Step potential and touch potential are very essential factors in the designing of the earthing system of any substation. Step potential and touch potential also decide the overall safety of personnel working in the substation. As for humans, 25mA current can prove fatal hence calculation of step potential and touch potential is very important.
STEP POTENTIAL:
When a conductor is grounded or a fault occurs in a substation or a transmission tower, the potential that is developed between two points in the ground because of variation of soil resistivity is known as step potential.
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As the fault occurs, the current will enter the earth. Based on the varying distribution of soil resistivity, the voltage distribution will take place. The voltage drop in the soil surrounding the grounding system will present a hazardous situation for any personnel standing in the vicinity. Personnel stepping in the voltage gradient will be subjected to a difference in potential between his/her feet, which will result in the flow of electricity through them.
To avoid the situation proper calculations must be carried out by the utility engineers. Any personnel in the zone must be wearing the safety shoe electrical grade as it gives millions of ohms resistance when dry. Instead of taking big steps out of the zone, it is advisable to move in little steps, the heel must not move ahead of the toe.
TOUCH POTENTIAL:
In case of an electrical fault, current will pass from the metallic object/ conductor to the ground. Any person touching the metallic object in the vicinity of the ground potential rise (GPR) will be subjected to the touch potential.
It is basically because of the difference in potential of the ground network and the personnel touching the metallic part, current will pass through the personnel to the earth.
Careful analysis is required to determine the acceptable fibrillation currents that a body can withstand if a fault is to occur. Engineering standards use a one-meter (3.28ft) for calculating touch potentials. A two-meter (6.54 ft) reach is also used when two or more objects are inside the GPR event area.
Calculation of Step Potential and Touch Potential
The allowable potential difference between any two points can be calculated according to the parameters of the circuit and the allowable body current. According to the theorem of energy equivalency, when the external circuit that connects two contact points is connected in series, the body current between two points is equal to the current generated by the voltage on the body resistance.
Strictly, this external current has two parallel paths, one is the direct path through the earth, and the other is through the external parallel circuit. Because the fault current is very high as ranging from several hundred to several thousand amperes, while the allowable body current must be limited to a milliampere level, the influence of the body on the applied voltage therefore can be ignored.
Assuming all the potential differences before the electric shock are unchanged. When a person walks on the ground, the touch resistance RF between the two feet and the ground surface and the body resistance RB are connected in series, and the permissible step potential US between the two feet is:
US = [ RB / (RB + 2 RF) ] VS
Where VS is the potential difference between two points where no person stands.
When a personnel stands on the ground and touches any grounded conductor, the touch resistances between the two feet and the ground are parallel, and the actual voltage, that is the permissible touch potential UT between one hand and one foot is
UT = [ RB / (RB + 0.5 RF) ] VT
where VT is the difference in potential between two points where no person stands.
When the feet come in contact with the ground, the touch resistance RF between one foot and the ground surface has a strong influence on the body current. Usually, one foot is considered as a round plate with a radius b (in cm) and then, in uniform soil, RF can be calculated by
RF = ρS / 4b
where ρS is soil resistivity (in Ω m). Usually, b = 8 cm, and then the touch resistance of one foot is 3 ρS (in ohms).
Let the resistivity of the surface soil is 200 Ωm, the body resistance as 1000 Ω, the touch potential RF becomes 3 * 200 = 600 ohm, the step potential US and the touch voltage UT calculated by Equations US = [ RB / (RB + 2 RF) ] VS and UT = [ RB / (RB + 0.5 RF) ] VT are:
US = 0.455 VS
UT = 0.769 VT
According to Equation IK50 = 0.116 / √t for calculating the allowable body current, the maximum permissible step potential and touch potential, US50 & UT50 respectively for a 50 kg human body in a power system with a large earth fault current can be obtained
V R I
US50 = ( RB + 2RF ) IK = ( 1000 + 6 ρS ) 0.116 / √t = ( 116 + 0.7 ρS ) / √t
UT50 = ( RB + 0.5RF ) IK = ( 1000 + 1.5 ρS ) 0.116 / √t = ( 116 + 0.17 ρS ) / √t
Similarly, we can obtain the maximum permissible step potential and touch potential US70 & UT70 respectively for a 70 kg human body
US70 = ( RB + 2RF ) IK = ( 1000 + 6 ρS ) 0.157 / √t = ( 157 + 0.942 ρS ) / √t
UT70 = ( RB + 0.5RF ) IK = ( 1000 + 1.5 ρS ) 0.157 / √t = ( 157 + 0.2355 ρS ) / √t
If we assume the maximum permissible step potential and touch potential are the allowable body voltage US, the maximum permissible step potential and touch potential, VS & VT respectively can be derived as
VS = [ ( RB + 6 ρS ) / RB ] US
VT = [ ( RB + 1.5 ρS ) / RB ] UT
If we take US = 50 V, the maximum permissible step potential and touch potential in a non-solidly grounded system (i.e., a small current grounding system) are
VS = 50 + 0.3 ρS
VT = 50 + 0.075 ρS
Influence of Resistivity of Surface Soil Layer on step potential and touch potential
According to the analysis above, the permissible step potential and touch potential are determined directly by the resistivity of the surface soil layer, which can be altered by increasing the resistivity of the surface soil.
