STEP and TOUCH VOLTAGES ASHPHALT

[ROMM]

Hi I am trying to calculate the allowable step and touch voltages outside a substation given that the soils resistivity is 20 ohm meter and using a surface layer of both dry road base and asphalt. Does anyone have any experience with these calculations and what values of resistivity to use for these surface layers as well as how to calculate the allowable step and touch voltages.

Thanks

 

[sanalv]

You could consult "IEEE Guide for safety in AC substation grounding. IEEE Std80-2000."

This standard states the procedure to calculate the allowable step and touch voltages and tables with typical surfaces material resistivities.

 

[mc5w]

If you have too much ground area under asphalt or concrete you need to consider burying a subsurface irrigation system such as Porous Pipe(R) and adding water so the soil.

Deep ground rods using rod couplings also help. You can get both threaded and threadless couplings from Erico among other manufacturers.

 

[MikeDB]

IEEE 80 is a very good reference.
Your 20 ohm-meter soil is an unusually low resistivity.
I don't think I would count on "dry" road base and it normally has too many fines to be a very good insulator. However, the asphalt is an excellent insulator, on the order of 10,000 ohm-meter, and should reduce step and touch potentials.

 

[cuky2000]

Question 1: …. using a surface layer of both dry road base and asphalt. What values of resistivity to use for these surface layers?

Asphalt is a good insulator with resistivity more than 3 times higher than crushed rocks. A typical range of asphalt resistivity values published by IEEE and other sources are as follow:
[sub]a- Dry Conditions: (0.2 to3.0)x10[sup]7[/sup] Ohm-meter
b- Wet Conditions: (1.0 to 600)x 10[sup]4[/sup] Ohm-meter.[/sub]

The lowest resistivity value of 10,000 Ohm-m is often used to determine the allowable step & touch potentials.

QUESTION 2: …the soils resistivity is 20 ohm-meter and. Does anyone have any experience with these calculations?

As mentioned in a previous post, 20 Ohm-m appear is an unusual low value. However, even if this value is 10 higher, the effect on the step and touch potentials values are not significant different because the high resistivity value of the asphalt.

QUESTION #3:…I am trying to calculate the allowable step and touch voltages outside a substation.

The allowable step and touch potentials in the area covered by asphalt is shown in the enclose site

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

Surface Material Step Potential Touch Potential
Asphalt (10,000 Ohm-m) ~ 6,900 ~1,800
Native soil (20 Ohm-m) ~180 ~170

[sub]COMMENTS:
1- The critical lowest potentials values are in the native soil.
2- Allowable step and touch potentials for native soil with 20 to 200 Ohm-m do not change significantly.[/sub]

 

[ROMM]

Hi Cuky2000, thanks for your reply, but I cant access the website link you provided. Also when calculating the allowable step and touch potentials do you have to consisder the layer of dry road base in between the asphalt and the 20 ohm meter soil and then calculate an average surface layer resistivity or do you just use the 80mm layer of 10000 ohmmeter ashphalt in the calcuations for allowable step and touch potentials regardless of how thick or type of dry raod base layer is underneath the asphalt?

 

[MikeDB]

ROMM,
I think it would be conservative and easier to assume one soil below the asphalt, at the 20 ohm-m. The road base can be wet and has enough fine (small) material to make a poor insulator anyway.

 

[cuky2000]

Sorry for the inconvenience accessing the website link. Please try the new link below.

http://cuky2000.250free.com/Step.pdf

The calc presented do not include the dry road layer since there is not information available regarding the resistivity, thickness and how keep this layer without contamination.

I concur with ROMM since there is a lot of uncertainties how to keep the dry layer resistivity under control in time. If still there is need to estimate the insulation effect of the dry layer thickness considers the following suggested relation:

h[sub]s[/sub]= h[sub]a[/sub]+k.(h[sub]a[/sub]).([δ][sub]d[/sub]/[δ][sub]a[/sub])

[sup]Where:
h[sub]s[/sub]= New surface thickness
h[sub]a[/sub]= Asphalt thickness
h[sub]d[/sub]= Dry material thickness
r[sub]a[/sub]= Asphalt resistivity.
r[sub]d[/sub]= Dry material surface.
k = Factor <1 to take in consideration contamination.

Notice that if the resistivity and thickness of the dry road layer is small compared with the asphalt or if k=0, its effect may be neglected.[/sup]

I hope this could help.

 

[jghrist]

You could model the road base as a soil layer. I modeled a simple grid (100'x60' with 20' spacing and 10' rods at each connection) and got a maximum touch voltage of 66.0 volts with 1000A grid current with a uniform 20 ohm-m soil. I used SES AutoGrid Pro software. Allowable touch voltage at 0.5 sec fault clearing for an 80 mm 10000 ohm-m insulating layer is 1671 volts. This is different from cuky's because it includes a decrement factor. Maximum touch voltage would exceed the allowable with a 25300A grid current.

With a one foot layer of crusher run of 1300 ohm-m resistivity, the allowable touch voltage becomes 1827 volts and the maximum touch voltage within the station becomes 72.8 volts for a 1000A grid current. Maximum touch voltage would exceed the allowable with a 25100A grid current.

In this simple example, modeling the road base is slightly more restrictive than assuming a uniform soil. Note that IEEE-80 does not consider a 2-layer soil when determining the allowable touch voltage; the upper-layer soil resistivity is used. This is why the allowable touch voltage is higher with the 2-layer model. In reality, I would expect the lower layer to affect the surface layer derating factor.

I conclude that it is not worth considering the road base.