HT SWITCHGEAR ERECTION ,TESTING AND COMMISSIONING (PART-II) -ABC OF THERMAL POWER PLANT

In my last dispatch I have indicated the number of panels required to be installed for HT connection .A picture of the same was also attached. Numbers of panels are erected aligned on line. Incoming and outgoing cables are laid through spreader room . These panels are erected by the side of the service bay through an opening kept for this purpose only. Once these panels are and aligned then pre-commissioning activity starts.

Opening has been kept at service bay side to lift panels as shown in the above picture.

Cable spreader room  erection of cable trays under progress. This is below the Panel room

This is the panel room ..civil works under progress.

Following testing are being carried on HT panel after erection.

 a. Insulation resistance measurement of bus bar (power ,control ),vcb

b. Busbar,contact resistance measurement

c. High voltage test – bus.

d. Primary injection test  - vts & cts.

e. Protection, interlocks and metering circuits .

Following instruments (calibrated)  , systems , qualified testing personnel  ( Accident while testing is very common, this may be fatal at HT lines) etc are required to be lined up.

5000v insulation tester (motorized) ; 3 phase 415  ac 50 hz system with variac (3 ph and  1 ph)

Dc control supply (preferably from permanent DC source;  Hi pot ac/dc source

 Measurement kit ; Current injection kit ; Micro ohm meter; Milliseconds timer ; Multimeter ( 4 and half digit ) digital ; Ananlog Multimeter ; Volt and Ampere meter for measurement; Tong tester;  Number of long testing leads with end connectors ; Supplier’s tools for the panel etc.

Safety precaution is paramount before and after testing a few points are given below:-

The Engineer-In-Charge of the testing  team will inform each member of his team as to the exact nature and extent of his contribution and responsibilities towards the tests carried out.

 Only nominated person will take part in the tests. 

All persons must acquaint themselves with the use and location of fire  extinguishers and first aid posts.

Adequate lighting facilities will be provided at all times.

Each test will be carried out under the control of the test team leader.

A strict and safe method of applying test leads shall be practiced.  Bare wires shall not be inserted into sockets.

A record of any alternation made during the test such as shorts or disconnections, shall be made and annexed to relevant test procedures .

Adequate number of poster ,signboard are placed strategic location for safety.

All procedures are recorded and checked with the drawings . A single line diagram or conditions to energise the particular panel and also for tripping may be made before testing .This will be immense help. This is not a must condition but helps during testing as ready reference.

CHECKLIST FOR THE FOLLOWING PANELS ARE CHECKED

i. Incomer panels (1 & 2) ;ii. Outgoing feeders; iii. Bus coupler; iv. Tie breaker

- All incoming and outgoing cables to incomer panels are completed.

-Interlocks for closing and protection for tripping are to be checked as per standard check list.

-11KV Bus Duct/Cable erection from station switch gear to unit switch gear complete in all respects.

-The Tie Breakers at station switchgear end of tie is tested and in withdrawn position with shutters locked.

All circuit breakers contained in 11KV unit switch gear in withdrawn position.

Clearance from Erection Department for carrying out tests on 11KV unit   switchgear and 11KV Bus duct/cable from unit to station switchgear obtained.

-11KV station switchgear associated with the energising of this unit switchgear is live.

-Safety Clearance notification issued  and effective for 11KV unit switchgear and 11KV tie bus duct/cable.

-The associated DAS/Annunciation scheme is complete, tested and in service.

-The control supplies are available.

COMMISSIONING PROCEDURE :

-Check and test all the panels of 11 KV unit switch board and associated equipments as per their standard check list.  Record in a format for future use.

Check and test the unit switchgear bus bars and the Tie bus duct/cable to the station switchgear as per their standard check lists and record.

Check and test supply cables for the DC control bus and the AC heater supplies as per their connection diagram and record.

Continued to PART-III --PRE ENERGISING TESTING ,AND COMMISSIONING

ELECTRICAL SYSTEM ( HT ) PANEL 11KV FOR EQUIPMENTS AND SYSTEM (PART -I)- ABC OF THERMAL POWER

Till last dispatch basics electrical system discussed which are essential for the erection and commissioning of the Power system. Earthling rods and flats are laid at different places when civil works under progress. Riser are kept at different places to get it connected. Slowly it becomes like a grid. There value etc are precisely monitored time to time. After that main plant DC system are getting erected .As it was told no AC system will run without  the DC input.

