Maintenance of Substation Equipment

Table of Contents

1.0 Introduction

The switchgear and protective relaying system should be always alert to operate against an unexpectedly fault. Switchgear which was in quiescent state has to operate immediately. For such an operation, regular and detailed maintenance is necessary. The lack of maintenance may result in failure in operation.

The maintenance requirements of static equipments like transformers, capacitors etc. are much less compared to other dynamic equipments like circuit breakers, motors etc.

The Switchgear manufacture supplies ‘Manual of Installation, Operation and Maintenance’. These manuals should be carefully studied by trained maintenance staff. The code of practice booklets published by the standards institution; regulations of electrical installations are also useful. Detailed programme of maintenance of switchgear should be prepared with pre-determined intervals between inspections. The period may be one to three months if switchgear is operated frequently and six months to twelve months if switchgear is operated rarely. After operation of the circuit-breaker on abnormal condition, inspection should be carried out as early as possible. Further, it is unwise to leave the circuit-breaker close for a period longer than six months without opening, because the mechanism may become sluggish and contacts may need cleaning. Hence during the periodic maintenance, the circuit-breaker is purposely opened and closed by manual command.

The maintenance schedule is usually in the form of log sheets on which weeks, months of the year are tabulated. Each equipment in the sub-station or the plant is provided with a column. The maintenance period is indicated against each equipment. Further each major equipment is provided with a history card. The details about inspection, operation and remarks are written in these cards.

The ‘spares’ are important for maintenance duty. The spares are kept in stock with proper inventory control.

The maintenance work is done according to the schedule. In case of difficult jobs the manufacturer is consulted. The manufacturer provides trained personnel on request and necessary payment.

2.0 Breakdown Maintenance Versus Preventive Maintenance

Maintenance is classified in two categories as follows:

  1. The breakdown or corrective maintenance activities are undertaken after failure of an equipment. Such maintenance results in outage of circuit and supply. In general, it consists of locating the trouble, repair and recommissioning.
  2. The preventive maintenance is undertaken to ensure smooth and efficient working of a system, equipment. Preventive maintenance is undertaken as per schedule before breakdown of a system or machine takes place.

A performance record of each critical component is maintained and basic decisions on the service life of the component and the total service it has put in. Repairs or replacements are made to ensure that no breakdown occurs at any time during the service.

Preventive maintenance is carried out in planned manner. Breakdown maintenance is carried out as and when necessary. For switchgear and protective equipment, preventive maintenance is recommended because failure of a switchgear cannot be permitted.

3.0 Inspection, Servicing, Overhaul

Maintenance covers a wide range of activities aimed at keeping the equipment in perfect working condition for performing its function as per assigned duties. The choice of activities and schedule depends upon local requirements.

  1. Inspection: This refers to the maintenance activity which comprises careful observation/scrutiny of the equipment without dismantling it. It usually includes visual and operational checks.
  2. Servicing: This refers to cleaning, adjustment, lubrication and other maintenance functions without dismantling the equipment.
  3. Examination: This refers to inspection with necessary dismantling, measurements and non-destructive tests to obtain data regarding the condition of components/sub-assemblies.
  4. Overhaul: This refers to the work done with the objective of repairing/replacing worn-out parts and defective parts. The equipment, sub-assemblies are dismantled partly or completely. The condition of components is inspected. Dimensions of worn-out components are measured. The components worn-out beyond acceptable limit are replaced. The assembly is followed by functional checks and measurements to ensure satisfactory operation.

4.0 Guidelines for Maintenance of Switchgear

The requirement of inspection, servicing examination and overhaul vary with:

  • Environmental aspects such as dust, chemical fumes, moisture/humidity, ambient temperature variations, etc.
  • Operating duty; frequency of operation, rated current.
  • Switching duty severity, e.g. repeated operations.

Manufacturer gives general guideline. It is not possible to obtain exact maintenance schedule meeting local requirement of each site. Hence maintenance schedule is determined after initial periodic inspection at each site. In case of switchgear and control and protection panels; distinction should be made between the maintenance of fixed devices like busbars, insulated enclosures etc. and maintenance of switching devices like circuit breaker, isolator, earthing switch, contractors etc. having moving parts.

The fixed parts need regular inspection and servicing for removing dust damp, corrosion etc. Moving parts need regular inspection and periodic replacement of worn-out parts. The functional readiness of switching devices should also be ensured.

The maintenance of switching devices is related mainly with the wearing-out of contacts, deterioration of quenching medium and mechanism components. The maintenance requirements of vacuum circuit breakers SF6 circuit-breakers are quite modest as compared with those of oil circuit-breakers, minimum oil-circuit-breakers. In vacuum circuit breakers interrupter is a permanently sealed unit and the contracts have long switching life. Puffer type SF6 circuit-breakers have long switching life and the gases do not need replacement. Hence the present trend is to use maintenance free vacuum and SF6 circuit-breakers.

Table below gives recommendation regarding the period of maintenance of contacts and quenching medium in terms of number of load operations and number of short circuit operations on rated short-circuit breaking current. The schedule should be established for each site by checking the contacts of one pole after every three months and observing the rate of erosion.

Maintenance of Quenching Medium Contacts
Type of C.B.Maintenance of Quenching MediumReplacement of contactsK (∑I²n)
Load* OperationFault OperationLoad** OperationFault Operation
Air C.B.--300010 to 15-
Bulk Oil C.B.20006200062000
Air Blast C.B.--150002515000
SF6 C.B.500025150002515000

* Shelf life 20 years.

** Mechanical Endurance Test should be performed with specified number of operations on no load to confirm suitability of mechanism.


During every breaking operation, contact looses some material and the quenching medium gets decomposed. The decomposed products gets deposited on the internal insulating parts of the circuit breakers. The deterioration of contacts and internal insulation is proportional to K, the cumulative sum of the product of the number of breaking operation and square of the breaking current in KA (∑I²n) and is taken as a guide to the inspection of contacts, internal insulation and the quenching medium. The value of K depends upon the type of circuit breakers (Refer table above).

Life of contacts become supposedly bad after certain operations on load and or after clearing faults on rated short circuit current an indication of which is shown in table 2.

