• The correct oil level is indicated by the circular glass indicator which will be looking 'RED' when level is full. The minimum level acceptable during service is when the circle looks 'Half red' and 'Half White'.
• If the portion looking 'RED' becomes less than half, topping up of oil is essential. Oil used for topping up, should stand for 50kv/min on 4mm gap with 12mm spheres.

• Excessive dust collection at the control valves on the breaking unit and around the operating shaft may be due to oil leakage at these places.
• Oil leakage from the control valves can be arrested by proper tightening of valve fixing bolts and replacing the damaged gasket, dowdy seals. To arrest oil leakage from the operating shaft, oil from the breaking unit has to be drained out. Operating arm and bearing cover are to be removed and the unit is to be pressurized by compressed air/nitrogen gas to about 2 Kg/Cm2 so that the bearing on the shaft is pushed out. The guide with O-rings is now accessible for replacing the damaged O-rings.

• Low and high pressure ranges are marked 'RED' in the manometer. Working range is indicated by white colour. Pressure in the interrupter is satisfactory when the manometer needle lies in the middle of working range.
• If the needle goes to 'RED' zone, corrective action is essential. Low pressure in interrupter might have resulted on account of leaking safety valve and leakage through manometer. High pressure building up in the interrupter may be due to non functioning of safety valve, (Dismantling, cleaning and retesting of safety valve is needed.)

• Open the mechanism doors and check the operation of heater. Replace the damaged/defective heaters.

Visual Inspection of Breaker and Operating Mechanism :

• Interrupters and insulators in the breaker are to be examined for excessive dust collection on insulators. Checking of operating mechanism for damaged/broken, loose and rusted components is essential.
• Replace the damaged/broken parts, tighten the loose parts properly. Exposed metal parts are to be provided by a thin coating of rust preventing oil/grease after dusting. If dust collection is excessive, cleaning with non fluffy cloth at the earliest opportunity is essential. for removing oil/grease and carbon deposits use Tricholoro Ethylene or Acetone.

• Breaker should be operated electrically from local and remote control few times at no load. Tripping of breaker with mechanical push button is to be checked. In this process any coatings developed between the sliding surfaces, get them removed.

• Oil leakage from oil dash pot will be due to defective/damaged O-Rings in the cylinder lid.
• Replace the damaged/defective O-Rings by dismantling the oil dash pot as instructed in the manual.

• Record no. of short circuit Trappings if any and identify the pole which has cleared the short Circuit fault current.
• Following maintenance work is to be carried out after 3 interruptions of 100% rated short circuit current or 10 interruptions at partial short circuit current or 1000 interruptions at rated normal current.
• If fault are occurring in one particular phase trip base overhauling may be necessary.
• Condition of oil may be checked after about 7 interruptions of partial short circuit current.

• Oil sample from breaking unit of each pole should be taken out by connecting a pipe to the bottom control valve and opening the valve very slightly. Dielectric strength is/to be determined as per BS-148/IS-335. Oil shall withstand at least 35 kv./minute in a standard testing bowl with 4mm gap between electrodes. If sample from one breaking unit is good, oil in the other breaking unit in the same pole need not be checked.
• Dielectric strength of oil in service be tested as given below :-
• Voltage more than 170 kv-50kv/1 min.
• Voltage between 70-170 kv-40kv/1 min.
• Voltage less then 70 kv-30 kv/1 min.

However minimum permissible value should be taken as 30 kv/1 min at 4mm gap.

• Filtering/replacement of oil is essential if withstand value is less then 30 kv a min.

• Remove the dome and close the breaker, now moving contact is visible.
• Contact fingers tips and arcing ring of fixed contact are to be examined for burning. Check the contact measurement as indicated in manual BHEL Manual Reference BHE/IM/SWG/001. If the distance is 179mm, replacement of moving contact tip is needed.
  If burning is very slight removing the burn beads and polishing the surface is sufficient. Replace the tips and arcing ring when burning is heavy. Replace the moving contact tip with the help of tool 6896706. Make sure that the contact surface, at the joint is clean and loosening and tightening of tip few times before final tightening is done. Instructions given in the manual for fixed contact reassembly are to be followed.

• Inspection of extinguishing chamber with reference to the burning of moving contact, guide hole and slot in the top washer, (surrounding the fixed contact and portion) soot and sludge formation between the washers. The guide hole and a slot original dimensions are 29 mm and 25 mm respectively. If these dimensions is increased by 2 mm due to burning further dismantling and checking is necessary. If the increases is negligible rinse the extinguishing chamber with transformer oil for soot and sludge removal. If the condition of extinguishing chamber is good in one breaking unit the extinguishing chamber of the other unit of the same pole need not be inspected. Vernier caliper or  steel scale with accuracy to check the dimension 25mm + 2 is required.
• Extinguishing chamber is to be removed from the breaking unit as suggested in the manual & rinsing with transformer oil should be done keeping the chamber up side down. If the condition of top washer indicates severe burning dismantle the chamber, replace the burned/damaged parts. At the time of re-assembly, the use of tool to Drawing No. (9750787), and torque wrench is advisable. The screws are to be tightened uniformly quarter turn a time. The final tightening torque is 4.5 Kgs.
• Nitrogen, to be used, should be of more than 99.98% purity and preferably. 'White spot' or IOLAR-2 Brand.

