Step Potential Higher than Touch Potential ? 3

[Romanko]

I have recently been advised by the Electrical Approval Authority, that the Step Potentials within the Electrical Substation could exceed the Touch Potentials.
The gentleman quoted a journal paper by Dr Dawalibi that is also referenced from the IEEE 80 standard.

Has any body heard of it and/or encountered it, or can give me a quick pointer?

thankfully
ROMANKO

 

[ColinSp]

When an earth fault current flows back to a transformer neutral it passes through the mass of earth. The resistance of the earth path will depent on the soil composition, resistivity etc. The resistivity of the soil will be proportional to the area. As the earth path nears the transformer earthing point, the soil area through which the current passes reduces by a factor of the distance squared, thus the resistance path increases. As the resistance increases, the volt drop across that path for a given current increases. The step voltage is the voltage gradient across a 1 metre span of earth for a given earth fault current (1 metre being a typical human step). This is a particular problem if high step voltages go beyond a substation boundary (safety issue).
The touch voltage is the voltage difference during a fault between any two surfaces that may be touched simultaneously. One surface may be at a different potential because it is earthed at a different location, example a gas pipe run from another area.
Equipotential bonding of conductive parts will reduce touch voltage levels.
If step voltage levels are too high, the earth fault path resistance may need to be reduced by insertion of additional earth mats or better conductive mediums. In the UK, step voltages should typically not exceed 430V for systems below 132kV, or 650V for 132kV and above.

 

[stevenal]

IEEE 80 can be purchased from IEEE and elsewhere. It is available hard copy or pdf. Step potential is from foot to foot. Touch potential is from hand to foot. Touch potential has the heart more directly in the path, so voltages must be lower than for step potential to avoid fibrillation.

 

[Kiribanda]

Hi Romanko,

If we go the basics, it can be proved that the Thevenin equilvalent impedances (Z TH) for the two cases as follows.

1) Step - 2Rf (Two legs are apart by 1 m. Hence resistance are in series)

2) Touch - Rf/2 (Rf are in parallel to each other)

where Rf is the ground resistance of one foot of a person standing on the substation soil.Therefore when you proceed with the derivation for the STEP and Touch voltages (E=IxZ TH) finally we will end up with a higher potential for STEP than TOUCH.

Hope this will be helpful

Kiribanda

 

[cuky2000]

If the Step Potentials exceed the Touch Potentials within substation, this facility is not safe, according with the design criteria on the IEEE Std 80.

It is still unclear to me how the voltage difference between a grounded (metallic) surface in the substation with respect to remote earth (at zero reference potential)is lower than the voltage difference between two points 1 m apart on the substation surface covered with insulated material (ex. crushed rock).

Let us take a second review of the potential definitions and a remark from the standard:

[red] Because the mesh voltage is usually the worst possible touch voltage inside the substation (excluding transferred potentials), the mesh voltage will be used as the basis of this design procedure.[/red]
<sub>Step voltages are inherently less dangerous than mesh voltages. If, however, safety within the grounded area is achieved with the assistance of a high resistivity surface layer (surface material), which does not extend outside the fence, then step voltages may be dangerous. In any event, the computed step voltages should be compared with the permissible step voltage after a grid has been designed that satisfies the touch voltage criterion.
DEFINITIONS
3.24 step voltage: The difference in surface potential experienced by a person bridging a distance of 1 m with the feet without contacting any grounded object.

3.29 touch voltage: The potential difference between the ground potential rise (GPR) and the surface potential at the point where a person is standing while at the same time having a hand in contact with a grounded structure.

3.20 mesh voltage: The maximum touch voltage within a mesh of a ground grid.

3.14 ground potential rise (GPR): The maximum electrical potential that a substation grounding grid may attain relative to a distant grounding point assumed to be at the potential of remote earth. This voltage, GPR, is equal to the maximum grid current times the grid resistance.</sub>

It should be noted that the allowable step potential is always higher than the step potential. From the safety stand point, it is inconsequential if the actual step or touch potentials one is higher than other is since each of them must be compared individually with the allowable values.

 

[jghrist]

Kiribanda,

Your analysis might make sense if the step- and touch- potentials were produced by current going through a body. They aren't. They are produced by current going through the soil. The current going through a body is the result of the body getting the step- or touch- potentials across it.

