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