Ground Potential Rise Vs. Touch Potential 2

[nchrista]

Hello All,
I created a ground grid for a multi-peaker unit site with soil resistivity in the neibhborhood of 10.7 ohm*m. There is approximately 7000 meters of 4/0 copper with some 500 kcmil in the heavy fault (60kA) area. The grid area is in the neighborhood of 300km^2. My calculated GPR is around 550V and my touch potential for a 0.5 second clearing time and a 50 kg person is around 50V. The calculated worst case touch potential is around 410V. Should the 410V value be greater than the GPR?

Can someone tell me whether I am reading this correctly?

My concept of the grounding calculation was that as long as the touch potential is greater than the GPR then we were covered. But after adding more ground and stiffening the grid altogether the touch potential has gone down substantially whereas the GPR has only gone down a fraction of a percent.

 

[rcw retired EE]

I think there may be confusion between Allowable and Calculated Touch Potential. The touch potential must be less than the Allowable. If the allowable touch potential is more than the GPR, the system is very safe. Touch Potential will always be less than or equal to GPR. The GPR is the maximum voltage that can be impressed on the grid.

Calculations for allowable touch and step potentials are based on the fault clearing time, resistivity of the soil and of the surface treatment (gravel covering), and estimated size of the person. The calculation result is the maximum touch voltage that will force enough milliamps through the body and heart to cause harm. If the GPR is less than the allowable touch voltage, no further grid or analyiss is needed.

If the allowable touch is less than the GPR, further design is needed until the maximum calculated touch potential is less than he allowable.

You say the allowable is 50V and the calculated maximum is 410V. The 410 V figure seems about right. But the 50V figure sounds like you do not have any surface covering like 6" of gravel or asphalt and are assuming the person is standing on wet native soil. Is there a gravel covering throughout the site?

Your comment on the GPR not changing much as you added conductors is understandable. With your low resistivity, the grid would have a low resistance to remote earth with just a few cross conductors. Adding additional cross conductors decreases the voltage variations between conductors which reduces the step and touch inside the grid. But the added copper does not have much of an effect on the already low (.01 ohms) total grid resistance.

 

[nchrista]

Though the entire grid was modeled, the grid area under concern here was confined to the area of the largest fault currrent, 60kA at one of the utility feeds. The 50V touch potential is graphically produced by the SKM GroundMat software. The site has been modeled, both in excel with approximations and Groundmat. The surface has been modeled with both 6" and 3" of wet crusher run granite with a resistivity of 1300 ohm*m. For this study I'm using the 6" layer. I guess you've answered my question as far as how to interpret the numbers, so thank you.

So to regurgitate your response to ensure that I'm understanding it, the calculated worst touch potential (410V) must be greater than this worst case grid area fault touch potential (50V). The GPR (550V) refers to how much potential the grid can withstand before things start to go awry and must be lower than the worst case touch potential (410V)...right?

Also, should I go to a lower clearing time, like 0.2 Seconds or even less? At 0.2 seconds, the Touch potential goes to 650V which is greater than the GPR. The 3D model hasn't changed anywhere though, it still shows the exact step, touch (50V) and GPR levels. Does that make sense?

 

[jghrist]

There is something wrong with the allowable touch-potential calculation for 0.5 sec. Using IEEE-80 Eq 32 gives 410 volts for 0.5 sec and 650 volts for 0.2 sec. The calculated touch-potential has to be less than the allowable touch-potential. The GPR is the voltage of the grid with respect to remote earth and will always be higher than the calculated touch-potential.

You cannot look at only the part of the grid close to the largest fault. You have to look at any point on the surface where someone standing can touch something that is connected to the grid (like a steel structure). The grid voltage will not vary much with distance from the fault point. The worst case will probably be near the middle of a large grid mesh because the difference between the soil surface voltage (where the feet are) and the grid voltage (the voltage of the steel structure) is higher when you are further from the grid wires.

Most software assumes that the grid wires are the same potential everywhere. This may be a bad assumption in your case however with low soil resistivity and a large grid.

 

[nchrista]

Ok, I think I see what's going on. Most of my confusion was coming from the 3-D model of the GPR. The safer arrow points in the direction of higher voltages, which intuitively didn't make sense. Following the definition of touch potential:

The touch potential is the potential difference between the GPR and the surface potential at a point where a person is standing with a hand in contact with a grounded structure.

Due to time constraints I'm going to have to put this one to bed with a less than complete understanding of exactly what is going on. It would be nice if SKM provided a value for the surface potential and the ability to model different rho's simultaneously. Not to mention an area specific fault current where multiple faults could be profiled. Certainly, the largest fault current is going to affect the area that it occurs in more than an area distant from it. For tight grid & touch potentials this could come in very handy. The approximaions are very close to the SKM results and, incidentally, were verified with a MathCad sheet and by hand.

