Maintenance of Sub-stations and Distribution System

Er. K.P.Krishanrajendra,

SEE(retired), KPTCL

Major Generating stations are at remote places and loads are far away from these plants.

Power is generated at 11kV/22kV level. Power generated is to be transmitted over long distances. So to transmit the power with less technical loss and economical way,

voltage is stepped up to EHV using generator transformers. At the receiving end voltage is again stepped down and power is delivered.

Power system Network

Sub-Station - Single line diagram Equipments in Stations-Outdoor equipments:

Lightning Arrestors Power Transformers Isolators Capacitor Banks Current Transformers 11kV MCVCB/PCVCB Potential Transformers Fire protection systems Circuit Breakers

Indoor equipments:

Control and Relay panels DG set RTCC 11kV control desk Battery charger RTU Battery set DCDB and ACDB

What is a Station? Different equipments are interconnecting in a station to deliver the power. Power can be delivered either at the same voltage level or at stepped

up/stepped down voltage level. Provides protection to each Bay/equipment. It is a link between the Generation plants and the ultimate consumers. Voltage levels are 400-220-110-66-33-11kV. Types of stations – Grid - R/s - S/s

Distribution system:-Single line diagram Distribution system :

11kV Over head lines HG fuses – LT box 11kV ABC and UG cables Line materials GOC/DOLO Capacitor Bank RMU/Auto reclosure/sectionalizer LT lines Distribution Transformer HT and LT consumers

Maintenance: Maintenance may be defined as the upkeep of the station equipments in proper and efficient condition to achieve:

Reliable and efficient operation. Optimum utilization. Detection of premature failure of equipments by condition monitoring. Reduction of down time resulting in increased revenue.

Type of Works in Maintenance:

Condition monitoring Preventive maintenance

Condition monitoring:

For condition monitoring, all the pre-commissioning test reports are be maintained and these values will be taken as reference values for future analysis of tests conducted. Different Equipments and their purpose: LAs: Provides protection against lightning. Isolator: It is group (gang) operated double break switch which can be operated only under off condition and helps to create safety zone for any maintenance work. CTs and PTs: Current is the basic input for both measurement and protection. CTs will reduce the higher magnitude of current to lower standard value and also provides isolation from higher voltages. Similarly PTs will reduce the higher voltage to lower standard values. Circuit Breaker: It is a switch which can be operated under load conditions also. It is also capable to open/close under fault conditions when the trip/close command is extended during faults. Transformer: it is the static equipment and step up/step down the voltage and deliver the

same power at the same frequency. Also the required voltage can be maintained by

providing tap changer (OLTC) on HV side. Transformer protection devices-BZ-PRV-OSR-WTI-OTI and fire protection scheme.

Capacitor Bank: Drawl of reactive power increases the load Current drawn and drop of voltage. Use of capacitor bank will help reduction of technical losses and improvement of voltage profile. C&R panel: Required for control and monitoring of the bay. It has Metering - control-Indication-Annunciation and protection scheme. RTCC: Is used to operate the OLTC of the transformer remotely from control room to maintain the voltage. Battery set and Battery charger: For operation of Breakers, relays, isolators, etc. reliable auxiliary power supply is required. AC supply cannot be considered as reliable power. Hence DC supply is used as auxiliary supply, for which Battery set and Battery charger are used.

Distribution system: GOS: It is a single break group operated switch which can be operated only under off condition and helps to create safety zone for any maintenance work. DOLO: used for spur lines instead of GOS. HG fuse units: This is the over current protection provided on the HT side of a distribution transformer. Distribution Transformer: Step down the voltage to 433-250V to cater the local LT consumers. Three earth electrodes are provided for each transformer center, one each for LAs, DT neutral and for all non- current carrying parts. Three earth pits with earth electrodes should be located such that they form an equilateral triangle of a side of 6500mm. Earth pits arrangement should be such that DP structure of the transformer is on one of the sides of the earth pits triangle formation. For earthing, two earth connections with 4sqmm conductor, one on either side of the transformer is to be run and connected to the same earth pit. Switch operating handle to be provided with coiled type earth connection(flexible). There shall be no joint in the earth leads/conductors en-route to earth electrodes. If joints are unavoidable, they shall be brazed, riveted or welded and painted with bituminous paint.

UG cables:

UG cable is a conductor insulated through its full length and designed to lay underground either buried directly or in conduit.

Designed to protect against moisture, rodents, etc. Cables are used in densely populated urban areas where there is no space to

run overhead lines and reliability is of important. Gives long life with less maintenance.

