater infiltration switch auxiliary contact leads to. The utility model also discloses a low pressure isolator.