Maintenance of Contacts
Type of C.B.Life of Contacts
Number of load operations on rated load currentNumber of fault operations on rated short-circuit current
Air break CB1,0001-6
Air Blast CB4,00015-25

5.0 Maintenance Schedule Check List

The check list of maintenance of switchgear gives the list of activities and there periodicity. The periodicity of above activities varies with:

  • Type of switching devices.
  • Frequency of operating duty and breaking current.
  • Environment.
Important Checks on Circuit Breakers during Routine Maintenance
ActivityQuarterlyHalf yearlyAnnual
1.Check insulation resistance of each pole of phase to ground across terminals.X  
2.Check and dielectrical strength.X  
3.Check Mechanical operations.X  
4.Check tightness of bolts.X  
5.Check oil level.X  
6.Clean procelain insulators.  X
7.Check contact length and simultaneous contact touch.  X
8.Measure contact resistance. X 
9.Clean cross jet pots, recondition contacts fill oil (OCB). X 
10.Measure contact speed.  X
11.Measure Break Time, Make Time. X 
12.Level of oil/gas pressure.X  

The check list and maintenance schedule for various types of circuit breakers are indicated in table below, and the following abbreviations has been used:

DI =Daily Inspection.
WI =Weekly Inspection.
MI =Monthly Inspection.
QI =Quarterly Inspection.
HI =Half yearly Inspection.
AI =Annual Inspection Maintenance.
OH =Overhaul.
N =Normal duty
R =Repeated operations on rated load current
Check List and Maintenance Schedule for Circuit Breakers
Maintenance activityAir-break C.B.Minimum Oil C.B.Vacuum C.B.SF6 C.B.Air Blast C.B.Indoor Metal clad MV Switchgear
Cleaning of external insulation.QIQIQI-QIQIQI-QiQIHI
Contacts, contacts shrouds (ABCB), Nozzles (SF6 CB) Arc Control Devices (MOCB), Ext. Contact position (VCB) Contact resistance (All)HIQIHIQIAIAI5YAI5YAI-
Quenching mechanism in circuit breaker and topping up--HIQI--AIAI--QI
Cleaning of internal insulation of CBAIHIAIHI--5YAI5YAI-
Terminal connections, earthing connections, structure, cabinet cable etc.AIHIAIHIAIAIAIAIAIAIQI
Sealing leakage of medium, Pressure of medium--QIQI--QIQIQIQI 
Main mechanism operating checks, interpole linkagesHIQIHIQIAIAIAIAIAIAI 
Operating checks and Insulation resistance measurement, Contact resistance measurementAIHIAIHIAIAIAIAIAIAI 
Post fault maintenance after No. of fault operations10650252525
Overhaul after no. of years55501010 
No. of fault opearations111055 
No. of load operations200010001000050005000 

6.0 Maintenance of Circuit Breakers

The following steps are involved in maintenance of circuit breakers:-

  1. General Inspection: Observe the circuit-breaker visually. Note the cleanliness, terminals, earth connections, readings of counters, levels of quenching medium (in case of Oil Circuit-breaker), pressure of quenching medium in case of SF6 circuit breakers etc.

  2. Cleaning and Drying: Use CTCL or trichloroethylene or other cleaning agent recommended by manufacturer. The fluid should be compatible with the surface to be cleaned. Use air-pressure jet (3 kg/cm²) for cleaning. Use clean cloth which does not leave fibres or particles on the surface.

    Care should be taken to avoid falling of dust, iron particles, nut-bolts, washers etc. inside the breaker. Avoid water, moisture or dampness during cleaning. Internal dust and moisture causes gradual deposits on internal surface resulting in gradual increase of surface leakage currents and internal flashover by tracking. Congealed lubricants should be removed by means of solvents. The cleaning and sliding surfaces should be cleaned, relubricated. Before assembly of the circuit-breaker, the interrupter support porcelain components etc. should be cleaned in dry clean atmosphere.

    After assembly, evacuate the breaker pole to remove moisture, dust molecules etc. and then fill oil or SF6 gas. Grooves for O-rings on flanges should be cleaned with tricloroethylene, so as to remove hardened grease and dust. Such a dust or grease will give the uneven sitting of the new O-ring and cause gradual leakage of SF6 oil. No dust, chalk-marks fibres, hard grease etc. is allowed on the O-ring grooves.

    Terminals should be cleaned of dust, oxide coating if any by emery paper without iron particles.

    Circuit breaker pole can be internally dried by circulating dry hot air or by evacuating to 2 mm of mercury. In case of SF6 circuit breaker or porcelain clad vacuum circuit-breaker, the drying of pole units should be carried out before filling SF6 gas or dry nitrogen. SF6 gas or dry nitrogen does not remove the water drops and dust deposited on the internal surface. Hence drying and evacuating is necessary.

    A small portable vacuum pump with Teflon hose is connected to the valve. The breaker is kept under vacuum for a few hours. Thereafter the SF6 gas / dry nitrogen is filled. The moisture is eliminated due to application of vacuum. Drying is recommended before filling of fresh SF6 gas / nitrogen / oil in the breaker.

  3. Insulation Surface: Inspect visually, carefully for signs of cracks, tracking or any other defects. Clean the internal insulating surfaces and external insulating surfaces as mentioned in 2 above. Insulation should be free from electrical or mechanical defects. Perform insulation resistance measurement tests after cleaning. Insulation resistance measurement gives indication about the health of the insulation.

    In case of oil circuit-breakers and minimum oil circuit-breakers, the internal insulation should be cleaned thoroughly by means of trichloroethylene, clean cloth and air jet. The deposition of sludge and carbon particles, conducting dust particles shall be removed before reassembly.

    In case of SF6 circuit-breaker, the decomposition products (gray colour) get deposited on internal surfaces of insulators. These are non-conducting when dry. If the circuit-breaker is dismantled during moist atmosphere, these surfaces are not cleaned before assembly, the internal flashover is likely to occur despite the good properties of SF6 gas. In case of vacuum interrupters only external cleaning is possible.

    In case of air blast circuit-breakers, no internal cleaning is generally necessary as fresh medium is used for arc-quenching. In case of porcelain-clad outdoor vacuum circuit-breaker, the pole units should be internally clean and dry to avoid internal flash-over by tracking. Glass fibre pull rods should be cleaned thoroughly.

    Particular attention should be paid to the nozzles, arc control pots, arc control plates. They should be cleaned. If burnt or disfigured, replace them.

    After cleaning and drying measure insulation resistance by megohm meter (megger) between two terminals of each interrupter and between the terminal and earth. Insulators of circuit-breakers installed in heavily polluted areas and sea-shores need frequent external cleaning. The method of measuring the insulation resistance is shown in the end of this chapter.

