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Hints for Maintaining Allis-Chalmers/ Siemens-Allis Type BZO HV Oil Circuit Breakers

May 2004- Volume I-Issue 2

Siemens and Allis-Chalmers manufactured oil circuit breakers as early as the 1920's under the model name BZO, SDO, FZO, OZ, TDO & LBO rated from 15-242 kV. In the late 1970's, Siemens purchased the rights to the Allis-Chalmers design and continued to manufacture many of them until 1996. Although most oil circuit breakers are over 30 years old, thousands of them are still in service operating reliably.

Thorough, periodic inspection is important to satisfactory operation. Inspection and maintenance frequency depends on installation, site, weather and atmospheric conditions, experience of operating personnel and special operation requirements. Because of this, a well-planned and effective maintenance program depends largely on experience and practice.

It is recommended that the breaker be inspected mechanically and electrically at least every twelve to six months. Depending on the results of the six to twelve month inspection and the accumulated amount of interrupting duty, it is recommended that the oil be drained and a thorough major inspection performed every five years.


Interrupter Hints

Maintenance TIps for Interrupters Used on Types TDO, BZO-4, & BZO-7 Oil Circuit Breakers

FAILURE TO PROPERLY MAINTAIN THE EQUIPMENT CAN RESULT IN DEATH, SERIOUS INJURY OR PRODUCT FAILURE. THE INSTRUCTIONS CONTAINED HEREIN SHOULD BE CAREFULLY REVIEWED,  UNDERSTOOD AND FOLLOWED. THE FOLLOWING MAINTENANCE PROCEDURES MUST BE PERFORMED AS RECOMMENDED BY SIEMENS:

1. Disassembly of the Interrupter

a. Remove the two sets of screws, one at the top and one at bottom, from the interrupter.

b. Loosen the fiber on the bottom plate of the interrupter by use of the spanner wrench supplied with the breaker major inspection kit. Several turns of this plate will relieve the stack pressure.

c. With the fiber loose, the complete interruptor can be easily screwed off the contact casing.

2.Inspection of Fiber Parts, Shell, and Fingers

a. Fiber, Phenolic, Plastic, and Permali Wood Parts

     i.  Fiber washers which are cracked or show erosion of .25" or more from the parts' original contour should be replaced.

     ii.  Nozzle assemblies which show erosion more than .25" from parts' original contour must be replaced.

     III. Phenolic tubes and check tubes which are cracked or which have burn marks that cannot be cleaned off should be replaced.

     iv. Permaili wood which is cracked or delaminated must be replaced.

     v. Cracked plastic finger buttons must be replaced.

CAUTION: Do not wash the finger buttons with cleaning fluid. A chemical reaction with the Lexan material will cause them to crack.

b. Interrupter Shell- inside and outside surfaces must be clean and free of deep burn marks. They may be cleaned with solvent and light sanding.

c. Stationary Contact Fingers- fingers which show erosion of .125" (Elkonite tip) for the parts' original contour must be replaced.

3. Assembly of the Interrupter

a. Prior to the assembly of the interrupter, all fiber and phenolic parts should be dry. If in doubt, dry for 20-28 hours at 150 degrees C.

b. Refer to instruction book provided with the breaker or the typical outline for correct assembly of internal parts. Assembly of parts is best accomplished by starting from the bottom and working towards the top.

c.The shell with washers and nozzle can now be screwed onto the contact casting. The top thread on the contact casting must be in line with the top of the shell plus/minus half a turn. With the thread of contact casting lined up and part of the interrupter facing in line with the lift rod, the top set screw is then inserted through the hole in the top of the shell and into one of the three tapped holes in the contact casting.

Warning: When remounting the interrupter, make sure the exhaust ports face outward and are in line with lift rod when the top set screw is in place.

d. In order to assure a snug baffle assembly, use the spanner wrench to tighten the bottom plate until a firm stacking of fiber parts is achieved. The bottom plate should be further tightened until one of the four holes in the bottom plugs lines up with a single hole in the bottom of the shell. The bottom set screw is then inserted into one of the tapped holes in the bottom plate.

4. Conditions that Can Result from Improper Assembly

a. Loose fiber stack

     i. Housing and tube on stationary contact assembly is free to float electrically producing corona.

     ii. Gases produced during interruptions are free to escape from between the fiber washers and, consequently, burn the inside of the shell.

b. Wet Fiber and Phenolic

     i. Vaporization during interruptions will unbalance dielectic integrity of interrupter

     ii. Swelling of fiber due to moisture will cause distorted parts.

     iii. Drying will cause the stack to loosen.