Types of UG cables: PILC –Paper insulated lead sheathed cables. PVC insulated and PVS sheathed cables XLPE insulated – Crossed linked polyethylene – PVC sheathed cables. Cable has conductor – insulation (dielectric) – sheath. Sheath does not allow the moisture to enter and protects the cable from all

external influences. Maintaining of cable route map. Cable route marking slabs.

DAS RMUs: DAS Ring main units are used in urban area to maintain continuity of power

supply during any contingency with minimum interruption period. By implementing distribution automation scheme (DAS), the interruption

period can be reduced. With DAS we can monitor, have supervisory control and operation of the 11

kV distribution network. Fault passage indicators are provided in OD to know the faulty section of

the network , which will help in restoring power supply with lesser time. Auto reclosure: Are the circuit breakers with built-in automatic reclosing and

opening capability under fault conditions. AR feature is useful in avoiding the

permanent shutdown in case of temporary faults in one section of the line.

Sectionalizer: are normal load break switches with built-in capability to open

automatically. The sectionalizers are designed to open based on the “fault count”

setting. The sectionalizer trips in no-load condition after auto-reclosure has opened

and before it recloses again.

Maintenance of Station equipments: Before taking up any work, creating safety zone is very much important. Creating safety zone means, connecting the equipments/system to earth at

both the ends using earth isolators/Earth rods. Without creating safety zone, no staff shall be allowed to work. It is the responsibility of the team leader to ensure that all safety

precautionary measures have been taken before directing his team to start the work.

When Electric Shock is experienced?: Electric shock is experienced when the two points of the human body

becomes the path for flow of current. When current flows? When there is a potential differencebetween the two points and a load is

connected across these two terminals, current flows. What restrict the magnitude of the current?

Resistance of the circuit restricts the magnitude of the current that intends to flow in the circuit.

Human body resistance varies from person to person. Wet skin human body resistance is around 1000 ohms and it may be as

high as about 100000 ohms when the skin is dry.

When electric shocks occurs, severity of the shock depends on : Amount of current flowing through the body. Period of current flow – duration of time. Path of the current flow through the body. Magnitude of the voltage which causes current to flow. Weather condition.

Causes due to electric shock:

Muscle contraction. Skin burns Heart stoppage

Since magnitude of current depends on the body resistance, high voltage shocks burns the skin faster resulting in reduction of body resistance to as low as 500 ohms and increases current.

Human body contains about 70% water. Water is an excellent conductor of electricity. When you touch any live part or a faulty appliance, electricity flows through

your body as a shortest path. When current flows through heart, muscle will cause ventricular fibrillation.

Under this condition, heart will be in dis-organized condition and it will not pump blood, may result in fatal.

Threshold current is 1mA. If the current is 1-6mA, they are called let go current and they do not impair

the ability of a person holding an object to release it. Current up to 25mA are painful and impair the ability to release the object. Current less than 60mA are not permanent and disappear when current is

interrupted. Current above 60mA may lead to ventricular fibrillation, injury or death.

Advantages of creating safety zone: when a person working in safety zone experiences accidentally an electric shock, the current flowing through his body will be very less, as there is an alternate very low impedance path created before starting the work. The injury, if at all, may be limited to mechanical injuries only. So earthing plays an important role in safety and reliability of power supply. What is Earthing?

Earthing Means an electrical connection to the general mass of earth. Earthing is done to ensure safety to personnel and equipments and for

protection scheme. Earthing is done to provide safe passage to fault current so as to enable the

protective devices to operate under abnormal condition to isolate the faulty section and provide safety to personnel and equipments.

Purpose of Earthing: Safety of personnel Safety of all equipments in stations. Protection of Installations. Improve reliability of power supply. Protection of communication exchanges in the vicinity.

Objectives of earthing:- To ensure that no part of equipments, other than live parts, should assume a

potential that is dangerously different from that of surroundings To suppress dangerous potential gradients on the earth surface which may

cause incorrect operation of protective devices and also may cause shock or injury to personnel.

It preventing excessive voltage peaks during disturbances and also provides protection against lightning surges.

While considering design of Earthing system of the station, care to be taken to keep the Step potential and Touch potential below the specified values.