Two more systems are coming after that one is HT ( 11kv/6.6 KV /3.3KV system ). It allows to distribute power in power plant as bulk and also the source running equipment like BFP , ID ,FD PA etc motors . Higher rating equipments are connected with HT ac voltage and this will reduce the cable size considerably and easy handle at site.

Here is brief description of panel in a 500 MW power station. There will be variation site to site but more or less these feeders will be there. This will give a good idea of number of HT panel to be commissioned. Viewers comments /value addition  will be appreciated.

11 KV Switchgear

Generally a few companies supplied these panels and they are BHEL . ABB , C&G ETC . these panels are installed in service bay near unit one side at an elevation of 8.5m .

1.         PLANT DETAILS :

       THIS SYSTEM CONSISTS OF  11 KV AND 3.3 KV HT SWITCHGEARS (UNIT SWITCH BOARD BA,BB,CA,CB FOR UNIT #1 AND UNIT #2 AND STATION SWITCH BOARD 0BA,OCA FOR 11 KV/3.3 KV  STANDBY SWITCHBOARD  SUPPLY IN CASE OF BOTH GENERATORS OUT OF SERVICE) .

MAKE: BHEL,BHOPAL

RATINGS VOLTAGE : 11 KV & 3.3 KV 3PHASE 50 HZ AC , CURRENT:2750 A

11 KV SWITCHGEARS

I.The 1BA Board consists of the following HT Panels :

1.Incomer Panel ( incomer from UT A )

2.PT PANELS ( line /feeder PT  and Bus PT )

3.         Tie breaker from standby switchboard (0BA)

4 .        OUTGOING FEEDERS    

            3240 KW Cooling Water Pumps 1 & 2

2450 KW Primary Air fan        A (Unit #1)

            16MVA Unit Aux. Transformer A

            16MVA Misc. service Transformer()

            2MVA  Unit service transformer A

            3MVA VFD transformer (ID Fan 1A Channel 1&2)

            1.25MVA Bunker area service transformer

            1.6 MVA ESP Service transformers

            1.25 MVA CHP transformer 1A

            0.75 MVA Track Hopper Service Transformer

            1.6MVA Fuel oil service transformer

            7.5 MVA Station  aux. Transformer.

            2 MVA Station Service Transformer

            6.3 MVA CHP Aux.

             5. Spare feeders

            a) 3240 KW Motor feeder

            b)2 MVA Transformer feeder

II.The 2BA Board consists of the following HT Panels :

1.         INCOMER PANEL

            Incomer Panel ( incomer from UT A ) (Unit #2)

2.         PT PANELS ( line /feeder PT  and Bus PT )

3.         Tie breaker from standby switchboard (0BA)

4.         OUTGOING FEEDERS    

            3240 KW Cooling Water Pump 4

            2450 KW Primary Air fan        A (Unit # 2)

16MVA Unit Aux. Transformer A

            16MVA Misc. service Transformer

            2MVA  Unit service transformer A

            3MVA VFD transformer (ID Fan 2A Channel 1&2)

            1.25MVA Bunker area service transformer

            1.6 MVA ESP Service transformers

            1.25 MVA CHP transformer 1B

            0.75 MVA Track Hopper Service Transformer

            1.6MVA Fuel oil service transformer

            7.5 MVA Station  aux. Transformer.

            2 MVA Station Service Transformer

            6.3 MVA CHP Aux.

            5.  Spare feeders

            3240 KW Motor feeder

            2 MVA Transformer feeder

III. The 1BB Board consists of the following HT Panels :

1.         INCOMER PANEL

            Incomer Panel ( incomer from UT B ) (Unit #1)

2.         PT PANELS ( line /feeder PT  and Bus PT )

3.        Tie breaker from standby switchboard (0BA)

4. OUTGOING FEEDERS    

            10000 KW MD BFP (Unit # 1)

            3240 KW Cooling Water Pump 3

            2450 KW Primary Air fanB (Unit # 1)

16MVA Unit Aux. Transformer B

            2MVA  Unit service transformer A

            3MVA VFD transformer (ID Fan 1B Channel 1&2)

