Insulating gravel vs coarse subgrade gravel

[basilasq1]

Hi,

We are designing a grounding system for a pipeline terminal which is being supplied by a 25kV utility substation. The grading drawings for the plant suggest that the backfill material is ‘225mm pit run gravel and 75mm thick coarse gravel layers’ for the whole plant area plus substation fenced area. While running the soil and touch voltage calculations I have the following concern:

The 75mm thick coarse gravel layer is being qualified as an insulating gravel layer (as recommended in Canadian Electric Code, 36-304) by the civil guys. However, from electrical grounding experience, gravel layer consists of clean washed rock which is free of sand and fines (3000Ωm resistivity as recommended by CEC), is un-compacted and is very difficult for vehicles to drive on. From the info we have, it seems that the material being identified as insulating gravel is compacted for easy vehicular traffic which may introduce fines thus decreasing its resistivity.

My question is, can we go ahead with considering this material as a suitable insulating gravel layer or should recommendations be made otherwise?

 

[davidbeach]

I don't do ground grid calculations, but all of our newer substation has one grade of gravel (small rock) around the electrical equipment where one might walk and a different grade (what I'd actually call course gravel) in the areas away from energized equipment where vehicles might be expected (between the gate and the control house). My personal car has no problems in the areas designed for vehicles but I'd hate to think of what might happen if I tried to drive it up to a breaker or switch.

 

[stevenal]

How was it qualified? Was in qualified in a compacted state? We use material from quarries that have passed a wet resistivity test.

 

[basilasq1]

@davidbeach
Thanks for sharing your experience.

@stevenal
By qualified, I mean that the civil guys are persisting that this coarse gravel top layer is enough to be considered as an insulating gravel layer in our grounding study. But as a grounding engineer, it's my job to recommend if additional insulating gravel with the right properties is required in the substation area.

 

[3I0]

The only way to actually know is to measure the resistivity of the material, preferably taking into consideration the effects of moisture content/temperature. Another consideration is making sure it is installed in such a way to prevent vegetation growth.

IEEE 80 provides guidelines on how to calculate safe touch/step voltage levels for a given surface layer resistivity, upon which the 3000 ohm-meter crushed rock allowable touch/step potentials in the CEC are based on.

 

[cuky2000]

Yes, you could go ahead considering this material as a suitable insulating gravel layer provide that meet the safety criteria for step and touch potential.
The resistivity of a compacted gravel with fine could be estimated roughly 1000 ~1200 Ωm. To obtain the desirable insulating level the thickness could be increased to obtain comparable insulating layer if use clean washed rock which is free of sand and fines. Work with your civil team to excavate extra few inches if require meeting the approved grading and backfill plan.
Although you might face some challenges in obtaining the budget for extra test, here is couple suggestions few steps that could be implemented to mitigate the uncertainties meeting the safety requirement:
a) Perform a wet sample resistivity test on the prospective material to be used.
b) Perform a step and touch potential test on site to verify the calculation.
I hope this help

 

[basilasq1]

@va3mnr
Thanks for your suggestion. We did try completing a test for top soil layers. However early winter frozen soil + highly compact top layer effectively rendered those measurements useless with no budget for further testing. And we cant wait til summer conditions to redo the test as the substation has to be commissioned before that time.

@cuky2000
That really helps going forward. Also it confirms my suspicions that the current gravel top layer will act a pseudo-insulating layer (lower in resistivity than a standard utility-grade gravel of 3000Ωm).

 

[stevenal]

Your civils are really engineers with this kind of cavalier attitude toward safety? If you haven't the budget to do the project safely, I propose you end the project. But my way is pretty cheap, just call around and find out who's tested which product. And I don't believe proper testing per va3mnr is all that expensive. Note that testing of rock prior to spreading is not the same as testing the soil already in place. Link

 

[basilasq1]

Actually, for the electrical and civil group doing the work, grounding was an after-thought. It was only after the client insisted that they agreed to get a grounding study done. Hence they have little idea or appreciation for grounding hazards due to a utility level fault current.
And thanks very much for the IEEE paper. Will definitely go through it.

 

[bacon4life]

Rather than washed or crushed, we use fractured rock. All faces of each stone are relatively flat, so they lock together well enough to drive on, while still having minimal surface area contact between each stone. Also, our typical thickness is 150 mm.

 

[basilasq1]

By 'we use' do you mean a in certain geographical area?

 

[bacon4life]

The utility I work for is in Washington State, USA.

 

[SalKhan]

I agree with cuky2000's suggestions.

Fines reduce the resistivity substantially. Using the tested resistivity of the compacted gravel with fine, determine safe touch/step voltage levels.

I work for utility in Canada, and our standard practice is to use fractured rock with minimum wet resistivity above 3000 ohm-meters. If a tested gravel does not meet minimum resistivity requirements then new gravel source is considered.

 

[pwrtran]

You need to have a specification. Other than the resistivity test you may consider aggregates gradation test either by the quarry or by a third party.
There are always some smalls and fines, the gradation test will confirm whether the test samples meet your specification.
Specify clear crushed stones not rock. Crushed stones have no rounds and they can set and pack while the rocks don't.
When testing the resistivity you have to use de-ionized water not tap water. No excess water csitting on the bottom of the test box and the way you damping the stones has to be consistent otherwise the results vary a lot.

 

[jghrist]

The insulating crushed rock layer increases the allowable touch potential. You can use lower resistivity rock, or even no rock if you design the ground grid for a lower allowable touch potential.

 

[waross]

Actually, for the electrical and civil group doing the work, grounding was an after-thought. It was only after the client insisted that they agreed to get a grounding study done.

Now a qualified grounding engineer has done a grounding study and the civil group won't accept the results?
How do you spell HUBRIS.
Point out to the civil group that they are not qualified to comment on or to change the grounding specs.


Bill
--------------------
"Why not the best?"
Jimmy Carter