I can get the touch and step potentials to far outstrip the GPR by limiting the time of fault (or by adjusting the area of the grid in the GPR calculation...since it's not symmetrical). This tells me that a person can be exposed to the lareger fault provided it's for a very short duration. The shorter the duration, the larger the fault that can be tolerated. So there is a relationship between the touch and GPR, but to say the touch cannot exceed the GPR doesn't make sense as there is a time element involved in the touch. Any more thoughts on the matter would be appreciated.

 

[rcw retired EE]

nchrista- GPR is the driving voltage that creates the touch potential. Etouch < GPR. Etouch = k x GPR where k is <1.0. Refer to the quick estimate graphs in IEEE 80 for an example.

I believe we are still having communication problems and confusion about Etouch, Etouch Max Allowable and GPR.

GPR is the maximum voltage rise the grid sees relative to remote earth during a fault. GPR= Igrid x Rgrid.

Etouch Max Allowable is not related to GPR but is a function of the fault clearing time, effective foot resistance and body weight. Grid layout or fault current do not change the Allowable Touch Voltage.

Etouch is the calculated touch potential between the grid and a point on the surface. It should be calculated for every point in the plant and compared to the maximum allowable.

It is possible and desirable to have the Etouch Max be greater than the GPR. But the worst case calculated Etouch cannot be greater than the GPR. If the calculated or simulated Etouch exceeds Etouch Max, more grid or a redesign is needed.

 

[jghrist]

You had it right when you said

The touch potential is the potential difference between the GPR and the surface potential at a point where a person is standing with a hand in contact with a grounded structure.

The surface potential is only zero at remote earth (the place where all the fault current flows to). Anywhere else, the surface potential is positive, so the difference between the GPR (the grid potential) and the surface potential is always less than the GPR.


The voltage in the soil is reduced by current flowing from the grid through the resistance of the soil. The surface potential is just a special case of the voltage in the soil.

 

[nchrista]

I understand the equations and the relationship between GPR and Touch Potential. I believe what GroundMat has shown me is correct only had to adjust my thinking to interpret the results correctly. GroundMat shows a safer arrow going in the direction of increasing potential. What the good folks over at SKM are saying is that the grid itself is better (more robust) in the direction of increasing voltage. The 3D view shows this quite plainly. They are not trying to say that higher voltages are better, only that the more voltage your grid can withstand the better.

As far as the touch potential values are concerned, the limit is the limit, no brainer there. The lower values (~50V) shown on the 3D contour curve is saying the actual voltage values in this region are only 50V and as long as these values don't breach the touch limit of 410V then your ok. That is why the safer arrow points in the direction of decreasing voltage on the 3D contour map. Thanks to all for your input.

 

[jghrist]

Without actually seeing what the GroundMat 3D graph is showing, I'll hazard a guess. It is probably showing surface voltages. The safer arrow going in the direction of increased voltages means that the higher surface voltages are safer. That's because they are closer to the grid voltage and the touch-potential is the difference between the grid voltage and the surface voltage.

If this is correct, then the lower values of voltage are not safer. A surface voltage of 50V represents a touch-potential of 500V if GPR = 550V.

The touch-voltage graph will be an inverse of the surface-voltage graph and will look like this:

Note that the safe arrow is down, toward lower voltages.

 

[Ikol]

If you drive grounding rods of varing depths throughout the grid, you can effectively render the grid electrically neutral,(at surface level),as opposed to differing "hot-spots" due to concentrations of activity.

The process virtually reverses the topographical electromagnetic potential by transferring it deep in the surrounding earth, and replacing it with a calculated "mean" value. Current will be induced to flow underground, and this will further isolate the grid above by creating a barrior via polorization.

We have been using this procedure for decades in the former USSR.

When driving the numerous deep ground rod system, each rod's conductivity to its closest neighbour must fall between 13.6 and 14.3% of that at ground level.

Be sure that no one can calculate the final configuration, angle and depth of the grounding rod matrix required to fit the bill here, and in order to effect optimal results, the "tip area" of all of the rods driven, (summed by triangulating all combinations of the two dimentional areas drawn between the entire ground-rod matrix.) This summed area must fall between the square root of 2, and the square of 5 of 0.333 of the surface grid area. You have to Choose a rod as primary first, then build on that: Drive, test, calculate: Drive, test, calculate: Drive, test, calculate. Many comrade become crispy critter in former USSR untill we solve problem. Good luck: Wear rubber.

 

[nchrista]

jghrist,
Can you also post the GPR contour map. As I've stated, it makes sense to me that the touch would be safer in the direction of decreasing voltages. As you've stated, the GPR contour is the inverse of the touch, and it shows safer in the direction of increasing voltages. I believe this is because it's saying your grid is stronger and safer as its ability to withstand higher voltages improves.

 

[jghrist]

The image in my previous post is just an example from the SKM website. I have no way of posting the Earth Potential map.

You are trying to read too much into things. It is very simply why touch voltage is safer in direction of higher earth surface voltage.

Vt = GPR - Vs

where: Vt is touch voltage, GPR is the grid voltage, and Vs is the earth surface voltage.

For a given GPR, if Vs goes up, Vt goes down.