  4. Interrupter: Study the operation and maintenance manual of the circuit-breaker. Note the important settings and measurements of moving contact, other movable parts with reference to fixed flanges and the allowed tolerance in the settings. Check simultaneous touch of 3 poles if slow closing is possible. Main activities in the interrupter maintenance include:

    • Observation, cleaning, replacement of main/arcing contacts; PTFE nozzles, arc-control pot plates etc.
    • Cleaning the other parts as well.
    • Replacing hardened O-rings, worn-out sliding parts.
    • Removal of carbon/metallic decomposed products.
    • Cleaning of venting systems to ensure free passage of oil/gases. The vents should be made free but not enlarged.
    • Cleaning terminals and sliding contact surfaces.
    • Assembly with proper settings of components.
  5. Mechanism: Check operation ‘open’; ‘close’; ‘closing followed by opening’ locally. If operation O, C, CO are satisfactory, the mechanism is satisfactory and does not need any major repair/maintenance.

    Check operation counter. If the mechanism has operated more than 1000 times, it needs very close observation and may need overhaul. Check the condition of springs and dashpots. Two important tests to determine the health of the operating mechanism, linkages and moving contact settings include:

    • Checking simultaneous opening and closing of 3 poles.
    • Checking no-load times characteristics of moving contact for O, CO, O-CO operations.
    1. Method of obtaining Time-Travel Characteristic on no-load: In case of MOCB or SF6 CB poles, this characteristic is extremely important because the breaking capacity is related with the time/travel characteristics of moving contact. No-load characteristic gives sufficient indication about the health of mechanism linkages.

      For satisfactory are interruption the moving contact should open and travel with optimum characteristic. Slow initial movement indicates excessive friction between sliding parts. Slow movement during middle of stroke indicates very high dynamic load during arc quenching. Slow movement during final part of the stroke indicates excessive damping or low energy of operating mechanism during opening.

      A straight rod is connected to the moving contact or movable part (contact or mechanism). This rod is in turn connected to the curvo-roller or rectilinear transducer (travel recorded). Curvo-roller is a specially designed motor driven drum mounted on the top-hood of the MOCB. The pencil attached to the rod fixed on the moving contact touches the paper on the drum. The motor is driven electrically. The drum rotates at known speed. The graph sheet fixed on the drum has definite circumferential speed. During the opening stroke and closing strokes, the pencil gives the time/travel characteristic marking on the graph sheet on the curvo-roller.

      For high speed SF6 circuit-breaker the curvo rollers are not suitable. Rectilinear transducer is fitted suitably on the breaker frame. It has a cylindrical resistance with internal moving piston. The piston is attached to the rod with and swivel joint at the end. The swivel joint and rod of the rectilinear transducer are connected to the moving rod attached to the contact movement system. In case of SF6 or oil CB, a suitable rectilinear seal should be designed to permit movement without leakage of SF6 / oil. The rectilinear transducer is connected like a potentiometer. The central terminal gives variation proportional to the travel of the contact.

      The output is given to UV recorder. The trace of time / travel is obtained on UV recorder. The trip signal / closing signal is also recorded simultaneously. The time / travel characteristic should match with that obtained in the manufacturers works.

    2. Method of checking the Contact Setting: Follow manufacturer’s instruction booklet. In case of MOCB and SF6, measurement of distance from the top flange surface to the tip of closed contact may be possible. In case of vacuum interrupter, a gauge with pointer may have been provided by the manufacturer to indicate contact erosion.

      Measurement of Contact Resistance: The resistance between terminals of each interrupter and each pole is measured by means of micro-ohm meter. The resistance should be within a few tens or micro-ohms. A pair of contacts has a resistance of about 15 μΩ.

      Contact resistance is inversely proportional to the contact pressure. Low contact pressure may be due to week springs or worn out contacts. High contact resistance cause excessive heating of contacts while carrying normal current and possible welding during through short circuit.

    3. Mechanical Assembly: Check that all the nut bolts are in their position and check their tightness. Check circlips, split pins. Clean and lubricate sparingly. Oil dashpots should be checked for current level and operation. Air Dashpots should be checked for current operation.

      Clean inspect and replace worn-out parts during overhauls. In case of spring operated mechanism check the ratchet wheels and prawts for broken or chipped teeth.

      Valves of pneumatic mechanism or hydraulic mechanism should not be disturbed unless the diagnostic tests indicate the need for their readjustment.

    4. Inter-pole linkages: For ensuring simultaneous operation of 3 poles (within pole discrepancy of 5 ms), the inter pole linkages should be checked for deterioration of springs, other components. Linkage pins, circlips, nut-bolts, etc. should be checked. Verify that fixings are tight and pivot pins are secure.

      Maloperation of mechanism, interpole linkage, dashpots etc. can be revealed by the time/travel record of 3 poles plotted on UV recorder as described earlier.

      Congealed lubricants should be removed from sliding, rolling surfaces. The parts should be relubricated as per the instructions of the manufacturer.

      The moving contacts of each phase should meet the fixed contact practically simultaneously. An electrical method of checking the contacts is illustrated in figure 1. A low voltage supply, lamps are needed. Simultaneous glowing of lamps indicates simultaneous making of contacts.

  6. Check and Maintenance of other important accessories: 

    Main Connections. Ensure that good contact is maintained and connections are right and secure.

    Secondary Wiring and Fuses. Ensure tight and secure connections, cleanliness and freedom from dust and moisture. Heater in the cabinet should be checked and repaired, if necessary.

    Earth Connection. The main and secondary earth connection should be tight and free from dust and rust.

    Heater. The heater provided in the control cabinet should be in working condition.

    Safety Shutters. The shutters mechanism in metal-clad medium voltage switchgear should be verified after pulling out of drawout unit and deenergizing the busbars.

    Busbars and Busbar Chambers. The busbars and the busbar chambers should be checked for cleanliness of insulators, tightness of joints and freedom from dust, damp and foreign materials/insects etc. There shall not be any loose joints or signs of overheating, melting sparking.

    Auxiliary Switches, Indicating Devices and Interlocks. Auxiliary switches shall be kept in clean and sound condition because the correct functioning of other items of equipment, including protective gear depends on auxiliary switch.

    Inspect the contacts and clean or renew if necessary, where possible verify correct contact force and correct timing of contacts. Indicating devices such as mechanical ON and OFF indicators, semaphores etc. shall be inspected to ensure that they are in good order and operating correctly.

    Interlocks and locking devices shall receive particular attention especially those associated with earthing and testing facilities. A defective or worn device may result in a dangerous condition. It shall be verified that any incorrect operation is satisfactorily inhibited lubricate as necessary.

    Particular attention shall be paid to the required timing of the auxiliary contacts controlling the trip circuit.

    Isolating Contacts. Clean inspect for signs of overheating, renew or recondition if necessary, lubricate as required.

    Overload Devices and Protective Relays. Routine maintenance should be carried out at correct intervals.

    Instruments and Protective Transformer. Routine maintenance should be carried out according to instructions.

    Control Relays or Contractors. Inspect mechanical parts for free movement with control and main solenoid or motor circuit isolated clean arc chutes. Inspect contacts and renew, if necessary.