Step potential: Means the voltage between the two feet of a person standing on the floor of the sub-station with 0.5metres spacing between the two feet during the flow of fault current through the earthing system. Touch potential: Means the voltage between the raised hand fingers touching a faulted structure and the feet of a person standing on the floor of the sub station. Equipments connection to earth mat: Direct connection to Earth mat:

Current transformer earth points. Isolator earth points. Transformer Body earth points. Breaker structures and mechanism box Control and relay panels

Connection to Earth mat through Earth Electrodes: Transformer Neutral. Voltage Transformers All Lightning Arrestors.

Maintenance of Sub-Stations:

Engineer incharge of the station and all the shift engineers should know about all the equipments provided in their station and the protection scheme adopted.

The maintenance staff shall also be educated about the station and the main key factors to be monitored regularly.

Maintenance means inspection, monitoring and carryingout works on the equipments.

Such works may be daily, weekly, monthly, quarterly, half yearly and yearly. Following charts are to be displayed in the station

SLD of the station Monthly peak load chart Monthly interruptions chart Quarterly maintenance schedule Safety procedures to be followed. First aid to be give. Monthly Shift duty staff details

Following Registers to be maintained: Equipment register Interruptions register Line clear book Maintenance works Monthly Energy consumption register Battery set maintenance register Observations book – these are to be checked by the incharge engineer and if

the trouble could not attended by them same to be attended through RT staff Daily:

General cleaning of panels and other equipments in the control room. Inspection of Battery charger and Battery set. Transformer oil level and humming sound No. of OLTC operations

Out door yard for any arcing (night time) Running of DG for at least 10 minutes Review of log books/registers/reports Checking of cooling fans

Weekly: Electrolyte level- top up if required. Draining condensed water in air compressors. Checking auto start/stop of compressors

Monthly: Cleaning of terminals of all cells and applying petroleum jelly. Similarly all other period maintenance works are to be done.

Transformers:

Quarterly: Bushings cleaning Air release from main tank, B/Z relay, bushing turrets. Cap of capacitor bushings Fans and pumps local.remote/auto operations Oil leaks OLTC- manual/local/remote operations Earth connections All transformer mounted protection devices Vermin proof Fire protection scheme has to operate only when there is severe problem in

the transformer to quench the fire and to avoid spreading of fire. It has to operate only when the adopted logic conditions are fulfilled.

½ yearly:

IR test- while conducting IR test of a transformer, recording of oil temperature is very important. Minimum IR values for one minute can be determined using the formula R=CE/√kVA Where R is IR in mega-ohms, C=1.5 for oil filled transformers, E is the voltage rating of the transformer in volts.

Polarization index (PI) is measurement of variation in the IR value and is the ratio of 10 minutes to one minute IR values. Low value of PI indicates that the oil might be contaminated with dirt or absorption of moisture.

DC voltage to be applied to measure IR values depends on the voltage rating of the equipment under test.

Rated voltage of the equipment

<600 1000-2500

2500-5000

<5000

DC test voltage 500 500-1000

1000-2500

2500-5000

BDV of oil for new & in service oil

< 72.5kV <170kV >170kV

40kV 30kV 50kV 40kV 60kV 50kV

Whenever gas traps frequently in the relay, gas is to be collected and tested

by passing the collected gas through silver nitrate solution. With change in colour of the solution cause for generation of gases can be

ascertained as below White - Destroyed paper Yellow - Damaged wood Black or Grey -Dissociated Oil

Physically seeing the colour of the Oil, condition of the Oil can be ascertained as tabled below.

Oil Colour Oil quality

Yellowish/Transparent/ Sparkling

Very good

Yellow/dull Good

Brownish Bad

Brownish/Black Contaminated

Black Discardable

Oil level in OTI & WTI sensor pockets All quarterly maintenance works

Yearly:

DGA and PPM of oil Tan-delta and capacitance test of windings and Bushings. All quarterly maintenance works If recommended, the transformer is to be taken out of service after 5-8 years

and check the windings, core, core clamps for proper position and tightness. Clean the tank and replace the gaskets Overhauling of OLTC diverter switch Above works are to be carriedout with the technical assistance of the

manufacturer. Circuit Breaker Quarterly:

Leakages in Air and Gas pipes. Lubrication of operating and linkage mechanisms. Tightening of clamps. Checking of breaker for local/remote/ SCADA operations Antipumping and PD operations Checking lockout scheme. Vermin proof

Anti-pumping scheme: Capacitance trip device:

Pole discrepancy trip:

½ yearly:

IR test between poles and between poles and earth. Measurement of contact resistance All quarterly maintenance works

General:

After recommended years of Breaker in service, carryout the overhauling of the Breaker through the manufacture.