• Carryout maintenance works as given under sub-paras of weekly and Quarterly maintenance.

• Connect the breaker to earth in OFF position and discharge the springs of operating mechanism.
• Clean the mechanism base frames and link boxes (in side and outside) with non fluffy cloth. Rust is to be removed from the metal surfaces and unpainted steel surfaces are to be smeared with grease (Shell M.P. no.2). The interrupter heads and insulators are to be cleaned with non fluffy cloth, oil, grease and carbon deposits can be removed by Trichloro Ethylene or Acetone. Lubricate all the points provided with grease nipples in mechanism and breaker with shell M.P. grease No. 2. gear wheels are to be lubricated with the same grease, avoid excessive lubrications. Care should be exercised to keep the friction clutch free from lucrication. Sliding parts, catches, rollers and operating magnet are to be lubricated with shell tellus-29 or LUC DTL oil (medium)

Checking of Locking Elements of the Rod System, Outer Mechanism (On Interrupter) and in The Operating Mechanism :-

• Observations for broken/missing split pins, springs washers, B-Rings and circlips should be done.
• Replace the damaged locking elements with new ones. During assembly after overhauling all the locking elements coming in the moving parts should be replaced though they are not damaged.

• Connect the breaker to earth in OFF position and discharge the springs in operating mechanism. Checking/tightening of foundation,/structures bolts and all fixing screws/bolts in the mechanism, rod system, insulator assembly and interrupting units is to be carried out.
• Satisfactory movement of links and arms should be checked after tightening the bolts by slow operations a described in the manual Remove earth connection before taking the breaker into service.

• Correct closing position is indicated when the gap is 50+.5mm and the contact measurement is 173+2.0 on new breaker which has not gone in to service. Burning of moving contact tip during service will increase the dimension from 173 mm to maximum 179mm. This measurement is to be done with scale (tool no. BF-46).
• If the control dimension are deviating from the given values. corrective action as indicated in the manual is essential. Adjust the gap for 50+0.5mm by adjusting the pull rods between mechanism and link boxes maintaining the symmetrical movement of axle in the link box with reference to the link box sides. Adjustment of the gap to 50+0.5mm can be done by varying the length of operating links on the interrupter.

• Check each wire by slightly pulling out at the terminal block, and observe for oxide coating/broken strands wherever loose jointing between terminal and wire is found.
• Tighten the terminal connections properly after removing oxide coating, if any. In addition to loose terminals, tightening of all other terminal connections in general is essential.

• Undamaged operating links, uniform contact wipe in close and open position, no broken insulating parts in the assembly and clean contacts are essential for correct functioning of auxiliary switches.
• Clean the auxiliary switch contacts with C.T.C. and replace the broken parts if any, make sure that the adjustment of auxiliary switch is unaltered.

• The sequence of operation of limit switches with reference to the spring bridge movements should be as described in the manual. The tendency of clutch slipping and spring bridge falling-down at the final charging position, to be observed.
• If the sequence of operation of limit switch is not as stipulated in the manual, check for the disturbed setting on limit switch (operating screws rod main operating stud). Care should be taken while re-adjusting the main operating stud, so that the locking of inter locking plates with pin is possible and there is possibility of moving the stud further about 1-2 mm petrol, replacing the worn-out friction studs, and adjusting the clutch for correct torque is essential. (Torque for BLG 10=2 Kg.m. BLG 20=2.5kg.m. BLG 30=3.5kg.m.)

• Commutator for excessive carbon deposit and brushes for wear and tear are to be examined.
• Clean the commutator with C.T.C. and replace the worn out brushes.

• Check whether tripping coil operates at 50% of its rated voltage, if not, then check the coil and remove the defect.

• Measure and compare resistance with initial values.

Overhaul the circuit breaker by carrying out the maintenance works listed above. The subsequent overhauls should be carried out every fifth year unless services during the first overhaul motivated other intervals. Testing of breaker for the functions listed in sub para (20 C-22 F) and (20 C-23 F) is essential after every overhaul. Slow operations after overhaul should be carried out as described in the manual before attempting fast operations.

• Opening and closing speeds of moving contact in each pole (for breakers with 3 operating mechanism) be measured and compared with original test certificate values, after every overhaul.
• Speed measuring equipments is to be mounted as described in the manual on the first breaking unit near the mechanism variation of about +0.2m/sec. in the specified range need not be viewed seriously. Variation of +0.5m/sec. indicates abnormality in the breaker operations, hence checking for misalignment during re-assembly, excessive friction in the coupling of rod system, missing disc. spring washers/disturbed setting of tripping unit (it dismantled for overhaul) and variation in closing spring bridge movement due to disturbed stopper bolts, should be done.

• Contact resistance of each breaking unit as described in the manual to be measured after every overhaul. The value for breaking unit is 45 micro-ohms. A variation of +10% is normal.
• If variation is more than + 10% checking of fixing bolts of extinguishing chamber, fixed contact, proper tightening of moving contact tip and proper cleaning of the surface in the current transferring joints in the path is essential.

• Operating timings of the breaker, after every overhaul need to be measured with the help of synchronous clock/oscilliograph or a timer. Variation of + 5% over specified limits need not be considered as serious.
• If the variation of operating timings is more than + 5% inspection for disturbed operating coil setting, incorrect operating voltage, variation in closing position indicating gap (50 + 0.5mm), and control dimension 173+2-0mm is essential.