IEEE Ground Grid

[timm333]

I have question about IEEE-80 ground grid. IEEE-80 says that the total depth of the horizontal ground grid below grade should be 18 inches. But it does not give information that how much should be the depth of the soil, how much should be the depth of crushed-rock, and how much should be the depth of the asphalt.

Is 18 inches sufficient for the vehicle-traffic inside the substation? Thanks for help!

 

[mivey]

the grid is 18 in down in the dirt. The rock has insulating properties and is a layer on top of the dirt that reduces step potential.

add: With 18 in of dirt and a layer of gravel we have never had a problem.

add2: I've never put asphalt in a sub.

 

[timm333]

IEEE-80 9.2-b(1) states that the depth is 18 in below grade. Will the layer of rock/gravel be above the grade level?

For most of substation the gravel top would be Ok. But vehicles cannot move on the gravel, so some roads with asphalt are needed for the vehicle traffic. How much should be the thickness of back-fill material below the roads?

 

[kingpb]

IEEE-80 9.2-b(1) states that the depth is 18 in below grade. Will the layer of rock/gravel be above the grade level?

For most of substation the gravel top would be Ok. But vehicles cannot move on the gravel, so some roads with asphalt are needed for the vehicle traffic. How much should be the thickness of back-fill material below the roads?

The depth of the grid burial is measured from top of finished soil, prior to adding the crushed rock or other surfacing. The rock added is in addition to the 18in. consequently an asphalt road is also in addition to the 18 in of soil. Refer to Table 7 under Section 13.1 to get an idea on the high value of ohm-m for asphalt, and refer to Section 7.4 for the value of adding the surface material.

 

[timm333]

Thanks. So for the substation-yard there would be 18 in of soil above the horizontal ground grid, plus 6 in of gravel (total 2 feet). And for the substation-internal-road for vehicle traffic, there would be 18 in soil plus some thickness of sub-base, some thickness of base-course, and some thickness of asphalt (total 3 feet). The main problem is that there is difference in the height of the substation-internal-road and the substation-yard (3 feet height of the road compared to 2 feet height of the yard).

How to handle this situation to make the substation-yard and the substation-internal-road are at the same height?

 

[timm333]

I think thermal resistivity of soil is not much important for ground grid, as there is no heat generated in ground conductor under normal conditions. Is it correct?

 

[Julius Right]

I don't see somewhere in IEEE 80/2013 a such limitation of the grid depth.
From IEEE 80/2013 in clause 9.2 the "shallow depth
(usually 0.3 m to 0.5 m [12 in to 18 in] below grade)" is required indeed, if the freezing it is not plausible and it is explained why do we need grounding rods.
In clause 16.2.5 Grid geometry it is written: "typical grid depths range from 0.5 m to 1.5 m." and in clause16.5.2 Step voltage (Es): For the usual burial depth of 0.25 m < h < 2.5 m...
We need hs[gravel or surface material coverings, usually about 0.08 m to 0.15 m (3 in to 6 in) in depth] in order to calculate Cs[formula 27] and it is connected to ρs [Surface layer resistivity] and h is connected to ρ [average soil resistivity].

 

[timm333]

I think I am not able to explain the question. Let me put it this way. If we have 0.5 m of soil and 0.15 m of asphalt, the top of the asphalt would be at total height of 0.65 m (above horizontal ground grid) and it will fulfill the requirements of IEEE-80. But the question is that are the vehicles allowed to run on this asphalt, or do we need some different special arrangement for vehicles? What should be the minimum height of the top of the asphalt (above horizontal ground grid) if vehicles are present?

 

[mivey]

IEEE-80 9.2-b(1) states that the depth is 18 in below grade. Will the layer of rock/gravel be above the grade level?

For most of substation the gravel top would be Ok. But vehicles cannot move on the gravel, so some roads with asphalt are needed for the vehicle traffic. How much should be the thickness of back-fill material below the roads?

The gravel is above grade. This helps keep dirt from washing into the gravel and destroying its insulating properties.

Vehicles travel fine on gravel; cars, trucks, line trucks, cranes, etc. are not a problem. Never have been. We use a #57 gravel on top. Small, roundish river rock or pea gravel is too smooth IMO and does cause travel issues. Not really sure what kind of travel issues you have had but something might be wrong with your design.

 

[ron]

I think thermal resistivity of soil is not much important for ground grid, as there is no heat generated in ground conductor under normal conditions. Is it correct?

Don't sweat the thermal resistivity of the soil, but the soil resistivity itself and how it relates to the overall ground resistivity and step / touch potential calculations for IEEE 80 is the issue. Testing is IEEE 81.

The last project I did at high voltage 345kV, we had to work with a consultant and a software program to estimate the step / touch potential calculations, then they tested.

 

[Julius Right]

NEC Table 300.50 Minimum Cover Requirements column 6 Areas Subject to Vehicular Traffic,Such as Thoroughfares and Commercial Parking Areas : 600 mm[24"]
note a) Cover is defined as the shortest distance in millimeters (inches) measured between a point on the top surface of any direct-buried conductor, cable,conduit, or other raceway and the top surface of finished grade, concrete, or similar cover.

 

[Tony S]

The few intake substations I’ve been involved with the “earth mesh” was about 2ft below the soil level, the infill, a compacted mixture of soil and Kerphalite. 9in of limestone chippings layed on top.

Kerphalite and Marconite retain moisture, Marconite can be added to cement to improve the conductivity of an Ufer earth.