I'm not sure how the step-potential could exceed the touch-potential. Step-potential is the maximum difference in surface voltage between two points 1 meter apart (the step distance). Touch-potential is the difference between the grid voltage and the surface voltage at a point where someone can touch something that is bonded to the grid.

The surface potential is always less than the grid potential. If the difference between two surface points is Va - Vb = Vstep, the touch potentials at the two points would be Vgrid - Va = Vtoucha and Vgrid - Vb = Vtouchb. If you subtract these last two equations, you get:

Va - Vb = Vtouchb - Vtoucha = Vstep

Vstep could only be larger than Vtouchb if Vtoucha were negative.

It follows that you don't even have to calculate Vstep except in an area where you don't worry about Vtouch because it is not close enough to touch anything bonded to the grid. Like outside the station.

 

[cuky2000]

Kiribanda,

What you are describing is the maximum allowable step and touch potentials. See the enclose link.

http://cuky2000.250free.com/Step_Touch_Pot.jpg

I believe that the issue is how the actual substation step potential is higher than the touch voltage. We know the worst case for step potential is:
E_s= (r.Ig.Ks.Kf)/(0.75.Lc+0.85.Lr) [IEEE Std D.12]
_{Where:
r = average ground resistivity
Ig = current injected into the ground.
Lc= Length of grid conductor.
Lr=length of total rods
Ks & Kf are irregularity and mesh corrective factors}

For touch potential, need further investigation to compare the value and in what conditions the following relation is valid E_s > E_t.

 

[Kiribanda]

Hi jghrist,

Yes you are absolutely correct. Many thanks for the correction. I should have said that "TOLERABLE" Step potential is higher than the "TOLERABLE" Touch potential at a location.

But as cuky2000 and you have pointed out that I also donot know how the "CALCULATED" Step potential is higher than the "CALCULATED" Touch potential in a substaion ground grid installation.

Thanks!

Kiri

 

[stevenal]

I am taking the word "could" in the original post as "permissible" per IEEE 80. What is actually possible in a given situation might be different, but is hopefully less than the maximum permissible levels. (Cuky, "always" is a stretch in this context.)

From IEEE 80 the permissible potentials for a 70kg person:

Estep70=(1000 + 6Cs*?s)0.157/sqrt(ts)
Etouch70=(1000 + 1.5Cs*?s)0.157/sqrt(ts)

These are equations 30 and 33 from IEEE 80 2000. They share the same constants, so the allowable touch voltage is less than the allowable step voltage. This is for the reason I stated above.

jghrist,

Worst case Estep can easily exceed worst case Etouch. The safest and most economical use of copper tightens the mesh distance around equipment and fence for the touch potentials and increases the mesh distances away from equipment where step potential rules.

 

[Romanko]

I guess there are a few issues we can all agree on:

1) Tolerable Step Potential is always higher than tolerable Touch potential (actually the Canadian Electical code has it pegged around 3 times higher for Crushed rock cover).

2) Touch Potential is the Difference between the GPR (or the "Grid Voltage") and a place on surface anywhere near a metallic struture (actually the code refers to the normal horizontal reach considered to be 1 m)

3) Step potential is the difference between two points on the surface separated by a distance of one pace (assumed to be 1 m).

I also have to agree with the simple equation given by jghrist to derive the Step Potential as a difference of two Touch potentials (assuming they are apart <= 1 m).

Furthermore, if we look at a graphical representation:

The following is an actual simulation of a ground grid (it's not symmetrical as you might notice)

NOW, the Red peaks in the Absolute potential graph (corresponding to actual ground rods) represent the points on the surface that reach almost full GPR: 4600V.
The "valleys" correspond to space between the interconnecting grid elements.

The worst case Touch potential would seem to be, when someone stands in the "valley" and touches something that has full GPR (i.e. 4600V). - That is clearly shown in the second picture.

The worst case Step potential would be, when someone walks across the steepest gradient on the grid.
The thing is, I can not fathom how this gradient difference could ever be more tham the differnce between the deepest "valley" to the highest peak (GPR)??
This differnce is in fact the wost case Touch potential.
I really don't understand stevenals statement that "Worst case Estep can easily exceed worst case Etouch."