            1.6 MVA ESP Service transformers

            1 MVA Switchyard Service Transformer

            2 MVA Ash Sys Compressor Plant Transformer

            1 MVA Ash Water Power House Service Transformer

            2 MVA Off Side Plant Service Transformer

            2 MVA WTP Area Service  Transformer

            2 MVA Station Service Transformer

            0.5 MVA Raw Water Pump House Service Transformer

            1.6 MVA Administration Building Service Transformer

5. Spare feeders

            2450 KW Motor feeder

            1 MVA Transformer feeder

IV.The 2BB Board consists of the following HT Panels :

1. INCOMER PANEL   

            Incomer Panel ( incomer from UT B )  (Unit #2)

2. PT PANELS ( line /feeder PT  and Bus PT )

3. Tie breaker from standby switchboard (0BA)

4. OUTGOING FEEDERS    

            10000 KW MD BFP (Unit # 2)

            3240 KW Cooling Water Pump 5

            2450 KW Primary Air fanB (Unit # 2)

16MVA Unit Aux. Transformer B

            2MVA  Unit service transformer A

            3MVA VFD transformer (ID Fan 2B Channel 1&2)

            1.6 MVA ESP Service transformer

            1 MVA Switchyard Service Transformer

            2 MVA Ash Sys Compressor Plant Transformer

            1 MVA Ash Water Power House Service Transformer

            2 MVA Off Side Plant Service Transformer

            2 MVA WTP Area Service  Transformer

            2 MVA Station Service Transformer

            0.5 MVA Raw Water Pump House Service Transformer

            1.6 MVA Administration Building Service Transformer

5. Spare feeders

            2450 KW Motor feeder

            1 MVA Transformer feeder

Above description is typical of coal based super thermal power station feeder list. There may be variation site to site and of uses. But this will given an idea of HT drives and transformer required for HT drives.

Recently i was visiting a super thermal power station and there HT panel erection is going on. Some other picture on this will be on next dispatch.

continued to pat -ii

220 VOLT DC SYSTEM-BATTERY CHARGER -COMMISSIONING PART -I I-ABC OF THERMAL POWER PLANT.

COMMISSIONING PROCEDURE:

     TESTING OF BATTERY CHARGERS:

The insulation value shall be checked with 500 V DC Megger. During this test, all low voltage circuits  & Relays etc, shall be  bypassed ( shorting)

The panel shall be switched on at no load. For this, first make input switch ON & check the voltages at all phases indicated by lamps  & voltmeter by selector switch . Now the voltage is connected to the primary of the Main transformer which steps the voltage down as required for the specific DC output Voltage & isolates AC Input line from Battery circuits. The transformer's secondary is connected to the Full wave rectifier assembly.

The bridge is standard  three phase full wave half controlled silicon bridge. The voltage control is accomplished by the application of a positive pulse to the gate terminals of the SCR at the desired time. Before the pulse is applied , the SCR is open  and no current flows in the circuit.  As soon as SCR is fired, it operates as a standard silicon rectifier until the forward current is reduced to almost zero.

Check the no load output voltage at the output terminals.

Ensure the output voltage is steady without any trouble for 15 minutes.

Check for annunciation interlocks  etc.

If load is available, the equipment shall be loaded progreesively upto rated value

Check for voltage regulation, free from oscillation, ripple etc.

Check for dropping characteristics at higher loads.

Run the equipment for 4 hours and observe of stability and check for local heating.  When unit is loaded the ripple voltage level (peak to peak) may be observed . Follow the advice of Service engineer.

II          FIRST CHARGING OF BATTERY:

 Thumb rule says DC  Voltage needed for battery charging will be 2 times the no of cells in battery bank. The initial charging will take approximately 16 to 24 hours since the batteries are already charged and sent as maintenance free ones.

k board is firmly coIt is of utmost importance that the terminal of the battery is connected to +ve load of charging source. So polarity should be ascertained.

Commence charging not later than 24 hours after connecting all battery cells.

Give the first portion of charge at a rate of 100 Amps for 10 Hours

Discharge it using Water resistance or halogen lamps as recommended by supplier

Give the second charging at the rate of approximately 20 Amps till the signs of completion  of charge observed.

Select any one cell (except the end cell or regulating cell) as pilot cell out of every rack as given in the layout of the battery bank.

Voltage and specific gravity reading from pilot cells will indicate the state of charging of the whole battery system. Follow supplier’s recommendation

All cell terminals & connections are to be applied with pertroleum jelly to reduce contact resistance. No adjustment should be made which will disturb charging.