Further, paving a high-resistivity layer on the ground surface is favourable to prevent the fault current from flowing into the surface layer and the surface voltage also remains approximately close to the voltage without the surface high-resistivity layer. As there is increase in touch resistance, the current passing through the body reduces.
In substations, usually a layer of gravel or asphalt concrete pavement with a thickness of 15–38 cm is laid on the surface of the ground so that even on rainy days, the gravel or asphalt concrete can keep a resistivity of 5000 Ω m.
Great attention should be paid that a normal concrete pavement must never be used to increase surface resistivity because normal concrete has a property of water absorption, whose resistivity will reduce to several tens of Ω m on rainy days.
The touch resistance RF relates to the ratio of the resistivities of the high-resistive surface layer and the soil also it is related to the thickness of the surface of high-resistivity layer. As per recommendation from IEEE Std80-2000, using simplified method considering the influence of the high-resistivity layer works best.
With the high-resistivity surface layer, the grounding resistance of one foot of a person standing on the ground is amended by the following formula to consider the influence of the high-resistivity layer
RF = CS ρS / 4b
where CS is the correction coefficient of the foot grounding resistance while considering the high resistivity layer.
The correction coefficient can be calculated using the following empirical formula
CS = 1 – {a(1- ρ/ ρS)} / {2hS + a}
where hS is the thickness of the high-resistivity surface layer (in m), ρS is the resistivity of the high-resistivity layer (in Ω m), ρ is the resistivity of the uniform soil under the high-resistivity layer (in Ω m). a is a constant which is equal to 0.09m.
The formula above is suitable for the condition that K is in the range from 0 to – 0.98 and hS is from 0 to 0.3 m. Computer models have also been used to determine the value of CS. Compared with the numerical result in a solid line, the error of the Equation above is less than 5%.
If the thickness of the high-resistivity surface layer is 0.2 m, its wet resistivity is 2000 Ω m, the soil resistivity is 105 Ω m and a = 0.08 m, then the calculated correction coefficient is CS= 0.792, K= – 0.9. From Figure below we can get the same result. So, the grounding resistance of one foot is RF = 4950V.
After taking into account, the effect of the surface-layer material on the foot grounding resistance, the formula for calculating the maximum allowable step potential and touch potential, US50 & UT50 respectively for a 50 kg human body in a power system with a large ground short-circuit current changes to
US50 = ( RB + 2RF CS ) IK = ( 1000 + 6 ρS CS) 0.116 / √t = ( 116 + 0.7 ρS CS ) / √t
UT50 = ( RB + 0.5RF CS ) IK = ( 1000 + 1.5 ρS CS) 0.116 / √t = ( 116 + 0.17 ρS CS ) / √t
Similarly, the formulas step potential and touch potential for a 70 kg human body change to
US70 = ( RB + 2RF CS ) IK = ( 1000 + 6 ρS CS) 0.157 / √t = ( 157 + 0.942 ρS CS ) / √t
UT70 = ( RB + 0.5RF CS ) IK = ( 1000 + 1.5 ρS CS) 0.157 / √t = ( 157 + 0.2355 ρS CS ) / √t
SOME MITIGATION TECHNIQUES:
Mitigation of the step potential and touch potential is usually accomplished through one of the three major techniques:
Reduction in Ground resistance of the system.
Reduction in the ground resistance of a system helps in reducing the step potential and touch potential below or at least up to the tolerable limit. We can opt for various processes for reducing the ground resistance like chemical charging in the soil, moisture charging, etc which are discussed in detail in the article Soil Treatment for reducing earth resistance.
Proper placement of grounding conductors.
By altering the spacing of the grounding conductors we can greatly reduce the grounding resistance of a system. As by the reduction in space of the grounding conductors, the grounding system will act as a metal plate and these dangerous high step potential and touch potentials can be avoided.
The addition of resistive surface layers.
By addition of a resistive layer of aggregates 15-38cm thick we can greatly reduce the occurrence of the high step potential and touch potential. This is because the high resistive surface layer resists the fault current flow into the surface layer.
CONCLUSION
We now understand the potent risk of step potential and touch potential which can cause electrocution. Therefore it is equally important to take measures to mitigate the high rise of voltages. Equipment shall therefore be marked or properly fenced so that they remain only accessible to trained personnel with proper safety gear and no general public.
