Sl.No Nature of Trouble Work Attended
1. Nonworking of bell a.   Loose connections
b.   Disturbing of bell adjustment
c.   Burning of bell coil
d.   Alarm D.C. Fuse blown
2. Failure of trip healthy indication a.   Fusing of bulb
b.   Loose connections
c.   Resistance found open circuited
d.   Alignment of Aux. Contactors of C.B. getting disturbed
3. Non working of O.L.T.C Control a.   Failure of Ac
b.   Failure of Aux. Transformer 220/110KV
c.   Putting remote switch and operating locally
d.   Putting local switch operating remotely
e.   Contact problem in Lower/raise button
f.    Tripping of miniature circuit breaker of AC
4. Non auto start of compressors a.   Pressure switch faulty
b.   Loose connection
5. Non Tripping of Feeder Breaker a.   Loose Connection
b.   Open Circuit of trip coil
c.   Mechanical trouble
d.   Alignment of Aux. Contractors of CB getting disturbed
6. Noise in CT a.   Open circuit of secondary of C.B. getting distrubed
b.   Loose Connection in C.T. secondary C.T. ckt.
7. Soring Charging Mortor does nor start a.   Failure A.C. Supply
b.   Failure of limit switch/C.B. Aux. switch
c.   Defective mortar.
8. Non Closing of Breaker a.   Loose Connection
b.   Open circuit in closing coil
c.   Mechanical trouble
d.   Alignment of Aux. contractors of C.B. getting disturbed
e.   Spring was not charged
f.    Non resetting of trip/master relay
g.   Non closing from remote and in local/
remote switch on
9. Flash overs and damage of the
a.   Failure of insulation
b.   Short circuit to lizard/rats etc.
10. Tripping of transformer on differential a.   Failure of L.A.
b.   Loose Connection
c.   Providing improper core CTS
d.   Failure of LV Cable box
e.   Failure of C.T
f.    Failure insulation within deferential zone
11. “Flasher on” without fault a.   D.C. Leakage
12. Tripping of transformer on Buchholz a.   Due to surge
b.   Due to moisture /water inside the connecting Box
c.   Improper operation of Buchholz relay
d.   Shorting off terminal due to failure of Bakelite plates
e.   Low oil level
13. Low oil level indication of panel was appearing continuously a.   D.C. leakage
b.   Sticking of low oil level pointer
c.   Rain water inside the connection box
14. Flag not resetting a.   Mechanical defect
15. Non resetting of Bell a.   Accept Switch fauly
b.   D.C. Leakage
16. Non resisting indication a.   D.C. leakage
b.   Push Button faulty
17. Non tripping of OCB from remote end a.   The Local/remote switch on local position
b.   Loose connection
c.   Trip/Close switch faulty
d.   Alignment of Aux. Contract of C.B. getting
18. Capacitor bank tripping a.   Faulty capacitor unit
b.   Blowing of H.T Fuse of CXapacitor
19. Tripping of Breaker without indication a.   Due to shorting of DC (+)ve
b.   Leakage of DC
c.   Due to low pressure
20. Continuous ringing of bell a.   DC Leakage
b.   Adjustment of Aux. relay contract is disturbed
21. Abnormal noise in current
a.   Winding is open
22. Mal-operation of relay a.   Defects is relay
23. Battery Charger not working a.   Failure of Transformer
b.   Failure of Diode
c.   Failure of A.C Supply
d.   Switch faulty
24. Transformer cooling Fans Auto start not working a.   Defects in Temp. Indicator
b.   Defects in Contractor
c.   Loose connection
25. Indication lamp not indicating a.   Fusing of Lamp
b.   Defects in Aux. Switch
c.   Loose connection
26. Semaphor not working a.   Defects in Aux. Switch
b.   Loose wire
c.   Semaphor coil burnt
d.   Non availability of Aux. contractor for isolator



(1000 Watts used for 1 Hour =1.KWH = 1 Unit)