    Any flexible braids shall be inspected, especially for fraying at the terminations, and renewed if necessary. Where exposed to external atmosphere, the braids shall be treated with a suitable protective compound which will not impair their flexibility.

    Pressure Gauges and Pressure Switches. The readings of pressure gauges are checked against a standard gauge. The operation of pressure switches should be checked against their setting.

  7. Final Verification: Before returning to service after the overhaul, the circuit-breaker is subjected to operational checks by performing C, O, CO operations from local control cabinet and from the control room. Simultaneous touching of the contacts of three-phases is verified. Insulation resistance is measured between the terminals of open interrupter and between the lower terminal and earth. Insulation resistance of auxiliary wiring is also measured.

  8. Typical Maintenance Record Card: History card is kept for each circuit breaker.

    • Circuit breaker S.N.:
    • Make:
    • Year of Manufacture :
    • Ratings:
    • Making/Braking Capacity:
    • Short Time Rating:
    • Inspection date:
    • Permit to work number :
    ComponentObservationAction takenInitials
    Quenching medium
    Main contacts
    Arcing contacts
    Final verification

7.0 Maintenance of Air Break Circuit Breaker, Fusegear for Low and Medium Voltages.

The schedule of maintenance depends upon the frequency of load operations and fault operations. For frequent load operations / fault operations, maintenance requirement is high. For indoor, dust-free installation with infrequent load operations, the following schedule is recommended:

  • Inspect as often as possible with maximum interval of 12 months.
  • Examine at 5 years interval.
  • Overhaul when examination and diagnostic, tests indicate need. Maximum interval of 15 years.
Maintenance for air circuit breaker and switchgear
Maintenance operationRoutine MintenancePost fault maintenance
InspectionExamination and overhaul
Operational checks.XX 
Visual Inspection.XXX
Cleaning. XX
Insulation resistance.XX 
Circuit-breaker enclosure. XX
Main/arching contacts. XX
Arc control devices and Interphase barriers. X 
Mechanisms.  X
Auxiliary switches, indicating devices and interlocks. X 
Isolating contacts. X 
Overload devices and protective relays.XXX
Instrument and protective transformers. X 
Control relay or contractor. X 
Main connection. X 
Secondary wiring and fuses. X 
Earth connection. X 
Heaters. X 
Switchgear spouts. X 
Busbars and busbars chamber. X 
Weather shields. X 
Final verification.XXX
Maintenance of auxiliary equipment. X 
Diagnostic testing X 

8.0 Maintenance of Oil Circuit Breakers (BOCB, MOCB)

The schedule for inspection and maintenance depends upon the frequency of load operations and fault operations. Oil circuit-breaker installed outdoor and in damp / rainy areas need frequent inspection. The main requirement is inspection of oil, contacts and internal insulation.

  1. Under normal conditions for frequent operations: Once in 6 months with maximum period of 9 months.
  2. For frequent operations: Once in three months.
  3. After fault clearance. If fault level is high; inspect as early as possible. If fault level is low, inspect after fault opening operations.
  4. Overhaul: Once in 5 years for normal and once in 3 years for repeated / frequent fault duty.
  5. During the periodic check up the following checks should be made:

    1. Check the level and condition of oil.
    2. Clean the insulators with fine fabric cloth that will not leave fibres. Do not use cotton waste in any case. For removing oil, grease, carbon deposit use trichloroethylene or other chemical recommended by the manufacturer.
    3. Check contacts.
    4. Check operating mechanism.
    5. Check oil.
    6. Check auxiliary switches.
    7. Tighten nuts, bolts etc.
    8. Test insulation resistance by means of high voltage (1000 V DC) megger in case of high voltage circuits and by 500 V megger in 220 V control circuits.
    9. Carry out tests according to the specifications.
    10. Take the steps as mentioned in the subsequent paragraphs.
  6. When the breaker operates on fault, the internal and external inspection should be carried out as soon as the operating schedule permits.

    1. Examine the oil. If badly deteriorated, change it.
    2. Check arcing contacts. Clean with smooth file. If badly damaged, replace them.
    3. Inspect the insulation, carefully check the surface.
    4. Check the arc control device. If damaged, replace the plates.
    5. Check the tripping circuit and operating mechanism.
    6. Be sure that no tools are left in the tank. Some further details are given below.
    7. Check the dielectric properties of oil. In oil circuit-breakers and minimum oil circuit-breakers, oil serves as insulation and arc extinguishing medium. Insulating oil for transformers and switchgear (IS 335 : 1963) (BS 148 : 1959) is recommended in these applications. Properties of transformer oil (BS 148 : 1959) are given in the Table 6.

      Properties of Dielectric Oil
      CharacteristicLimiting wave
      Sludge (maximum)1.200 %
      Flash point (closed minimum)295 °F (146 °C)
      Viscosity at 700 °F37 CS
      Electric strength, 1 minute minimum40 kV rms.
      Saponification value (maximum)1.00 mg KOH/gm
      Copper discolorationNegative
      CrackleShall pass test

      Impurities have a bad effect on the properties of oil. The oil sample is taken in a standard oil testing cup (90 mm × 60 mm × 100 mm high). Electrode are polished brass sphere 12.7 mm diameter mounted horizontally. The gap spacing is 4 mm + 0.02 mm. Dielectric strength of oil is very important.

      A sample of insulating oil is taken from the bottom of the circuit-breaker tank. Dark and brown clouds indicate deterioration. Oil in good condition has pale yellowish uniform colour. It is tested by means of portable oil-testing set which consists of a auto-transformer, voltmeter, tripping device etc. The voltage can be gradually increased. Oil in good condition should withstand 40 kV rms. for one minute in a standard oil testing cup with 4 mm gap between electrodes. A gap of 2.5 mm, should have break down value above 40 kV. The oil should be tested during the periodic check-up and whenever the circuit-breaker clears a fault.

      Oil Maintenance: Oil maintenance is carried out in accordance with the standard titled “Code, Practices of Maintenance of Insulating Oils”. The code refers to the contamination of oil and determination of whether the oil is suitable for further service. It gives the procedure of treatment of oil at site.

      Contamination of oil due to moisture or solids can be dealt with satisfactorily at site. Centrifugal separators are effective in removing free water and fine solid impurities. Filters remove the solid impurities. The oil is heated to about 700 °C by means of electric heater. The purification set consists of purifier, heater, deaerator, oil pumps, strainer and other accessories. The oil is circulated through the purifier till the desired dielectric strength is obtained.