This enhances the life of the equipment. CTs and PTs:

Quarterly – IR values, oil level, tightness of connections Yearly – Tan-delta and capacitance, IR values, tightness of secondary

terminals at the CT/PT secondary termination box. Isolators:

Linkages for simultaneous operations Copper flexibles Mechanical interlock for isolators with earth switch Operations/indications/lubrication

Capacitor Bank: Fuses – Oil leakage – bulging – capacitance measurement – leakage current

–connections. Why capacitor bank is required? Advantages of reactive power compensation – losses and voltage How to be the series and parallel combination of the capacitor units? What to do when any one or two units fails

Capacitor Bank:

72.5kV 30MVAR two bank capacitor Bank Each unit 333.33kVAR, 8.37kV Total No. of units =30000/333.33 = 90 No. of units per bank = 90/2 = 45 No. of units per phase = 45/3 = 15 Rated per phase voltage = 72.5/1.73 = 41.8 No. of units in series = 41.87/8.37 = 5 No. of units in parallel = 15/5 = 3 For each phase, 3 will be in parallel and such 5 parallel connections are to be

connected in series. What to do when one or two units fails?

When some units fails, care to be taken to select number of units to be serviced, such that the particular bank is balanced and the units are not subjected to higher voltage.

The capacitor bank capacity will change as follows.

No. of units failed 1-15 16-30 31-45 46-60

Capacity available 25 20 15 10

LAs:

Check for any cracks. Cleaning Record leakage current Measurement of IR values If the leakage current is almost twice the old value, clean the LA. If there is

no change in the current value, LA may be suspected and may be required to be replaced.

11kV Switchgears:

Quarterly – DC and control circuit wiring, operations, anti-pumping, indication, annunciation, tightness of connections, vermin proof arrangements.

½ yearly – IR values of CTs, PTs, Breakers and Bus Yearly – calibration of relays and energy meters.

Battery charger and Battery set:

Check the charger for Float/trickle/ boost charging both in manual mode and auto mode.

Check the voltage and specific gravity of pilot cells daily. Measure the voltage and specific gravity of all the cells monthly.

Checking healthiness of the Battery set • Select some of the cells as pilot cells. • Record SG and voltage of pilot cells and the DC load voltage. • Switch off AC supply to Battery charger. • Record the pilot cells readings and DC load voltage at 30 minutes interval. • If there is no change in the readings for 8 to 10 hours, the Battery set is in

good condition. • This test is to be carried out once in 3 or 6 months.

Fire protection:

Fire is due to combustion of materials and are categorized as Class A-B-C and D fires.

If the fire is due to combustion of solid state non-metal like paper, wood, cloth, Plastics, etc, such fire is catageriozed as class-A fire.

If the fire is due to combustion of liquid state materials, such fire is catageriozed as class-B fire.

If the fire is due to combustion of gases, such fire is catageriozed as class-C fire.

If the fire is due to combustion of metal materials, such fire is catageriozed as class-D fire.

For fires in electrical equipments, carbon dioxide extinguishers are to be used.

For other fires, Dry chemical extinguishers are to be used. Training on operation to be given to staff The fire extinguishers are to be kept in good condition and at suitable

locations. They should be refilled at recommended intervals.

Distribution Transformers: Supports, connections HT and LT fuses Load on each phase and balancing if required.

Check for oil leakage Check for any bushing damage Earth connections Breather condition

Monthly inspection: Check supports and connections Check fuses for proper tightness and rating Check the load on the transformer. If over loaded action to be taken to

transfer the load or to provide additional transformer. Check for any leakage of oil and arrest it. Check the bushings for their healthiness and clean them. Check the condition of silicajel and recondition it. Check the tightness of both neutral and body earth connections. Check the availability of barbed wire and danger plates Check the explosion vent for any crack or damage.

Preventive maintenance: Monthly schedule – as stated above for inspection Quarterly Schedule: Tighten all the connections and replace any damaged or loosen bolts and nuts if required. Replace old fuses by new fuses of right capacity. Check the oil level and top up if necessary. Arrest oil leakage if any.

Clean off oil dirt, paint or other deposit on bushings. Examine for any cracks. Adjust the arcing horns for alighnment and proper gap between the rods. If tap changer is provided, measure phase to phase and phase to neutral

voltaes during peak hours and if required change the tap position. Half yearly schedule:

Select the schedule such that one should be before and other after monsoon season.

