BZO maintenance


Allis-Chalmers manufactured BZO oil circuit breakers as early as the 1950s rated from 121 up to 245kV. In the late1970s, Siemens purchased the rights to the Allis-Chalmers design and continued to manufacture them until 1996. Although most BZO oil circuit breakers are more than 30 years old, complete circuit breaker replacement may be costly and not necessary in most situations. Siemens is committed to supporting you with OEM parts and factory-trained field service support throughout your breaker's life-cycle.


Maintenance intervals


Siemens recommends that the breaker be inspected mechanically and electrically at least once every six months. Siemens recommends that major maintenance, a comprehensive internal investigation of the condition of the interrupter, contacts, and other internal components be performed, depending on the following factors:

  • One full-fault

  • Five times the accumulated interrupting rating (faults less than breaker rating)

  • 250 mechanical operations

  • Five years of service

  • Accumulated experience of breaker characteristics.

Be sure that the circuit breaker and its operating mechanism are disconnected from all electric power before starting any inspection or any repair work.


Inspect the breaker operating mechanism periodically and lubricate all bearing surfaces regularly with a good quality light, non-gumming lubricating oil having a pour point below -40 degrees C.


See that the oil is kept at proper level in the tanks and that the proper dialectric strength of the oil is maintained. It is recommended that the oil be checked at least yearly and more often if necessary depending on the severity of service.

Operating the mechanism several times each month will insure continued trouble free operation of the mechanical parts and reduce the accumulation of oxidized oil on the breaker contacts. It is recommended that this practice be followed by all users of oil circuit breakers.


On pneumatic or pneu-dralic operated breakers, be sure that the pressure in the air tank or the accumulator has been bled off before inspecting or repairing any part of the breaker and its equipment.


Also, Siemens has designed an uprate solution for BZO6 115 – 138 kV and BZO 242-3 kV high-voltage oil circuit breakers to increase fault duty rating from 40/50 kA to 63 kA.