      The dielectric oil should never contain suspended particles, water soluble acids and bases, active sulphur or colloidal carbon. These impurities accelerate deterioration of the insulating properties of the dielectric oil resulting in internal flashovers. The dust particles, carbon particles and sludge in the oil which is in suspended form in the oil gathers along the conductor and insulator surfaces in presence of electric-field. There by a thin conducting layer is formed gradually along the surface of internal insulation during service. The flashover can occur along the surface of the insulation due to tracking. Dust and other particles are removed by means of filters in the oil filtering unit.

      Moisture in oil lowers the dielectric strength. Thereby causing internal flashover. Moisture is introduced in the circuit-breaker through defective seals. Sometimes, oil filled in breaker may itself have moisture content.

      Viscosity indicates fluidity. Oil with low viscosity have more fluidity and gives better cooling and shorter opening time. At lower ambient temperature the viscosity sharply rises and the speed of contact travel reduces. Hence viscosity should be measured at various temperatures and the characteristic of contact travel should be plotted at various temperatures.

      Higher, flash point (145 °C) is preferred. Flash point indicates tendency to evaporate. For flash points lower than 135 °C, the oil tends to evaporate rapidly, thereby the viscosity rises and volume of oil reduces.

      When filling switchgear tank with oil it is a good practice to first rinse the tank and the immersed part with clean oil. It is essential that the tank and the surfaces of conductors and insulator be kept free from fibres and moisture as contamination which lower the electric strength of the oil.

      There should be as little aeration of oil as possible during the filling of the tank. A standing time should be allowed after the tank is filled before commissioning the equipment. Hot oil should not be used to fill switchgear.

      In addition to the above, a regular inspection should be made of oil levels.

    8. Silica gel Breather: In case of MOCB’s and OCB’s, porcelain-clad VCB, silica gel breather may be provided. It is installed at a slightly higher level than top chamber of MOCB. During higher loads the oil in the tank expands and the level of oil in the top chamber raises. The air cushion in the top chamber permits expansion and contraction of the oil in the pole without contact with moist-external air. The air cushion in the top chamber is connected to the external atmosphere via a silica gel breather ensuring dryness of incoming air during the breathing. In case of porcelain clad VCB, the breather may be fitted at the bottom of pole. Silica gel breathers are fitted with a sight glass through which the colour of the crystals can be seen. The colour changes from the blue to pink as the crystals absorb moisture. When the crystals get saturated with moisture they become pink and should, therefore, be reactivated.

      The crystals should be baked at a temperature of about 2000 °C until the whole mass is at this temperature and the blue colour has been restored. Clean the breather and replace the dry crystals and renew the oil in the sealing cup at the bottom.

      Silica gel breather may not be provided with some oil circuit breakers in which the upper chamber is filled with dry nitrogen permanently.

      Maintenance recommended for OCB, MOCB are summarized in Table below.

      Maintenance for Oil Circuit-Breakers in Metal enclosed Switchgear
      Maintenance operationRoutine MaintenancePost fault maintenance
      InspectionExamination and overhaul
      1. Operational check C, O, COXXX
      2. General inspectionXX 
      3. Cleaning X 
      4. Insulation, Insulating oilXXX
      5. Circuit-breaker enclosureX  
      6. Main arcing contactsX  
      7. Arc-control devices XX
      8. Isolating contacts XX
      9. Venting and gas sealsXXX
      10. Mechanism XX
      11. Auxiliary switches, indicating devices and interlocks X 
      12. Overload devices and protective relaysXX 
      13. Instrument and CTs, VTs  X
      14. Control relay or contactor X 
      15. Insulating oil XX
      16. Tank and tank lining XX
      17. Tank lifting mechanism X 
      18. Main connections X 
      19. Secondary wiring and fuses X 
      20. Earth connections X 
      21. Heaters X 
      22. ShuttersX  
      23. Switchgear spouts x 
      24. Busbars and busbar chambers X 
      25. Weather shields X 
      26. Final verificationXXX
      27. Joints and seals  X
      28. General mechanical inspection  X
      29. Maintenance of oil switches etc X 
      30. High voltage fuse connection and associated linkage X 
      31. Maintenance of air-break isolating devices associated with oil switchgear x 
      32. Maintenance of auxiliary equipmentXX 

Post fault-maintenance of oil circuit breakers in metal enclosed switchgear: Depending on the design and duty of the OCB, MOCB, inspection may be necessary after about six fault operations or the number specified by the manufacturers and based on local fault level. When such an inspection is necessary special attention shall be given to the points listed below. Isolation of withdrawable metal clad circuit-breakers shall be delayed for atleast 10 minutes after operation on fault to allow for the disposal of any explosive gases.

  1. Cleaning. Insulation tank lining and other parts liable to disposition from metal vapour shall be cleaned and inspected for signs of cracking, burning or other damage. An inspection for signs of tracking should also be made.

  2. Contacts and Arc-control Devices. Contacts should be inspected for burning or other damage and reconditioned or renewed if necessary verify contact for alignment and wipe.

    To remove traces of metal deposits it will generally be found necessary to remove the arc control devices or recondition or renew them.

  3. Mechanism: It shall be verified that the mechanism operates correctly. Attention shall be paid to setting and clearances after contacts or arc control devices are replaced.

  4. Insulation Resistance: A test should be made of the insulation resistance before putting the switchgear back into service.

  5. Insulating Oil: If the oil is badly discoloured or show evidence of carbon particles in suspension, it may require changing.

  6. Joints and Seals: All joints and seals should be inspected for tightness and particular attention should be paid to tank gaskets where these are fitted.

General Mechanical Inspection: This should be made for mechanism structure, internal or external to the tank. The switchgear should be tripped and closed.

9.0 Maintenance of Air-Blast Circuit Breakers

The circuit-breaker uses fresh compressed air for arc quenching and does not require maintenance of quenching medium. Only the dryness of air should be ensured. The compressed air system needs routine inspection.

After specified number of operations contacts and nozzles in interrupters need check. The diameter of nozzle above 5 % calls for replacement. The maintenance recommended for air blast circuit breakers is summarized in table below:

Maintenance for Air-Blast Circuit Breakers
Maintenance operationRoutine MaintenancePost fault maintenance
InspectionExamination and overhaul
1. Operational checkXX 
2. General InspectionXXX
3. Cleaning X 
4. Air conditioningXXX
5. Insulation XX
6. Load control kiosk X 
7. Pressure gauge X 
8. Pressure switches X 
9. Main connection X 
10. Secondary wiring and fuses X 
11. Interpole linkages X 
12. Main mechanism X 
13. Auxiliary switches, indicating devices and interlocks X 
14. Sequence isolator XX
15. Local air receivers and pressure vessels X 
16. Interrupters XX
17. Isolating and arcing switches XX
18. Earth connection X 
19. EnclosuresXXX
20. Overload devices and protective transformersXXX
21. Instruments and protective transformersXX 
22. Control relays or contractors X 
23. Final verificationXXX

The following list gives operations which may be required to be carried out during maintenance:

  1. Cleaning: All the dirts should be brushed off and any joint or gasket shall be cleaned prior to commencing, any dismantling, to avoid ingress of dirt into any internal portion of the breaker. This is particularly important in the case of pneumatic valve gear since relatively small particles of dirt can cause a disproportional amount of damage leakage or malfunction.