Oil sample is to be got tested to know its condition. Load on each phase is to be checked and if required it has to be balanced on

all the phases. Check the earth connections for tightness and examine and replace broken

rods/conductors with proper size Yearly schedule:

Measure the earth resistance of transformer neutral and body earthings during the driest part of the year.

Check the lightning arrestors for their healthiness. When transformer serves for ten years or recommended period, it should be dismantled and overhauling to be done at repair shop which includes

Inspection of core assembly Inspection of windings Inspection of all clamping bolts and other bolted joints Cleaning of main and conservator tanks.

Replacing the gaskets

Reconditioning of oil or replacing the oil.

Painting of transformer body. Lines:

Smooth operation of GOS Replacing damaged jumps

Attending loose spans Replacing deteriorated jumps Earth wires Guysets

Underground cables: Periodical inspection of cable and cable joints which are accessible for

inspection. Patrolling of cable route Loading of cables.

Faults: Conductor faults – open circuit – insulation failure (short circuit) Sheath fault – due to stress at any point leads to insulation failure, cable

failure –failure of insulation in outer sheath – water/moisture ingress RMU:

Periodical calibration of relay and Meters Checking of FPI and supply healthy indicators. Local and remote operation of OD/VL. Communication system Battery charger and both the battery sets used for control and

communication purpose. Vermin proof

Condition monitoring:

Frequency response signature analysis of windings. Tan-delta and capacitance of windings and Bushings. IR values Temperature rise with reference to load over a time. Tan-delta and capacitance of CTs. Contact resistance and operating timings of breaker.

The values obtained during tests are to be compared with the reference values (pre-commissiong test / acceptance test values) to know the healthiness of the transformer and the tolerances for the equipment service are to be allowed during such evaluation.

Safety Manual:

Definitions:

Authorised person: One who is properly authorised to perform specific duties under certain conditions or who is carrying out order from competent authority

Unauthorised person: One who is not permitted to work on electrical apparatus except under the personal supervision of an authorised person.

Dead: 'Dead' means at or about earth potential and disconnected from any live system. Provided that the apparatus separated from a live conductor by a spark gap shall not be deemed to be 'Dead'

Note: The term 'Dead' is used only with reference to current carrying parts, when these parts are not alive. Earth: 'Earth' means a conducting mass of earth or of any conductor in direct electrical connection therewith.

Earthed: 'Earthed' means connected to earth in such a manner as it will ensure immediate discharge of electrical energy without danger at all times.

Emergency: 'Emergency' for the purpose of this code means an unusual condition which exists that endangers life and/or property.

Foreman: 'Foreman or supervisor' shall mean Assistant Engineer/Junior Engineer/the authorised employee directly in charge of workmen doing the work regardless of title.

Live: 'Live' means electrically charged.

Permit issuing Officer: 'Permit issuing Officer' is a person who is authorised for ensuring that all controlling switches and circuits have been isolated, made dead and inoperative and that adjacent circuits have been made safe for the work to be carried out and who is authorised to issue the 'Permit to work'.

Permit to work: 'Permit to work' means a form of declaration signed by and given by one authorised person to another authorised person in-charge of work to be carried out on or adjacent to any electrical apparatus, mains or service lines, for the purpose of making known to such latter person exactly what apparatus, mains or service lines are made dead and earthed and safe for working.

Protective Devices: 'Protective Devices' means devices such as rubber gloves, rubber gauntlets, line hose, rubber boots or other insulating devices, which are especially designed for the protection of workmen.

Low Voltage: Where the normal voltage is not greater than 250 Volts and in no circumstances exceeds 263 Volts.

Medium Voltage: Where the normal voltage exceeds 250 Volts but is not greater than 650 Volts and in no circumstances exceeds 683 Volts.

High Voltage: Where the normal voltage exceeds 650 Volts but is not greater than 33,000 Volts and in no circumstances exceeds 37,125 Volts.

Extra High Voltage: Where the voltage exceeds 33,000 Volts under normal conditions subject to 12.5% variation.

Fundamentals of Safety: Prevention of accidents

A careless man is a liability to the Organisation, dangerous to himself, his fellow workers, the public and the Organisation.

Accidents do not just happen. Accidents are the result of unsafe acts or unsafe conditions or a combination of both.

General Instructions for Safety: Responsibility of Individuals: 1. Safety to himself. 2. Safety to his fellow employees. 3. Protection to the Public. 4. Protection to the equipments.

