STEP AND TOUCH POTENTIAL: REDUCE ELECTRICAL HAZARD AND IMPROVE SAFETY AWARENESS

Step and Touch Potential

Understanding of Step and Touch Potential in Power Transmission System to Reduce Electrical Hazard and Improve Line Crew Safety.

The purpose of this article to focus on some less familiar but the most important safety induction  like- step potential or step voltage, touch potential or touch voltage. We also try to give you some pint-size attention on basics of reducing touch potential and why power grid substation yard needs washed clean gravels or stone rather than others.
 
 We are here using some less familiar literature; means, we everybody knows the importance of electrical safety. But todays discussing topics like step potential or touch potential is little professional and not well known to all, even not to all electrical engineer. Especially who deals with power transmission and distribution substation they are very well familiar with these terms step and touch potential.

What Is Step Potential or Step Voltage

Step potential simply is the potential or voltage difference between a footsteps of a person onto the electrical grounding yard. When fault occur near a metallic structure or steel tower then large amount fault current flows from tower to the earth ground. Consequently ground potential raises at the tower earthing point and voltage gradient develops depending on the soil resistivity near surrounding the tower earth point.  This voltage gradient is causes to develop voltage difference between two points like step between two feet of a person is known as step potential.
The Occupational Safety and Health Administration (OSHA) defines step potential as- the voltage between the feet of a person standing near an energized grounded object.
 

Why Step Potential Is Dangerous

Each electrical potential try to find some easy or less resistive path and when some voltage feel in between two feet of a person; if resistance through person’s body is less than the resistance of ground in between two feet of a person; then obliviously current will flow through the legs via human body and the person will be experience an electrical shock. Because human body acts here as a shunt with the ground resistance, and larger current flow through less resistive shunt. Here shocking current may not passes through the vital organ of the person, only passes through lower limbs of the body; but level of injury depends on developed voltage and duration of current flow.

What Is Touch Potential or Touch Voltage

Touch potential or touch voltage is equal to the difference in voltage between the energized object and a grounded point some distance away. Other way we can say, touch potential is the voltage between any two points on a person’s body like – hand to hand, shoulder to back, elbow to hip, hand to foot and so on touching the energized and ground point.
Say, if an energized conductor falls on a metal fence, and a person touches that fence then current may passes from the energized fence through the person to the ground.How Measure Step and Touch Voltage
The touch potential or touch voltage can be approximately the full voltage across the touched grounded and energized point.
Why Touch Potential or Voltage is Dangerous for Electrical Shock
Each electrical potential try to find some easy or less resistive path and when it passes through a human body then it causes an electrical shock-injury and the degree of injury depends on the amount of current and duration of current flowing through it. In the above 1st figure shown in the cartoon at right-side, touching the grounded tower is experiencing electrical shock because the ground connection between the tower and the soil is high resistance that energized the tower itself. Touch potential or the voltage developed between the energized object (tower) and the feet of a person in contact with the object (soil).
Here current passes through the vital organ of the human body that may causes serious shock-injury.

How Reduce Step and Touch Potential Hazard: Improve Transmission Network Safety Awareness

Step and Touch potential actually a kind of hidden potential hazard that makes more dangerous than other typical electrical hazards. Touch voltage hazard may happen even away from electric tower like substation fence etc.
 
There are many way to reduce step potential and touch potential hazards, some of them are as follows:
 
§  Reducing ground resistance;
§  Ground conductor placing properly;
§  Using gravels or stone on surface area;
§  Using a good quality Person’s safety shoe, etc.
 
Reducing ground resistance
Reducing ground resistance is the best way to reduce the step potential and touch potential hazard, for example- if the fault current for a high-voltage line is 10 kA and the resistance to ground of the grounding system is 10-ohms, then ground potential will be 100 kV. If ground resistance reduced to 5 ohms then ground potential will become 50 kV which almost 50 percent, means fault current may increase 50 percent for same ground potential.
A typical soil resistivity is as bellow:
Type of Soil                                     Resistivity in (Ohm-m)
Moist Humus                                                     30
Farmland & Clay                                             100
Sandy Clay                                                       150
Moist Sandy                                                      300
Moist Gravel                                                     500
Dry Sandy or Gravel                                    1,000
Rocky Ground                                             30,000
 
Expected particulars permissible values of resistance is as bellow:
Location                                                       Resistivity in (Ohms)
 Power Stations                                                  0.5
 EHT Substations                                                1.0
33 KV Stations                                                    2.0
D/T centers                                                         5.0
Tower foot resistance                                    10.0
 
Ground conductor placing properly
Placing ground conductor is important to reduce step and touch potential hazard, generally to minimize touch potential a minimum 2/0 AWG bare copper conductor in direct buried as earth conductor, 3 feet or 1 meter from the perimeter of the object and 18 inches below grade is uses. But a soil with a dry and high resistivity conductor will be less effective than soil with wet and low resistivity. Soil study is very important before placing ground conductor.
 
Reduce Step and Touch PotentialUsing gravels or stone on surface area
 Adding more resistive surface layer in tower or substation is easy way to mitigate step and touch potential. So washed clean gravels or stone layer is the perfect solution to increase the uniform resistive surface and isolate from gradient soil layer.  This gravels layer isolate the person from current flowing earth and reduce the step and touch potential hazards, also control growing weed and oil splitting sleepy surface.
 
Using a good quality safety shoe
 A simple and easy way to mitigate step and touch potential hazards just wearing personal electrical hazard safety shoes. Safety shoes ensure some mega ohms of resistance, but notable that a wet and dirty shoes losses the resistance and current may bypass.
 
Finally we can say personal awareness is the key important to understand the step and touch potential hazard, and practicing safety rules every day in workplace can help to develop a professional line crew. Keep safe your coworker, share this article with them.

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