When all the cells have been greasing freely and when voltage and specific gravity of pilot cell has been constant over 3 successive hour readings, the cells can be deemed to be fully charged and the charging may be terminated. Then wipe down outside of all cells and clean the floor.

Put the battery system on trickle charging when charging completes.

The trickle charge should be adjusted to give optimum battery voltage of 1.53 - 1.67 volts per cell.

III         ENERGISING OF DISTRIBUTION BOARD:

Take final I.R. value of the board. Check ea
ethi ected with earth through wire or strip as shown in the drawing.

Ensure that all outgoing circuits are locked in OFF position.

Ensure that all safety procedures have been observed.

Check that polarities of D.C switch board battery charger and battery are matching.

Close the incomer switch of board to charge it.

Measure D.C Voltage at the Input.

Observe D.C board for any abnormality.

If board condition is satisfactory, check that all remote end fuses of outgoing circuits are removed.

Switch ON outgoing feeders one by one. Record the following for each circuit.

i.)         Polarity  at both ends of circuit

ii.)        Voltage at both ends of circuit.

IV       EARTHFAULT RELAY ON LOAD TEST:

To carry  out on load test for the Battery E/F relay, connect variable resistance box of the range of 15 to 150 K Ohms between +ve  pole of the board and earth

Keep selectivity setting plug of E/F relay  at 3 ms setting.

Cut out resistance slowly to simulate an E/F  and observe the current in milli ammeter when the relay operates.

Record resistance and current in mA

Carry out this test on each available setting on relay.

While doing test observe and record the direction of deflection in the ammeter. Check whether it confirms the pole on which the fault is simulated.

Repeat  once a again on -ve pole of the board.

The test is satisfactory if the operation of the relay confirms with the setting.

Note : Electrical commissioning demands cool approach not rushed to wards the job. Any mistake may cause permanent damage and that was witnessed many a time.

All instrument need to calibrated so that during dispute you can rely. This is also a common problem.

All hand tool should be insulated one and value checked before using.

Electrical signboard /safety signboard should be displayed .

220 VOLT DC SYSTEM-BATTERY CHARGER PRE-COMMISSIONING PART -I -ABC OF THERMAL POWER PLANT.

220 volt dc system with battery charger commissioning

Till last dispatch I have discussed about the basic requirement of electrical system in apower plant . In continuation with that commissioning of DC 220v system will be described briefly.

The system consist of :-

i.          Battery Bank

ii.         Battery Charger - Main and Standby

iii.        DC Distribution board.

The battery is installed in the battery room in the HT switchgear room.  This provides a 220 V DC supply to the relay and control circuits in the Generator. Generator transformer and Unit transformer protection and control panels, 11KV  and 3.3 KV HT switchgear and 4.5 V LT switchgears Unit and station service boards and aux. Control panels. As told you earlier 220 volt DC system is backbone of the electrical system .To energise AC system you need DC system.

Battery Cells - Data Sheet ( Typical 500 MW power plant

            Guaranteed AH Capacity at 10 Hrs.discharge

            Rate 1 Volt / Cell at 27 C                                -           990 AH

            Designation                                                     -           KPH 990 P

            Recommended range of floating/trickle

            Charging voltage                                            -           1.4 -- 1.42 V/Cell

            Recommended range of boost charging

            Voltage                                                            -           1.53 -- 1.67 V/Cell

            Trickle charging current                                 -           1 to 2 MA /AH

            Boost charging current                                   -           198 A

             Open circuit voltage of cell when completely

            Discharged at 27 C at 5 hrs discharge rage   -            1.0 V

            Dimension (mm)                                           -            522 L x 192 W x 405 H

            Amount of specific gravity or electrolyte

            Per cell  required for first filling at 27 C         -          18 Lit / cell, 1.19 +- 0.01

            Specific gravity of cell when fully charged                1.19+- 0.01

            Max. electrolyte temp. that cell can withstand

            Continuously                                                  -           50 C

            Net dry cell weight                                        -           45 Kg

            With electrolyte                                             -           68 Kg

            Cell container material                                  -           Polypropylene

            No of cells / Bank                                         -           169

             Charger - I & II

            Input Voltage                                                  -           415 V+- 10%

            Output Voltage                                               -           220 V DC

            Float Voltage                                                   -           236 V DC

            Boost Voltage                                                  -           287.3 V  DC

            Rectifier current limit                                     -           400 A

            Battery current limit                                       -           198 A

            Auto charge over current                               -           40 A

            Steady state output voltage Regulation (For +- 10% I/P

            Voltage variation                                            -           +-1%

            0 - 100 % load variation

            at a time).