Type of Appliance Wattage No.of units for month of 30 days
1 2 3 4 5 6
Lamp 25 0.38 0.75 1.5 2.25 3 3.75 4.5
  40 0.6 1.2 2.4 3.6 4.8 6.0 7.2
  60 0.9 1.8 3.6 5.4 7.2 9.0 10.8
  100 1.5 3.0 6.0 9.0 12.0 15.0 18.0
Ceiling fan 36”/48” 50 0.75 1.5 3.0 4.5 6.0 7.5 9.0
56” 60 0.9 1.8 3.6 5.4 7.2 9.2 10.8
60” 70 1.05 2.1 4.2 6.3 8.4 10.5 12.6
Table Fan 12”/16 40 0.6 1.2 2.4 3.6 4.8 6.0 7.2
  550 8.3 16.5 33.0 49.5 66.0 82.5 99.0
Electric iron 750 11.3 22.5 45 67.5 90 112.5 135
Dhobi iron 1000 15 30 60 90 120 150 180
Geyser 2000 30 60 120 180 240 300 360
  3000 45 90 180 270 360 450 540
  6000 90 180 360 540 720 90 1080
Storage Heater 1 KW 1000 15 30 60 90 120 150 180
1.2 KW 1200 18 36 72 108 144 180 216
1.5KW 1500 22.5 45 90 135 180 225 270
Mixer (Big) 200 3 6 12 18 24 30 36
Mixer (Small) 50 0.8 1.5 3 4.5 6 7.5 9
Washing machine 200 3 6 12 18 24 30 36
A/C 1 Tonne 1400 21 42 84 126 168 210 250
1.5 Tonne 1800 27 54 108 162 216 270 324
Small 200 3 6 12 18 24 30 36
Big 300 4.5 9 18 27 36 45 54
T.V 150 2 5 9 14 18 23 27
Water Pump 1.0H.P 11 22 44 66 88 110 132
  2.5 H.P 28 56 112 168 224 280 336
  5.0 H.P 56 112 224 336 448 560 672
  7.5H.P 84 168 336 504 672 840 1008
  10.0 H.P 112 224 448 672 896 1120 1344
Cooker  1500 22.5 45 90 135 180 225 270
Toaster 1000 15 30 60 90 120 150 180
Oven 2 Plates / 2KW  3000 45 90 180 270 380 450 540
  5000 75 150 300 450 600 750 900


Sl.No. Particulars Required Satisfactory Result
Checking    of    oil    level    in    conservatorand
bushing examine for leaks
No leaks
2. Checking for unusual noises 1) No unusual noise2) No sparks
3. Nothing the loading in amperes  
4. Checking   for   leakage   of   water   into   cooler
(Forced cooling system)
5. Checking relief diaphragm for cracks  
6. Cleaning of bushings Blue Clour
7. Checking the colour of silica – gel (Replacement or re-conditioned if necessary)  
8. Ensuring that oil comes out when air release value is opened With air bubbles
9. Inspection and cleaning breather Vent  hole  should  not  be  blocked.  Small quantity of oil should be in the bottom.
10. Measuring insulation resistance of windings with 1000 V. Megger See Chart
11. Cleaning    up    of    temperature    Buchholz alarms for correct operation Alarm    should    come    when    points    of Thermostat touches Set point
12. Nothing the oil level ranks in the inspection glass of Buchholz relay CC level shall be full
13. Testing of oil from rank and conservation for Di-   electric   and   testing   strength   (Above 1000KVA) 30 KV – 60 Sec
40 KV- instant
  EHT 40 KV – 60 Sec
50KV - instant
14. Checking    Buchholz    relay    for    any    gas collection and testing the gas collected If  gas  collected  switch  off  and  intimate TRE.
15. Testing of Oil for Di- electric strength of tap charger 30 KV – instant 40 KV – 60 Sec
16. Meggar testing of mortars (pump) of forced cooling system -
17. Check  Transformer  ground  connection  for tightness -
18. Cleaning of water jackets (Forced cooling) -
19. Testing oil from tank and conservator for Di-
electric strength for transformers below 1000 KVA (or before after wet season)
30 KV – 60 Sec
20. Checking   of   gap   settings   of   bushings   of transformers -
21. Pressure    testing    of    oil    coolers    (Forced cooling system) -
22 Testing   mortars,   pumps   and   calibrating pressure gauges etc. (Forced cooling) -
23. Calibrating  of  Temperature  indications  by MRT +2.5%
24. Testing of oil in the conservator and tank for acidity (ocutralisation value) 0.3 MG KOH Pet Gram of air
25. Testing the di – electric strength of oil in oil 40 KV – 60 Sec
  filled   bushings   whenever   the   Di   electric strength     is     unsatisfactory     filtering     of transformer of oil should be done. 40 KV – 60 Sec.
50 KV – instant
26. Checking  operation  of  Bucholz  relay  by air injection Alarm shall come
27. Tap Changer maintenance
a) Over hauling b) Checking up of contracts
c)   testing   oil   for   acidity   d)   Filtering   or renewal of oil (yearly or after 1000 operation or when the test results are poor)
28. Major     over     haul     (complete)     of     the transformers    with    capacity    and    below
should  be  done  whenever  test  results  are unsatisfactory.





Required Satisfactory result


Checking of tank, bushing etc.