  2. Maintenance of Compressed Air Plant: Compressed air plants should be inspected as part of the regular sub-station inspection. It is good practice to carry out periodic start/stop tests on the plant.

There are statutory obligations, to inspect, test and certify air receivers (pressure vessels) periodically. Safety and reducing values, should also be inspected during the same time. Guidance on the maintenance of this equipment given by the manufacturer should be followed. The driers should be inspected. If necessary, the drying agent should be changed.

10.0 Maintenance of Vacuum Circuit Breaker

Vacuum interrupter is sealed for life and does not require any replacement of contacts for several thousands load operations and about 50 operations on rated short circuit. Mechanism needs periodic lubrication as recommended by the manufacturer. The other parts need cleaning and general inspection. The details are given in Table below:

Maintenance for Vacuum Circuit-Breaker
Maintenance operationRoutine maintenancePost fault maintenance
InspectionExamination and overhaul
1. Operational check X 
2. General Inspection X 
3. Cleaning XX
4. Opening device (trip) X 
5. InsulationXX 
6. Circuit-breaker enclosure (Interrupter) X 
7. Main connections X 
8. Secondary wiring and fuses X 
9. Mechanism X 
10. Auxiliary switches, indicating devices and interlocks X 
11. ShuttersXX 
12. Switchgear spoutsXXX
13. Isolating contactsXX 
14. Vacuum interrupter XX
15. Isolating and earthing switchgear X 
16. Earth connectionXX 
17. Overload devices and protective relaysxX 
18. Instrument protective transformers X 
19. Control relays or contracts X 
20. Busbars and busbar chambers X 
21. Final verificationXX 

11.0 Maintenance of SF6 Circuit Breaker

During periodic maintenance, the gas sample from SF6 circuit-breaker is collected and tested for moisture and other impurities (IEC 376). The gas is circulated through filters containing activated alumina. The activated alumina absorbs the impurities like S2F2, SF4 moisture etc. The gas can be used again after regeneration.

For installation and maintenance of SF6 circuit breaker a gas handling unit is necessary. This consists of a vacuum pump, valves, pipings, a compressor and a service tank.

The SF6 gas in the breaker gets decomposed during quenching process. Most of the lower fluorides recombine, but some remain (S2F2, SF4 etc.) in the decomposed form. They are partly absorbed by activated alumina filters and dissicants. SF6 gas subjected to arcing becomes corrosive, irritant and has bad odour. It should not be inhaled or left in atmosphere. It is collected into service tank of gas handling unit by means of the compressor.

The breaker poles are not dismantled before reclaiming the SF6 gas in the service tank of the gas handling unit. Spare cylinders of SF6 gas in sufficient quantity should be arranged in advance before starting the maintenance work of SF6 circuit breakers. If the sub-station has only a few SF6 circuit breakers, the simple smaller gas handling unit is adequate. For large sub-stations having several SF6 filled equipment, a larger gas handling unit is necessary. A chemical laboratory for testing SF6 gas is also recommended.

Pole Unit and Interrupter: At a suitable interval one interrupter per pole to be examined to establish the rate of burning and erosion of the contacts and the general condition in order to assess the necessity for further maintenance etc. This work must be carried out under dry weather conditions and precautions taken to avoid the ingress of any moisture dirt into the pole units.

Slight burning of copper or copper alloy contacts should not cause any trouble but heavier burning should be removed with a fine file (emery or carborundum paper should not be used). Copper alloy and other arc resisting metal contacts should be inspected for any signs of excessive burning. In general, considerable burning of contacts can be tolerated before replacement becomes necessary but it is recommended that where contacts require dressing, the minimum amount of material shall be removed and the manufacturers recommended profile maintained.

It is imperative that the force between contact shall not be materially reduced. Any burning away from the arcing area should be noted and investigated. Transfer contacts shall be inspected for any signs of burning and cleaned as necessary.

The nozzle of the interrupter is usually made from PTFE. It shall be examined for excessive burning or erosion and this can be done by comparison with a new nozzle. In general the dimensions and profile of the nozzle are not as critical in SF6 circuit breaker and, therefore, a greater amount erosion can be tolerated before replacement becomes necessary (5 %). However, the manufacturers recommendations in this respect should be carefully followed.

The insulation adjacent to the arcing area should be cleaned as necessary. Burning of this insulation will indicate a misplaced arc and if found this must be investigated.

The entire interrupter is generally filled with thin dust of erosion of teflon nozzle combine with fluorides of contact material. This dust is insulating when not exposed to SF6 gas. Immediately after dismantling of the breaker, this dust absorbs moisture and becomes conducting. Hence it should be wiped-out completely by means of air-jet, cloth, trychloroethelene. This cleaning is essential in case of circuit-breakers. If assembled without cleaning, the insulation resistance goes down and internal flashover occurs during the normal closed position between the live part and earth due to surface tracking along internal insulation.

The important steps in the maintenance of SF6 circuit breaker include internal cleaning and replacement of SF6 gas. The operating linkages should not be disturbed unless diagnostic tests or a visual examination indicate that this is necessary. The setting dimensions should be verified.

Filters and Dessicants: Filters and / or dissicants are installed in SF6 circuit-breaker to filter out or absorb some of the breakdown products. As a last operation prior to closing up the chambers of the circuit-breaker the filters and dessicants should be replaced. Under no circumstances should untreated filter or dessicant material removed from the circuit-breaker after service be heated.

Maintenance for SF6 Circuit Breakers
Maintenance operationRoutine MaintenancePost fault maintenance
InspectionExamination and overhaul
1. Operational checksXX 
2. General InspectionXX 
3. Cleaning XX
4. Opening device (trip)XX 
5. Circuit-breaker enclosure x 
6. Gas systemXX 
7. Sulphur hexafluoride gasXXX
8. InsulationXXX
9. Local control kioskXX 
10. Pressure gauges X 
11. Pressure switches X 
12. Main connection X 
13. Secondary wiring and fuses X 
14. Earth connection X 
15. SF6 gas heaters X 
16. Interpole linkagesXX 
17. Main mechanism X 
18. Auxiliary switches, indicating devices and interlocks X 
19. Interrupters XX
20. Local air receives and pressure vessels X 
21. Filters and desiccants X 
22. Overload devices and protective relaysXX 
23. Instrument and protective transformers   
24. Control relay or contractor X 
25. Bus bars and bus bar chambers X 
26. Final verificationXXX

Maintenance of SF6 CB in G.I.S.: After interrupting 25 short circuits at rated breaking capacity or after 5 000 interruptions of rated current or part load, the circuit breaker should be overhauled. For this reason the breakers is so designed that it can be easily removed from the installation.

