STEP and TOUCH VOLTAGES ASHPHALT
STEP and TOUCH VOLTAGES ASHPHALT
(OP)
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
Thanks






RE: STEP and TOUCH VOLTAGES ASHPHALT
This standard states the procedure to calculate the allowable step and touch voltages and tables with typical surfaces material resistivities.
RE: STEP and TOUCH VOLTAGES ASHPHALT
Deep ground rods using rod couplings also help. You can get both threaded and threadless couplings from Erico among other manufacturers.
RE: STEP and TOUCH VOLTAGES ASHPHALT
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.
RE: STEP and TOUCH VOLTAGES ASHPHALT
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:
a- Dry Conditions: (0.2 to3.0)x107 Ohm-meter
b- Wet Conditions: (1.0 to 600)x 104 Ohm-meter.
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
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.
RE: STEP and TOUCH VOLTAGES ASHPHALT
RE: STEP and TOUCH VOLTAGES ASHPHALT
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.
RE: STEP and TOUCH VOLTAGES ASHPHALT
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:
hs= ha+k.(ha).(δd/δa)
Where:
hs= New surface thickness
ha= Asphalt thickness
hd= Dry material thickness
ra= Asphalt resistivity.
rd= 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.
I hope this could help.
RE: STEP and TOUCH VOLTAGES ASHPHALT
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.