No. leakages


Checking oil level in conservator tank

C.C. level and above


Checking earth connections

No rust & not loose


Checking colour of Silica –gel-and reactivating or replacing

Blue colour


Checking the diaphragm for cracks

No cracks


Renewal of H.G. fuses and L.T.Fuses

See Charts




Required Satisfactory result


Measurement of Voltage at Transformers and rail ends of the feeder

5 % of rated Voltage


Measurements of loads in ampehers on each feeder



Taking IR values of the winding

Not less than 25 Mega ohms at 60o C




Required Satisfactory result


Measurement of each resistance of the Transformer earth Pits.

5 Ohms


Checking the contracts of the A.B Switches

No sparks




Required Satisfactory result


Collecting oil samples for di-electronic strength test

0.3 mg KOH/gm of oil

Complete overhaul of Transformer and Acidity test for 5 years

Rated voltage of the winding

Minimum safe insulation resistance in maga-Ohma at winding temperature of

30o C




66 KV and above 600 300 150 15
33 KV 500 250 155 65
6.6 KV and 11 KV 400 200 100 50
Below 6.6 KV 200 100 50 25


  1. Before carrying out maintenance works on equipment should be isolated from the supply and grounded with grounding chain or wire. Only then the main in charge should allow persons as work on in.
  2. Meggering of particular equipment the wire from megger connected equipment for meggering should be hanging in the air. The equipment also should be isolated from wires, jumpers connected to it. After the work is over the jumpers connected to transformer bushing or breakers not be rigid. They should be flexible.
  3. Every day reading on pressure gauges connected to compressors air flow circuit, feeding breakers etc., should be noted down and oil levels, temperatures in Breakers, Transformers etc. should be noted.

Maintenance of grounds in the station is essential. Water the grounding pipes daily, tighten the connecting bolts. Check the earth resistance at least once in a year and take corrective action if necessary.

Batteries are like the heart and D.C system as like the nervous system. Maintain them well check the level of electrolyte every day. Top up wish distilled water if necessary. The batteries shall be trickle charged 5 to 7% more than normal D.C Voltage to make up for the normal discharges. They may be charges heavily once in there months or more frequently to make up for battery voltage and specific gravity. The amount and duration of charge (limiting to the battery capacity) should depend upon the existing condition of the battery (Voltage and specific gravity) for the lead acid battery specific gravity should always be between 1.18 and 1.20, Voltage of individual cells should not be allowed to fill below 1.8V.


  1. Check for the opening and closing of breakers both manually and electrically.
  2. Verify opening of the breakers for faults through relays.
  3. Check the joints for loose contact, pitting etc.
  4. Power break down ate mostly due to bad maintenance. Check the oil conditions insulation resistance, cooling arrangement, Bucholrz relay etc., of equipment. Remember undue temperature rise is a warning.


While collecting oil sample from equipment through sampling cock, allow small quantity of oil to be drained out initially and then collect the oil in the bottle for testing purposes. Before conducting oil test, cool the oil sample and remove any air bubbles present. Oil samples taken for testing in standard testing equipment. All air bubbles should be removed by a clean stirrer.


1 Breakdown voltage KV (Dielectric Strength) As per I.SS (For New oil)50KV (RMS)
2 Specific resistance at 27o C ohm-cm 500 x 10x2   
3 Water content    50 PPM     
4 Dielectric dissipation factor 9tan Delta at 90oC)   0.005
5 Neutralization value mg of KOH/gm of oil   0.03
6 Inter facial tension N/M at 27o C      0.04
7 Flash Point       140o C   
8 Sludge     0.1 percent by Wt.

Insulation Test: Insulation resistance values should be obtained with a standard megger. The leads should be suspended in air. The temperature at which the meggering is done should also be noted. Frequency oil and Megger rests twice a year once before and once after monsoon.

Other Tests on Transformers

Insulation Test, Oil Test  To Know the condition of insulation
Core excitation current test      For deletion of shorted runs and heavy core damage
D.C. Winding resistance test  Indicates presence of moisture and /or contamination  of insulation
Transformer runs ration test   To detect existence of a fault


Sl.No Part Defects           Causes
1 Tank   

a. Leakage of Oil  

bDeformation                                                                      c..Overheating                      

 Correction/mechanical damage- Gaskets Worn out-Excessive internal pressure.Improve circulation of cooling-oil-a   inadequate ventilation.
2 Radiations

a. Leakage of oil

b. Deformation

c. Overheating

Corrosion/mechanical damage-Gaskets Worn out-Excessive internal pressureImproper circulation of cooling oil and/or  inadequate ventilation

3 Conservator

a. Leakage of Oil   

b. Deformation


Corrosion/mechanical damage-GasketsWorn out-Excessive internal pressureImproper circulation of cooling oil and/or inadequate ventilation
4.     Breather    


Inlet choked –silica-gel saturated
5    Explosion

Glass broken              

6.     Core     


 b. Increased loses      

 c. Excess Noise

Bolts loosening up-change in characteristicsDue to heating –vibration of stamping
7 Winding    

 a. Shot-Circuited

b. loosenin

 c. Insulation

Overloading – Airbubbles-Loss of Insulation Shrinkage displacement Overheatingbrittle decomposition burn out.
8 Oil  

a. Discoloration         

 b. High Acidity         

  c. Low                

 d. Sludge

Contamination – Increased moistureDecomposition – Chemical action with other partsB.D.V.      