Having switched off and earthed the breaker, the gas is first pumped using the service truck. Then the casing containing the operating mechanism, situated one end, can be opened. All operating elements, tripping devices, auxiliary contacts and so on are now accessible. The extinction chambers can be removed from the tank with a special device, having first withdrawn the isolating chamber attached to the casing containing the operating mechanism.

The parts of the breaker and isolators must be cleaned very carefully, through this task is easily performed. Having reassembled the breaker, it must first be evacuated before it is refilled with SF6 gas. As soon as the breaker has passed its functional test, it can be put back in service.

12.0 Likely troubles and essential periodic checks to avoid them

The point to be checked during the periodic maintenance include the following:

  1. The operating mechanism of the circuit-breaker should be in good working condition both mechanically and electrically.
  2. Insulation resistance phase to ground of each pole should be above 2 000 MΩ, (up to 1.1 kV); 10 000 MΩ for above 36 kV.
  3. Contact pressure is important. When the contact pressure is enough even a line contact can pass normal current without overheating. The resistance of pole unit should be less than 50 μΩ.

    Trip circuit and battery supply. Maintenance of the trip circuit and battery supply is essential for the satisfactory operation of all protective relays (shunt trip type). The battery should be inspected daily for correct voltage, specific gravity etc. and it should be kept on trickle charge. The inter-cell connectors should be in good condition. There should be a pilot lamp or alarm indication to draw the attention of the operator, if the trip coil battery voltage falls below certain limits.

    1. Electrolyte level must be maintained 10 to 15 mm above the plates for the proper reaction of the constituents.
    2. Terminal voltage of the cell must not be allowed to fall below 1.8 V.
    3. Battery should be charged to its rated capacity to increase its efficiency.
    4. Do not allow battery to remain in semi-discharged condition for long, otherwisethe life of battery will be reduced.
    5. Commercial sulphuric acid should not be used otherwise the active components will be damaged very soon.
    6. Use distilled water for topping up the level of electrolyte.
    7. Avoid excessive charging, as it reduces the life of the battery.
    8. Bare and insulated lead should be painted with recommended paint to avoid accidental short circuit.
    9. Battery should be kept clean and dry and battery room should be well ventilated to avoid suffocation.
    10. If acid falls out of the battery, wipe over with wet rag and dry thoroughly to save the components from being damaged.
    11. Terminal posts and connectors should be clean and free from corrosion to increase the life of the battery.
    12. Nuts and bolts of cell connectors should be kept tightened and smeared with Vaseline to avoid rusting.
    13. It should not be left in discharged condition for long, otherwise the defects of sulphasion will occur.
  4. Every relay should be tested once in six months, with suitable testing set and the records of such tests should be logged in a maintenance resistor. During tests a check should be made if any of the overload or time setting on the relay require change due to the increase or decrease in the load conditions since the date of last test.

    Following defects are possible: 

    1. Improper contact or misalignment of the contact prongs of the trip battery circuits, between the cubicle and the drawout truck.
    2. Point in an auxiliary wiring of supply from battery, or discharged battery.
    3. Circuit-breaker operating mechanism not being sluggish due to mechanical defects, or stiffness due to dust or rust, lack of lubrication, etc.
    4. Wrong current transformer connections.
    5. Wrong relay settings for the load conditions.

Preliminary Preparation: The preliminary preparations include study of drawings, acceptance, report checking certificates and test reports of the equipment, completion of civil engineering work arranging the tools, lifting gears etc. organizing the labour, prepare the schedule of installation, preparing sequence cards for erection of major items etc. Such cards indicate the sequence of operation items involved, procedure in brief etc.

Common Troubles and Remedial Actions
TroublePossible causesPossible remedial actions

1.Low insulation Resistance (below 2 000 MΩ) between

  • Phase terminal and earthed frame, with breaker closed
  • Phase terminals of a pole.
  • Moisture
  • Dirty insulation surface internal and/or external
  • Poor oil
  • Carbon/copper particlessticking to internal surface
  • Circulate dry hot air or oil through the breaker polefor 4 to 6 hours.
  • Dismantle, clean, reassemble Insulation resistance should be above 2 000 MΩ, for 1.1 kV and above 10 000 MΩ above 36 kV

2.Resistance between terminals of pole too high (above 100 μΩ) (15 μΩ per joint) contact

  • Reduced contact pressure
  • Loose connections
  • Contact surface damaged due to repeated operations
  • Insufficient contact wipe
  • Oxide film on contact surface
  • Dismantle, repair and assemble again. If necessary, replace the contacts

3.Unequal contact wipe and travel in 3-pole measured from top surface of interrupter flange and the contact tip by a simple rod with

  • breaker open
  • breaker closed
  • Contact erosion due to repeated load operations or short circuit operations
  • Unequal length due to wrong adjustments of linkages
  • Inspect contact tips
  • Replace if badly eroded
  • Adjust contact if lengths are unequal in three pole.

4.One of the pole does not close.

  • Pull rod for contact damaged
  • One the links of that pole broken
  • Contact of that pole severely damaged.
  • Dismantle the pole and repair the defect.

5.Breaker operation too slow during opening (timing from trip command to contact separation instant too large (60 ms instead of say 40 ms)

  • Excessive friction in the pole unit.
  • Contact grip to high
  • Trip coil operation sluggish.
  • Low battery voltage, hence higher trip coil pick-up-time.
  • Identify the cause
  • Take remedial action.

6.Breaker does not operate on electrical command.

  • Open control circuit
  • Spring defective
  • Trip circuit open
  • Trip latch/coil defective
  • Spring not changed

    If breaker operates with manual operation of trip release, the mechanism is O.K.

  • Check control circuit
  • Check closing spring visually
  • Identify the cause and take remedial action
  • Check supply to spring changing motor.
  • Check pressure switches, relays, control wiring.

13.0 Maintenance of Transformers

Transformers will give trouble free service for long periods, if attention is paid to the points listed in Table 13 and 14. Proper record of all the observations and repair and maintenance operations will fore-warn of any impending troubles.