THIS CHECKLIST DOESN’T REPRESENT AN EXHAUSTIVE SURVEY OF MAINTENANCE STEPS NECESSARY TO ENSURE SAFE OPERATION OF THE EQUIPMENT. PARTICULAR APPLICATIONS MAY REQUIRE FURTHER PROCEDURES. SHOULD FURTHER INFORMATION BE DESIRED OR SHOULD PARTICULAR PROBLEMS ARISE WHICH ARE NOT COVERED SUFFICIENTLY FOR YOUR PURPOSES, THE MATTER SHOULD BE REFERRED TO THE LOCAL SIEMENS SALES OFFICE. DANGEROUS VOLTAGES ARE PRESENT IN THE EQUIPMENT WHICH CAN CAUSE DEATH, SERIOUS INJURY OR PROPERTY DAMAGE. ALWAYS DEENERGIZE AND GROUND EQUIPMENT BEFORE MAINTENANCE. MAINTENANCE SHOULD BE PERFORMED ONLY BY QUALIFIED PERSONNEL. THE USE OF UNAUTHORIZED PARTS OR SUPPLIES IN THE REPAIR OF EQUIPMENT OR TAMPERING BY UNQUALIFIED PERSONNEL WILL RESULT IN DANGEROUS CONDITIONS WHICH CAN CAUSE DEATH, SERIOUS INJURY, OR EQUIPMENT DAMAGE. FOLLOW ALL SAFETY INSTRUCTIONS CONTAINED HEREIN, ON THE EQUIPMENT, OR IN SIEMENS’ OTHER INSTUCTIONS.

Recommended Contact Inspection and Cleaning Procedures

Contacts should be cleaned or changed when the contact resistance, bushing to bushing, exceeds manufacturer's specifications. The average values shown in the following list are the readings we should expect after a normal maintenance inspection. If the maximum values are exceeded, we recommend the contacts be cleaned or replaced.

The stationary contacts and upper contact rod assemblies must be inspected with the shells removed. To remove a shell, take out the 8 cap screws at the top of the shell and the 6 cap screws at the bottom of the shell. Slide the spacer from under the shell and remove the shell.

The lower contact rods are burned somewhat from arcing, but are still in useable condition, use a file to dress the arcing tips. Do not remove the silver plating from the copper surfaces.

If the contacts are burned and eroded from arcing, but are still in useable condition, use a file to dress the arcing tips. Do not remove the silver plating from the copper surfaces.

If the contacts are burned or eroded from arcing so that the contact rods do not make solid contact in the stationary contact assemblies, the contacts should be replaced.

Remove excess carbon from the contacts shells and shell baffles using a non-toxic cleaning agent.

Bushing "Waxing" Problem

Reports from the field have indicated that under certain conditions there is a ax-like material generated inside the sight glass of a small portion of Lapp 67000 Series POC Bushings.

The wax is produced by the polymerization of the oil in the bushing under the influence of high dielectric stress, and corona at the air-to-oil interface in the expansion chamber when a partial vacuum is present. The high stress is produced by the close spacing between the spring holder and the sight glass and is increased when manufacturing tolerance combine to reduce the nominal clearance. The vacuum results from absorption of gas by the de-aired oil used in bushings and reduces the dielectric strength of the air, thus increasing the possibility of corona and polymerization.

In recent tests, we found that vacuum must be present in addition to the high stress for wax formation. Relieving the vacuum restores the dielectric strength of the oil to the point that wax is no longer generated.

This wax forming, while admittedly unsightly, does not constitute a danger to the bushing and certainly will not cause bushing failure. The only problem is that in extreme cases over a long time, it could make observation of the oil level difficult. A loss of oil in the bushing, for any reason, could cause failure if not detected and corrected.

For B-67000 Series bushings currently in service which show indications of wax formation, further waxing can be eliminated by a program of venting or by the addition of an external corona shield which has been shown by tests to eliminate the problem and does not reduce the flashover of withstand ratings of the bushing. The venting is accomplished by momentarily loosening the oil filler plus at the top of the bushing on a dry day to relieve the vacuum and then retightening the plug. It will be necessary to vent a number of times- perhaps once a year for several years until no further vacuum is observed.

In late 1970's, a combination of 67000 Series Bushings sight glass redesign and revised manufacturing/ assembly procedures increased the spacing and reduced the dielectric stress so that the probability of wax formation was virtually eliminated. Venting of corona shields will not be required for these bushings.

BZO 242kV Upper Movable Contact Adjustment Bumper

One of our customers had brought to our attention the possibility of the bumper on the crossarm used to adjust the penetration of the upper movable contact becoming loose or dislodged from the crossarm.

Should the bumper come off, the upper contact penetration may be reduced to the point where contact is loast resulting in serious damage or loss of the interrupter. This can occur due to insufficient threads of the fastening nut being engaged on the stem of the bumper. A minimum of a full nut engagement will assure that the locking feature of the nut is engaged.

A thorough study of the breaker assembly was made and improvements have been made to eliminate the possibility of the bumper becoming loose. Improvements have been made which include lengthening the stem and threads by 1/4 inch and locking the bumper to the stem with a pen.

We recommend that the bumpers be put on your next regular check for tightness and replacement. Replacing the bumpers will give you added advantage toward longer life and easier adjustment as the breakers become older and adjustments on the penetration of the contacts are required.

BZO-242-3 Contact Rod Penetration Adjustment 

We have received a number of requests for additional bumper spacers and occasionally for longer threaded studs for the bumper. These requests are the result of difficulty in achieving the upper contact rod penetration adjustment.