A.B.C ( Always be careful) Failure to comply with safety precautions is an offence and is punishable. An accident may be defined as a sudden mishap that interrupts the

operation of an activity.

Direct Artificial respiration is the method whereby a person ventilates the lungs of an unconscious non-breathing victim by blowing his own breath directly into the mouth or nose of the victim.

Direct mouth-to-mouth breathing is by far the most effective method of artificial respiration, as proven by comparative studies conducted by research groups in the United States.

Procedure to be followed before starting the work:

Apply for permit to carryout the work and take line clear. After taking Line clear, obtain confirmation for the same from second

person. Line clear issuing person has to put the LC board on the panel. Create the safety zone. If the work is in a sub-station, close the earth switch provided on one side of

the circuit and earth the other side of the circuit using earthing rods. If the work is in lines, create the safety zone between the two points of the

work, by shorting all the phase and neutral wires and connecting to the earth using earthing rods at both the ends.

While earthing, first connect the earth terminal and then put the earth rods to the conductors.

By doing so, 100% safety is assured to the personnel working. Even accidentally, if the line is charged, the staff working will get only non-

severe electric shock and any injuries may be due to mechanical accident only.

Under such conditions, there are two paths for the fault current to flow to earth and since the resistance of the short circuit path is very less when compared to human body resistance, only fraction of the fault current will flow through the body and the personnel will get a shock.

Procedure to be followed after completing the work: Confirm that all the disconnections if made to carryout the work were

reconnected. Confirm from the team leaders that all the work is completed and all the staff

engaged in the work have returned from the work. Remove all the safety zone connections. While removing the earth rods, first remove the connections made with the

live parts and finally the connection of the earth. Return the line clear with permit number and confirm the same. After supply is restored, check the healthy ness of the system, by checking

the voltages, running of equipments, etc. First Aid procedure:

Do not touch the victim. Isolate the source. If required isolate the victim by any means of insulated sticks, dry wood

sticks, etc. Call for Medical held desk Lay down the victim in a comfortable position. Move the victim to nearby place, if there is no suspicion on major injuries. If the victim is not breathing, mouth to mouth resuscitation (awake & come

back to life again) is to be done. If no pulse, begin CPR(cardio-pulmonary resuscitation).

Ensure casualty is on a firm flat surface. Place your hands one on top of the other in the center of the casualty chest. Compress the chest (4-5cm depth) for 30 times at a rate of 100 compressions

per minute. Compression and releases should be at the same rate. After 30 compressions, open the airway again using head tilt/chin lift. Seal the nostrils with your thumb and fore finger. Blow steadily into mouth until you see the chest rise. Cover the victim with a blanket to maintain body heat. Arrange for immediate medical attention

Duties of Shift Engineers:

When charge is handed over, they should inform about the line clears pending, station troubles.

After taking charge, the Engineer should check the DC voltage, load on each transformer.

Go through the observation book for any work to be attended on priority. Check the system voltage and tap position.

DO’S:

Use all the safety equipments while working Before starting the work, make sure that the supply is switched off. Place safety/warning boards on main switch on which Line clear is taken

before commencing the work. Make sure controlling switches are switched off and locked. Don’t face the fire/arc directly when fire/arc occurs. Guard against arcs and high voltages. Remember burns from arc are more

severe. Discharge the cables before starting the work. Confidence in work will avoid any mis-happenings. Follow the given safety instructions strictly. Ensure that all the tools used for the work are collected after completing the

work and the connections removed were put back correctly. Even though we have the knowledge and confidence in the work, make it a

practice to note down the connections before disconnecting them and again reconnect them accordingly before switching on the circuit.

DO NOT’s:

Do not provide switch/fuse in the neutral circuit. Always provide it in the phase circuit.

Do not operate any circuit unless you have the knowledge about the circuit. Do not work on the live circuit, unless you are permitted to do so and take all

safety precautions if so. Do not make any changes in the earthing circuit without permission Do not make any tampering in the meter board knowingly or unknowingly. Do not expose your eyes when any arc is observed and turn your face and

protect the eyes. Do not close the switch slowly. Close it fastly. Do not touch the live part with wet bare hands.

IR values in megaohms

Sl. No.

Voltage Oil Temperature in deg C

20 30 40 50 60

1 66kV & above 1200 600 300 150 75

2 33kV & above 1000 500 250 125 65

3 6.6kV to 19kV 800 400 200 100 50

4 Below 1.1kV 400 200 100 50 25

*-*-*-*-*-*-*