Acidity. Acids are formed in transformer oil during operation. Their formation is particularly rapid at operating temperatures above 75 °C. Acids are corrosive in nature and damage the windings, core, tank etc. They are also responsible for sludge formation. Acidity is expressed in terms of the number of milligrams of potassium hydroxide required to neutralize the total free acids in one gram of the material.

The recommended limits for acidity are as follows:

Acidity below 0.5 mg KOH/g.Permissible, if oil is satisfactory otherwise.
Acidity between 0.5 and 1.0 mg KOH/g.Keep oil under observation.
Acidity above 1.0 mg KOH/g.Discard oil.

The acidity of oil should be checked every two years for transformers upto 1 000 kVA capacity, and annually for transformers above this size. The actual period may be varied depending upon the severity of the service and observation of results. This test may not be performed on pole mounted transformers because of the cost involved. Detailed procedure for determining acidity is laid in IS 1866 : 1960.

Recommended Maintenance Schedule of Transformers (Attended Sub-stations- Indoor or Outdoors)
Inspection FrequencyItems to be InspectedRemarks
(1) Daily
  1. Ambient temperature, Winding temp, Oil temp, Load, Voltage.
  • Adjust load if temperature is high.
(2) Monthly
  1. Oil-level
  2. Relief diaphragm.
  3. Dehydrating breather. Check that air passages are free. Check colour of silica gel, which should be blue.
  • If low, top up with dry oil. Look for any signs of leakage.
  • Replace if broken.
  • Change if colour is pink.
(3) Quarterly
  1. Lubricate bearings etc. of cooler fans, circulating pumps etc. if any
  • Replace if necessary
(4) Half-yearly
  1. Check oil for dielectric strength and presence of moisture.
  2. Check bushes, insulators etc.
  3. Check cable boxes.
  • Filter or change oil as necessary.
(5) Yearly
  1. Check oil for acidity, sludge, and flush point, if possible.
  2. Check relays, alarms, contacts etc.
  3. Check earth resistance.
  4. Check lightning arrestors.
  • Filter or replace, if necessary.
  • For methods of lowering earth resistance, see Chapter 3.
  • Lightning
(6) Five-yearly
  1. Over-all inspection including lifting of core and coils.
  • Wash by hosing down with clean dry oil.


The above schedule is only indicative and tests may be performed at more frequent intervals depending upon the severity of service. The opening temperature and the quality of oil are the most important factors in determining the life and performance of a transformer. Earth resistance should be checked twice a year, once in dry and hot season and once at any other time during the year.

Maintenance Schedule of Outdoor Unattended Sub-station
PeriodItems to be checkedAction
  1. Clean bushes of all dirt and oil.
  2. Examine condition of switches and tighten connections.
  3. Look for any leaks.
  4. Check condition of silica gel. (See that the air passages in the dehydrator are free.)
  5. Check oil level and top up, if necessary
  6. Look for broken bushes.
  1. Check dielectric strength of oil. Test oil for moisture.
  2. Check earth resistance.
  3. Check lightning arrestors.
Filter or replace, as convenient.
Five yearly
  1. Open tank for cleaning and reassembly.
Wash down with clean oil.

14.0 Care and Maintenance of Earthing

  1. Check the earth resistivity during dry season half yearly and maintain and compare the records with previous readings.
  2. Water should be poured at regular intervals during dry season at the small substations.
  3. When the system is expanded, the earthing system should also be expanded by using additional earth electrode and earth wire separately.
  4. Electrode should be places in the earth in the upright vertical position.
  5. The electrodes and the earth continuity conductor used in the circuit should be of the same material i.e. copper or GI.
  6. Check the earth connection and tighten them properly.
  7. Pass the earth continuity conductor through the galvanized pipe from being damaged.
  8. Inspect the broken or rusted earth continuity conductors and replace them with proper size.
  9. The earth resistance of the following equipment must be measured during the driest part of the year:

    • Transformer body and other metal parts.
    • Lightning arrestor
    • Neutral of the transformer.
  10. Mixture of salt and water may be added to improve the earth resistance.
  11. In the corrosive earth, the steel electrode should be replaced by copper coated steel electrode.
  12. The earth continuity conductor should be properly welded with the earth electrode.
  13. Avoid any jointings from earthing conductor.

15.0 Insulation Resistance Measurement

It is the responsibility of the user to ensure that the insulation of electrical switchgear has been tested and the result recorded before commissioning the equipment. During the life of electrical equipment insulation resistance testing will give a good indication of the condition of the equipment and if these tests are recorded can help in deciding maintenance requirement for the whole equipment.

Insulation resistance measurements between poles and earth are comparatively easily made and are the most suitable for routine tests, but to facilitate their proper interpretation systematic testing and recording methods are essential. Insulation resistance tests are strictly comparative, in that for each item tested a rejection value can only be fixed on the basis of experience by comparison with earlier results. For this reason the test equipment and methods used should be the same on each occasion.

Test values obtained should be logged on a standard form designed for the purpose, together with the humidity and temperature at the time of the test, and in general a steady fall of insulation resistance over a period of time is a more reliable indication of deterioration than is a relatively low value which remains reasonably constant.

On long pieces of insulation, such as pull rods on some circuit breaker deterioration may not occur uniformly and resistance measurements taken over the entire length may not reveal localized deterioration. Particular attention should be paid to the pull rods of circuit-breakers. Where practicable the insulation to be tested should be allowed to reach ambient temperature before resistance tests are made.

During overhaul of the electrical equipment, the insulation surface should be cleaned with trichloroethlene, clean cloth compressed air.

Insulation resistance is measured by means of megohm meter (megger). The megger comprise a megohm meter with built-in d.c.generator. The minimum reading is zero and maximum is infinity. The scale is in megohms. The two terminals of megger are connected across the insulation, i.e. one to the conductor and other to earth body. The handle is rotated by hand or motor. The insulation resistance indicated by the pointer in megohms.

For HV switchgear, 1 000 V or 5 000 V (DC) megger is preferred. The insulation resistance of HV circuit-breaker is very high (above 10 000 MΩ). Insulation resistance of control circuit, trip circuit, relay circuit, secondary circuit, etc. is measured by means of 500 V megger. Value obtained should not be less than 1 MΩ.

For primary circuit, the insulation resistance is tested with the breaker closed; between lower terminal and earthed frame for each interrupter. With breaker open, insulation resistance is measured between terminals of each interrupter.

Test voltage which could be applied to primary insulation when making resistance test varies according to the voltage rating of the switchgear and is shown in table below:

Test Voltages
3-phase system rating of primary insulating of switchgearTest voltage recommended for insulation resistance test (to earth and between phases) kV (DC)
Up to 1 kV1
Above 1 kV to 3.6 kV2
Above 3.6 kV to 12 kV5
Above 12 kV5


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