The basic purpose of laying grit or gravel in the switchyard to make the upper surface more resistive.
The gravel has higher resitivity in order of 59000-96000 Ohm-cm. The gravel size of 25-30 mm is used in the switchyrd. Why we require the upper surface more resistive?
Gravel Decreases the Step Potential, How?
The fault current flow though the ground and voltage is developed between two points across the switchyard. The technical term of voltage developed between the two points is called the step potential. The step potential is the voltage developed between the two feet on the ground or animal during short circuit current flows in the ground. It is desired that the step voltage must be as minimum as possible. The current flowing on the upper surface of the ground is limited by the gravel, and thus potential difference between two points decrease with increase in the resistivity of the gravel. This is how the step potential gets decreased. The current flowing through human body is;
Current through human body = step potential/ Body Resistance
The lesser the step potential, the more safe to work in the switchyard.
Gravel Decreases the Touch Potential, How?
Touch potential is the voltage developed between equipment body and the gound. The gravel provides very high resistance and it cause very less current through the upper surface which has gravel. The decrease in the touch potential cause less current though the human body when someone touches the structure of the equipment.
Gravel prevents growth of weeds
The stone/ gravel prevents growth of small weed, climbers and other plants inside the switchyard. The small weeds and grass may increase the step and touch potential because the resistance of the upper surface gets decreased with weeds and grass. When the weeds growth is hindered, the possibility of reptiles in the area can be prevented.
Gravel reduces the possibility of Fire in the Switchyard
The switchyard have oil filled transformers inside it. The spilled oil seeps through the grit /stone and does not remain on the upper surface. Thus in case of fire, the upper area does not catch fire and thus prevents spreading of fire in the switchyard.
Gravel increase the conductivity of the lower layer below the gravel/grit/stone
The rain water passes through the gravel and seeped into the soil, thus the resistivity of the lower layer gets decreased which is desired for power system protection. Also, after rain the stone/gravels gets dried up quickly and
again the resistivity of the upper surface gets increased.
1. NEVER GO INSIDE THE SWITCHYARD WHEN RAIN IS POURING BECAUSE THE STEP AND TOUCH POTENTIAL IS HIGH BECAUSE OF INCREASED CONDUCTIVITY OF UPPER SURFACE.
2. THE POSSIBILITY OF LIGHTNING INCIDENT VOLTAGE IS MUCH MORE DURING RAINING.