I guess the answer may lay somwhere in the equation:
Es= (r.Ig.Ks.Kf)/(0.75.Lc+0.85.Lr) [IEEE Std D.12] that cuky2000 has shown.
Possibly with the non-uniform current distributon in the grid elements or ground rods.

ROMANKO

 

[Romanko]

Sorry, it seems the pictures didn't seem to have come out
I'll try here again:

Hope this works now.
ROMANKO

 

[stevenal]

Your images are broken.

"I really don't understand stevenals statement that "Worst case Estep can easily exceed worst case Etouch.""

Jghrist's analysis was for two locations a meter apart where grounded metal was within reach. Estep is low and inconsequential for this location compared to Etouch. The worst case Estep, however, is not likely to be located here. It will be located where the grid designer has increased the mesh distance because grounded structures are not within reach. Etouch is not existent here, and Estep rules the design. Maybe your software gives you an Etouch value anyway, leaving it to you to determine if it's valid.

 

[jghrist]

With hand calculations based on simplified IEEE-80, you calculate Emesh which is the greatest difference between surface potential and grid potential within the grid mesh.

With computer calculations, you calculate Etouch everywhere, as shown on Romanco's graph.

The only way the original statement of Estep > Etouch can be true, I think, is to define Etouch as stevenal has done. There is no Etouch where there is nothing to touch. The really high values of voltage difference in the middle of large meshes with no above grade equipment are not defined as touch potentials.

 

[Romanko]

So I guess it's the common concensus that Estep can only be higher in areas where Etouch is not applicable (i.e. no grounded metallic structure above grade ? ! ?

ROMANKO

 

[cuky2000]

The actual maximum substation touch voltage is allways higher than the step potential. See the enclose figure.

I will attach leter the support documents for this statement.

 

[stevenal]

Not just my definition, jghrist. Here's the IEEE 80 definition:

touch voltage: The potential difference between the ground potential rise (GPR) and the surface potential
at the point where a person is standing while at the same time having a hand in contact with a grounded
structure.

I think the software assumes there is a grounded metal plate hovering about seven feet off the ground covering the entire substation. Not the best assumption. Even if you don't mind putting too much copper in the ground, the best practice would be to put the extra in the location it will do the most good.

 

[rezaa]

The software calculate the potential difference between the ground potential rise (GPR) and the nearest conductor as touch voltage.

 

[cuky2000]

The enclose information shows that the actual maximum touch potential inside a substation is higher than the actual maximum step voltage for standard design with grid parallel conductors separated greater than 5 ft.

Although it is uneconomical and impractical design the substation with small grid separation or solid metallic surface, may be situations that require special consideration.
For infrequent and unusual conditions such as the cases indicated below, the step potential may be slighter higher than the touch potentials.
It should be noted that the IEEE std 80 represent the human foot as a conducting metallic disc and the contact resistance of shoes, socks, etc., is neglected. That possible may be a factor to influence that the touch potential is higher than the step voltage for very low reflecting coefficient.

[red]The simplified analysis in the enclose link compared favorably with the IEEE Std 80 graphs above. [/red]

http://cuky2000.250free.com/GND-Ratio_Touch_Step_V_Calc.pdf

 

[vic3fan]

Oh c'mon guys... A round of applause for cuky2000!

 

[stevenal]

The IEEE 80 method uses even grid spacing with the worst case touch potential at the corner. Although it wastes a lot of copper, this method ensures step potentials toward the center are low. Even with this method they say to check step as well as touch potential:

"Step voltages are inherently less dangerous than mesh voltages. If, however, safety within the grounded area
is achieved with the assistance of a high resistivity surface layer (surface material), which does not extend
outside the fence, then step voltages may be dangerous. In any event, the computed step voltages should be
compared with the permissible step voltage after a grid has been designed that satisfies the touch voltage
criterion."

The even grid spacing method is not the best (safest) use of copper. Better to space out the conductors away from everything and tighten the grid near corners, fences, and equipment where it does more good. Software like Romenko has can allow you to do this easily and see the result. You need to know where the equipment is, though, something the software is evidently not modeling.