The above data was given from a data sheet of particular power station but this may vary marginally  power station to power station depending on system design and integration for which I am not expert.

Pre -commissioning  

- Battery,  battery charger and distribution boards are installed as per drawing.

- Cabling between the battery / battery charger and the distribution boards completed .

-     

      Basic testing .quality checks of components like insulation testing ,continuity testing , dimension checking etc are over. Charge the distribution board live at 220 V DC in  a safe and orderly manner without load and keep it charged for 24 hours.

- To carry tests and checks to ensure that all the components on the boards are operating satisfactorily including cables to all the remote points in the 400 KV substation and control room.

-  AC distribution board for input supply to be charged and commissioned.

-  Each module will be checked individually

-  Each outgoing supply will be tested up to the fuses at the remote end.

-  Battery E/F will be simulated for both the poles and E/F relay sensitivity will be

    checked.

-  Following  services will be  required during ,testing and pre –commissioning

 - 415 V,  3 Phases, 50 Hz supply ,Adequate running water, Suitable voltmeter, ammeter,

  

   thermometer and hydrometer,Variable resistance or water load for discharging the battery.

    Electrolyte for cells as per IS-266, Pure battery grade distilled water, Variable resistance box

    of 15 to 150 K Ohms, 1 No.  500 VDC megger, Multimeter , hand tools etc.

-Standard safety equipment for handling acid and battery .  Also fire prevention equipment like

extinguishsers , Exhaust fans to take out fumes if any , etc should be made available. Different

precautionary sign board in and around the panel battery room etc.

continued part -II battery charger commissioning.

ELECTRICAL SYSTEM 220 V DC SYSTEM FOR MAIN PLANT- ABC OF THERMAL POWER STATION

220 volt DC system

The 220V DC system supplies direct current as source of operating power for control, signaling, relays, tripping and closing of switchgears, emergency motors of most important auxiliary systems.

Under normal conditions of station generation, the storage battery units are kept floating in DC bus bars by means of the trickle chargers (also known as float chargers). The trickle chargers of each battery unit, which is a rectifier with AC input, is normally made to take all DC requirements of the power station without allowing the battery to discharge. This is achieved by maintaining the DC output voltage of trickle charger a few volts higher than the voltage of the battery.

A typical 500 MW Main DC Plant is described below. In a power plant Number of DC system are running parallel in different BOP packages like Air compressor ,CHP ,AHP etc. Low voltage DC system is also sunning as per requirement of the system like FSSS,DCS,ATRS etc. However we are describing only main plant. Switchgear DC system at switchyard also not considered.

Main Plant consists of:

i.                     Battery Bank

ii.                   Battery Charger - Main and Standby

iii.                  DC Distribution board.

The battery is installed in the battery room in the HT switchgear room.  This provides a 220 V DC supply to the relay and control circuits in the Generator. Generator transformer and Unit transformer protection and control panels, 11KV  and 3.3 KV HT switchgear and 4.5 V LT switchgears Unit and station service boards and aux. Control panels.

Battery Cells - Data Sheet

                Guaranteed AH Capacity at 10 Hrs.discharge

                Rate 1 Volt / Cell at 27 C                                                -              990 AH

                Designation                                                                     -              KPH 990 P

                Recommended range of floating/trickle

                Charging voltage                                                            -              1.4 -- 1.42 V/Cell

                Recommended range of boost charging

                Voltage                                                                            -              1.53 -- 1.67 V/Cell

                Trickle charging current                                                 -              1 to 2 MA /AH

                Boost charging current                                                   -              198 A

              Open circuit voltage of cell when completely

                Discharged at 27 C at 5 hrs discharge rage                    -              1.0 V

                Dimension (mm)                                                            -              522 L x 192 W x 405 H

                Amount of specific gravity or electrolyte

                Per cell  required for first filling at 27 C                     -                 18 Lit / cell, 1.19 +- 0.01

                Specific gravity of cell when fully charged                   -              1.19+- 0.01

                Max. electrolyte temp. that cell can withstand

                Continuously                                                                   -              50 C