Terminal      Bushing   


a. Breakage                

 b. Leakage of oil                                      

Strain –Gasket Worn out-looses fit.         
10 Tap Switch

a. Inoperative            

 b. Burnt contact

   c. Short circuit               

Mal-operations-Insulation failure of operation mechanism – overheating                        

Types of faults against which Buchholz relay gives successful protections

Visible or Audible alarm                                     

Visible or Audible alarm    Trip circuit operates
(Upper Float accurate)       (Lower Float accurate)
1. Core bolt insulation failure   1. Short Circuit between phases.
2. Short circuited core laminations          2. Winding earth fault
3. bad electrical contacts 3. Winding short circuits
4. Local over Heating           4. Puncture of bushings
5. Loss of oil due to leakage              
6. Ingress of air into the oil system  


Parallel Operation of Transformers

The Conditions that must be observed for the parallel operation of transformers both primary and secondary de 1) Same voltage ration 2) Same polarity 3) Same phase sequence 4) Zero relative phase displacement.


Transformer Oil consists of four major generic class of organic compounds, namely paraffin’s, naphthenic, aromatic and olefins. All these are hydrocarbons and hence insulating oil is called a pure hydrocarbon mineral oil. For good fresh insulating oil, it is desirable to have more of saturated paraffin’s, less of automatic and/or naphthenes naphthenic hydrocarbons. Such an optimum balance is struck by a carefully controlled refining process. Depending up on the predominance, oil is usually termed a of paraffinic or naphthenic base.

Table A- Schedule of Characteristics of Insulating Oil:



Requirements as per IS : 335-11993



The oil shall be clear and transparent & free from suspended matter or sediment


Density at 27o C



Kinematic Viscosity at 27o C Min

27e St.


Interfacial tension at 27o C Min

0.04G N/m


Flash point, Pensky – Marten (closed) min

140o C


Pour pint , Max

- 6o C


Neutralization value a) Total acidity, max

                                     b) Inorganic acidity/alkalinity

0.03 mg KOH/g


Corrosive sulphur



Electric strength (breakdown voltage) min

30 KV

  a) As received  
  b) After filtration  


Dielectric dissipation factor (tan – delta) At 90o C max



Specific resist (Resistivity) min


  a) At 90o C 35 x 1012 ? cm
  b) At 27o C 1500 x 1012 ? cm


Oxidation stabiliTY


  a) Neutralization value after oxidation max 0.4 mg KOH/g
  b) Total sludge after oxidation max 0.1% by Wt.


Presence of oxidation inhibitor

The Oil shall not contain antioxidant additives


Water constants of oil

50 ppm by Wt

Table B – Physical constants of oil 

Sl.No. Property Value
1. Permittivity 2.2
2. Specific heat 2.06 kj.mgC
3. Thermal conductivity 0.012 W/mo C
4. Coefficient of expansion 0.0078/o C


Lightening Arrestors are classified as  a) Station type b) Line type  c) Distribution type

a) Station type: Voltage rating 3 KV to 312 KV

    Discharging current, 100KA used for protection on Substation & Power Transformer

b) Line Type: Voltage rating 20KV to 73 KV

Discharging current 65kA to 100 kA used for protection of Distribution Transformers   and small power transformers

c) Distribution type: Voltage rating 8 KV to 15 KV

 Discharging currents up to 65 kA used for pole mounted substation of Distribution Transformers up to 15 KV and including 15 KV classification.

The main consideration in selecting the rating of lighting arrestor is the line to ground dynamic voltage to which the arrestor be subjected for any condition of system operators.



(For Isolators as per : IS 9921 (Part II)- 1982)

Rated VoltageUn\KV (rms) Lighting impulseWithstand Voltage
UW KV (peak)(To earth & between poles)
One minute Power frequencyWithstand Volatge
KV (rms) (To earth & between poles)
3.6 40 10
7.2 60 20
12.0 75 28
24.0 125 50
36.0 170 70
52.0 250 95
72.5 325 140
123.0 450 185
145.0 550 230
245.0 950 395
  1050 460


KV Phase to Phase (mm) Phase to Earth(mm)
220 3350 1675
110 1675 1015
66 1220 760
33 915 610
22 610 460
11 460 305