STEP POTENTIAL AND TOUCH POTENTIAL

WHAT IS STEP POTENTIAL AND TOUCH POTENTIAL?

The knowledge of step potential and touch potential is very important.If you work in the electrical field. In this article, we will talk about step potential and touch potential in substation, transmission and distribution system.

Step potential: The step potential of sub-station and transmission, distribution line is given below:

Step potential at sub-station: At the time of earth fault, the fault current flow in earth. Then the voltage generated between the feet of the person standing on the floor of the sub station is known as step potential. Because both the feet are at different potential. So, a potential difference developed between the feet..

This voltage should be so low that the person does not get electric shock.

Step potential in transmission line and distribution line: If at the time of breakdown, transmission and distribution line falls on the earth. Then condition of the potential gradient or voltage gradient will be generated.

Potential gradient (E) = V/L

V = Voltage

L = Length

→Potential gradient: The value of the voltage will decrease as the length increases from source point.

When a person comes close to it (near the broken line on earth) then the voltage is generated between his legs, because both legs are at different voltages. Due to which the potential difference developed between the two legs. This developed voltage is known as step potential. This voltage should be so low that the person does not get electric shock.

→This step potential can be developed in the range of 10 to 15 meters from the source.

Step potential and Touch potential

Touch potential: The touch potential of sub-station and transmission, distribution line is given below:

Touch potential in sub-station: At the time of earth fault means a metallic body carrying current. Due to the earthing of the equipment. All the current will be flow in earth. But at this time some minor voltage also persists on the metallic body.

At this time if a person touch the metallic body of equipment than a minor current (due to minor voltage) will be flow from the person body to earth. Because at the time of touching the metallic body, the feet of the person presence on earth means ZERO voltage, but hand of the person is at metallic body means at minor voltage. Thus a potential difference is developed between human body and a minor voltage is flow from hand to feet. This voltage is known as touch potential. This voltage should be so low that the person does not get electric shock.

Touch potential in transmission and distribution line: If at the time of breakdown, transmission and distribution line falls on the metallic tower or pole. Due to the earthing of the tower or pole. All the current will be flow in earth. But at this time some minor voltage also persists on the tower or pole.At this time if a person touch the metallic body of tower or pole than a minor current (due to minor voltage) will be flow from the person body to earth (zero voltage). Because at the time of touching the metallic body of tower or pole, the feet of the person presence on earth means ZERO voltage, but hand of the person is at metallic body of tower or pole means at minor voltage. Thus a potential difference is developed between human body and a minor voltage is flow from hand to feet. This voltage is known as touch potential. This voltage should be so low that the person does not get electric shock.

How we can reduce the value of step potential and touch potential?

The given below point is generally used for reducing step potential and touch potential.

• By suitable earthing of sub-station equipment and tower or pole.
• In the sub-station yard earth potential and touch potential reduced by using gravel. Due to high resistance of gravel it reduces the potential gradient. Thus it reduce the step potential and touch potential.
• By the use of safety shoes and gloves.
• By the use of insulating mat in sub-station.

Why we use gravel in sub-station yard?

The reason of using gravel in substation yard is given below:

• As we have discussed earlier that the use of gravel in the sub station reduces step potential and

  

  Gravel in substation yard

  touch potential.
• This makes the earthing system suitable.
• Water does not stop in the yard during the rains means all the water goes into the ground under the gravel.
• Due to this many types of animals such as rats, snake etc. cannot make home in the yard.
• Due to this very little grass grows in the yard.