If contact penetration is not enough and the length of the bumper stud is not long enough to addshims, the "pull rod turnbuckle" adjustment will usually solve the problem.

The "pull rod turn buckle" adjustment is used to reduce the crosshead clearance to 1.25 +/- .25". +/- .1 should be reset to achieve the desired crosshead clearance.

Mechanism Hints:

Pneumatic Operators-52aa Switch Replacement

An evaluation of the 52aa switch used on BZO and TDO pneumatic operators showed that the type "T" limit switch can be eliminated and replaced with a 52a contact from the auxiliary switch.

This is possible on wiring schemes by removing wire 7D and 7F from the 52aa switch. Replace with a 52a contact from the auxiliary switch and carefully adjust the 52a rotating element of this one contact. It should be rotated one increment (22 1/2 degrees) from the open position in a retarding direction (towards the closed position).

To check the adjustment of the 52a contact, place the maintenance closing device in position and jack the lower end of the plunger up until the 52a contact is just making contact. Measurement of the plunger from this position to the breaker closed and latched positions should be between 1.75 and 2.25 inches. This is the new adjustment "J". If this setting is not in tolerance, change the rotor position or adjust the yoke on the rod that operates the auxiliary switch arm.

Changing this yoke will adjust ALL contacts on the auxiliary switch.

A final check is to perform a "run down" test. The breaker should be capable of closing five times from normal operating pressure.

Caution: Before attempting any work review the operator instruction book for your breaker and observe the precautions listed.

Elimination of 63S Switch on PN-70T & PN-90T Pneumatic Operators

BZO & TDO breakers with the above type operators that utilize the pneumatic seal-in switch (62S) can accomplish the same function by using one of the normally open "x" relay contacts.

Hydraulic Power Unit Modification Kit to Submerge Pump

In conjunction with the development of the PH-33T-5 operating mechanism, an improved hydraulic power unit was designed enabling submersion of the pump. A kit for submerging the pump, plus other design improvements, is available fro PH-33E-5 and PH-37-E5 mechanisms.

Submerging the pump has two advantages:

1. Should the pump's shaft seal become worn, the pump will continue to operate.

2. The suction line, which could become a source of air leaks, is eliminated. This gives more reliable pump performance.

Hydraulic Power Unit- Relief Valve Modification

The power unit of PH 33E-5, PH 37E-5, and PH 33T-6 Pneu-Draulic operators has a relief valve built into the manifold. Over-tightening of teh relief valve adjusting screw can result in bending the inner spring. When this happens, the relief valve begins acting erratic and the system may not build up to operating pressure.

To prevent bending the inner spring by over-tightening, a design improvement has been made to reduce the load on the inner spring. Other improvements include the outer spring, which is made of square spring steel instead of round wire, and a washer, which has been added in order to distribute the load from the springs on the ball.

Pneu-Draulic Operator Accumulators

Over the many years we have noticed an increasing failure of accumulator gas bags. Many of these failures were at the top of the gas bag. Our investigations showed that most of the failures were caused by the improper tightening of the lock nut at the gas valve assembly (see diagram below).


The gas valve assembly is molded into and bonded to the bladder assembly. Any tightening of the locknut must be performed using two wrenches to prevent damage to the bladder by twisting the gas valve assembly. Apply the second wrench flats provided at the upper end of the gas valve assembly. When installing the valve guard, lubricate the seal washer lightly with vaseline. Install the valve guard and tighten by hand....snug lightly to compress resilient portion of the seal ring. Do not over-tighten - 40fl -lbs max.

Kenett Power Unit – Service Tips

Many of our customers have brought to our attention that while running, these power unit motors make a smooth hum, with the pressure bleed valve closed. To resolve this issue, you’ll need to localize the problem by lowering the reservoir tank, test the power pump for leaks, and check the pump element function.

Breaker Upgrades Available

Major Inspection Kit- All required stationary assembly, contact rods, washers, springs, mgaskets, etc...... to complete major inspection.

Replacement Contacts- Optimizes inventory by providing one contact for 1200-3000 Amp applications.

Power Unit Modification Kit- Replaces previous out-of-production hydraulic pumps with one modern design.

Control Valve Replacement- Replaces previous out-of-production control valves with one modern design.

Accumulator Upgrade Kit- Enhances reliability by replacing bladder-type accumulator with superior piston-type design.

Front End Modification Kit- Replace hydraulic cylinder, and possibly accumulator and power unit, with modern, more reliable design.

Impulse Cushion Replacement Kit- Ensures reliability by replacing outdated impulse cushion design with mechanically stronger interrupter.

Trip Mechanism Application Kit- Additional trip coils provide redundancy.

Current Transformer Expansion Kit- Increases current monitoring capability of breaker by adding additional current transformers.

Voltage Conversion Kit- Increases equipment flexibility and interchangeability by changing system from 48v to 125v AC/DC.

BZO Uprating- Allis Chalmers (now Siemens) oil power circuit breakers, manufactured after 1958 with a rating of BZO 115 KVA, 1200 Amps and 5,000 MVA interrupting capacity, are capable of uprating to 1600 Amps continuous duty and 10,000 MVA interrupting capacity.

05/01/2004