                Net dry cell weight                                                          -              45 Kg

                With electrolyte                                                               -              68 Kg

                Cell container material                                                    -              Polypropylene

                No of cells / Bank                                                            -             169

   Charger - I & II

                Input Voltage                                                                   -              415 V+- 10%

                Output Voltage                                                                -              220 V DC

                Float Voltage                                                                   -              236 V DC

                Boost Voltage                                                                 -              287.3 V  DC

                Rectifier current limit                                                     -              400 A

                Battery current limit                                                       -              198 A

                                Auto charge over current                                             -              40 A

                Steady state output voltage Regulation (For +- 10% I/P

                Voltage variation                                                              -              +-1%

                0 - 100 % load variation

                at a time).

Above is standard specification of DC voltage system  followed by BHEL in 500 MW EPC contract. Cell voltage sometimes changed into 2 V +/- 5% instead of 1 volt .

Following installation checks to be carried out during erection of the DC system

1.  Room to be checked thoroughly about level , acid resistance tiles etc

2. Check the battery about damage , condition of battery i.e dry or wait.

3. If it is dry then prepare the electrolyte for filling.

4.Check all the cable connection from battery bank to DCDB

5. Check the installation of Battery charger and cabling.

6. Follow all safety precaution listed in safety manual

7. Use calibrated instruments  for measurements of parameters.

8. As we will be handling acid solution extra precaution is required for safe handling.

9. Check the ventilation system including exhaust fans are working properly.

Continued part –ii  for commissioning of battery charger

ELECTRICAL SYSTEM OF A THERMAL PLANT CONCLUDING PART- ABC OF THERMAL POWER PLANT.

     Earth pit checking

Each earth pit is checked as mentioned in the standard check list of 

Switch yard earthing system

Check that the depth of the earth pit is 3.5 Mtr in case of pipe 

electrode 3 Mtr incase of rod electrode.

Incase of pipe electrode, earth pit must be having the layers of sand, gravel, salt, and coal with the thickness  as mentioned in the separate detail.

Thickness of the gravel must be 150mm  and the gravel must be spread over the switch yard area.

Main earthing and subsidiary conductors are to be ensured that 40/50 mm dia rod is used for the same.

GENERAL CHECKS :

Minimum spacing between electrodes shall be 6 Mtrs.

Earthing conductor shall be laid at least 300 mm below cable trenches and under ground service ducts, pipes, tunnels, railway tracks etc., if crossed.

Risers of 40/50 mm  dia MS rod shall be brought nearby equipment foundations :  1.  2 Nos. for each equipment earthing.  2.  1 No. for each structural earthing.

Risers end near the foundation shall be brought 300 mm above the ground level near foundation.

Earthing conductors along their run on cable trench ladder columns, beams, walls, etc. shall be supported by suitable welding/cleat fixation at intervals of 750mm .

Galvanized iron sleeves shall be provided for the passage of the conductors where ever it passes through the walls. Both ends of the sleeves shall be sealed to prevent the passage of water through the sleeve.

Earthing conductor embedded in concrete shall have approximately 50 mm concrete cover.

Auxiliary mesh of 1500 x 1500 mm size shall be laid at spacing of 300 mm below operating boxes of circuit breakers, isolators-main switches and earth switches.

All joints  shall be welded as per the drawing and IS code.

LAs, CVTs, and Earth wire down conductor shall be connected to the main earth mat through rod electrodes, rod electrodes shall be made out of 40 dia MS rods at site.

Lightning Masts down conductor shall be connected to earth mat through 1 NO. pipe electrode.

Transformer neutral shall be connected to earth mat through 2  nos. pipe electrodes.

Earth mat shall extend 2 mtrs outside the switch yard fence through rod electrode.

All equipment shall be earthed to at least two points on opposite side which shall be connected to different conductors of the earth mat.

Testing of earth pits  :  Earth pit resistance is measured with earth megger and it should be in the range of 1 to 1.5 ohms. The continuity between two earth pits connected together is checked using a multimeter and it should show exact continuity.

Earth resistance of earth mat is measured and it should be in the range of  0 to 0.05 ohms.

Check overlapping of joints are welded with no gap between two joints.

The connection of earthing network is checked according to the layout drawing. 

We will discuss 220V DC System  as this system is required to start AC system breaker. Therefore  a little knowledge on DC system will help to understand the electrical system.

 continued