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.

EFFECT OF ELECTRICAL QUANTITIES ON HUMAN BODY

WHAT IS THE EFFECT OF DIFFERENT ELECTRICAL QUANTITIES ON HUMAN BODY?

PRECAUTIONS AT THE TIME OF ELECTRICAL WORK?

Effect of dangerous currents on human body: The main reason of electric shock is the electric current that flows through human body.The severity of electric shock depends upon two given below factors:

1. Time of contact: The more is the time of contact of the body with electric current.The more severity of electric shock.
2. Nature of current (AC or DC): The AC current is more dangerous than DC for human body.

If we know the working voltage and resistance of human body.Then the value of current flow through human body can be find by the OHM'S Law.

Current flow through the body = Working voltage/resistance of the body

Value of current and its severity on human body:

1. 1 to 8 mA: In the value of current man can be bear.But he feels the electric shock and it is not dangerous.
2. 8 to 15 mA: In this value of current body receives painful shock, but the severity of shock depends upon the health.
3. 15 to 20 mA: In this value of current body receives painful shock and body cannot control muscle nerve.So, human body cannot bear this value of current.
4. 20 to 50 mA: In this value of current, contraction of all muscle nerve started.So, human feels difficult in breathing.
5. 50 to 100 mA: In this value of current, due to contraction of muscle nerve, human feels difficult in breathing and heart beat increase.So, in this condition death may be possible.
6. Above 100 mA: In this value of current body burns, due to contraction of muscle nerve, human feel difficult in breathing and heart beat increase.So, in this condition death may be possible.

Electric shock

The Effect of electric current on human body is described below:

1. Due to electric shock body becomes weak and feel laziness.
2. The human may be become impotent, due to damage of nerve system.
3. The human may be become fully or partially handicapped.
4. Due to burning, the face become ugly.
5. The human may be become deaf.
6. The human may be become blind.
7. Any parts of body may be break.
8. Full body may be burned and converted into ash.

EFFECT OF AC FREQUENCY ON HUMAN BODY:

We know that the time period of alternating current is inversely proportional to frequency.

T = 1/f

Means lower the frequency more the time period of alternating current and vice-verse.Thus time of one cycle of alternating current is more and at time of electric shock it flow through the human body long time as comparison to high frequency AC and the peak value of AC flow through the body for long time and will harm the body.Thus lower frequency harm the body severely as comparison to high frequency. 

ELECTRIC RESISTANCE OF HUMAN BODY:

The value of resistance of human body depends upon two factors.

1. Dryness
2. Wetness

The value of resistance of body in different condition is given below:

1. Fully wet body = 1 KΩ
2. Partially wet body = 5 KΩ
3. Average dry body = 50 KΩ
4. Fully dry body = 100 KΩ

According to OHM'S Law, the greater the value of resistance the less is the current.So,wet body feel more electric shock as comparison to dry body.

EFFECT OF DANGEROUS VOLTAGE ON HUMAN BODY:

We know that current is more dangerous than voltage for human body.We can find the dangerous voltage according to OHM'S Law.

Dangerous voltage for human body = Current*resistance of body

The value of dangerous voltage for children is 24V and for adults is 60V.

PRECAUTIONS: We know that electricity is very dangerous for human life.So, at the time of electric work the given below points keep in mind.

1. At the time of work done, if you feel uncomfortable or may be a small injury happens.Than take rest for some time.
2. If a man comes in contact with live wire.Than take a dry wood stick to break the contact between human and live wire.It is to good before doing this you stood on a wood or rubber sheet.
3. Every time should be alert at the time of work.
4. At the time of work do not be hasty.
5. At the time of work, working a zone should be earthed.
6. The communication should be good.
7. At the time of work safety dress is necessary.
8. If you have not knowledge about the work than doing not do it.
9. At the time of work do not talk with any person and fully focused on work.
10. Always used insulated tools at the time of work.

Conclusion: The conclusion is that electricity is very dangerous for human life.But it can be avoided by taking a little care.So, at the time of work follow the precaution that saves our life from electricity shock.

ELECTRIC QUANTITIES ASSOCIATED WITH HUMAN INJURY

WHAT IS THE SEVERITY OF DIFFERENT ELECTRICAL QUANTITIES ON HUMAN BODY?

WHICH CURRENT IS MORE DANGEROUS AC OR DC?

The electrical quantities effect (loss) the human body more severely.The current is more severe than voltage.Because electric shock that effect (loss) human body is mainly due to electric current pass through the body.

The different electrical quantities severity that effect (loss) human body is given below:

1.Current: The intensity of shock is depend upon magnitude of current.So,high magnitude current is more dangerous than low magnitude current.

→Severity: AC is more dangerous than DC.Because value of maximum current at AC is more than DC at same voltage.

In case of AC;

Imax = √2.Irms 

In case of DC;

Imax = Irms

2.Voltage: According to ohm's law the current pass through the body is directly proportional to applied voltage.

I ∝ V 

Means higher the voltage higher the current.Thus high magnitude voltage is more dangerous than low voltage magnitude.

3.Frequency: Low frequency voltage is more dangerous than high frequency voltage.

Time period (T) = 1/f (frequency)

By above formula we see that low frequency has more time period than high frequency.Thus the current wave pass through the body take more time and thus it effect (loss) the body more.

4.Resistance: Low magnitude of  resistance is more dangerous than high magnitude resistance.According to ohm's law current is inversely proportional to resistance.

I ∝ 1/R

Means lower the resistance of body higher the current flow through it and it is more effect (loss) the body.

Electrical Danger Plate Yard

SEVERITY OF AC AND DC:

We know that AC and DC,both is dangerous for human body.But AC is more dangerous as compare to DC.The root mean square value of AC and DC both are same for same magnitude of voltage.But at the time of electric shock the max value of AC also works and it is √2 times than DC at same voltage.

Imax = √2.Irms = √2.Idc

 Thus AC is more dangerous than DC for human body.

Conclusion: All the electrical quantities like current voltage etc are very dangerous for human being.So,at the time of electric work,at the time of use of electric equipment should be take suitable precaution.