How to add bentonite into IEEE 80

[Power0020]

I am designing an earthing grid in a soil with very poor resistivity (~3000 ohm.m), (calculation returns a resistance of 12 ohm) the resistance required shall be <0.5 ohm which seems impossible, I think we will need to use Bentonite but don't know how to calculate the required quantity to achieve such reduction?

any ideas?

 

[lume7006]

Hello,

You can add something to the soild, however, you should have in mind the following:

1) Despite adding something to the soil, this will be temporal. I mean, in some time the effect of the added material will have passed and the earth resistance will be againg very high.

2) I understand your soil resistance is extremely high, however, is that the value independently of the deepth?
What I would think is about teh possibility to go deeper in order to find a lower resistance lawyer that allows you a better value.

3) The other point could be:
What are the touch voltages calculated using the 12 Ohms?
Do they keep you on the safe side or not?

I think that the target value (0.5 Ohms) is because of the sensitive equipment, so what I recommend you is to look to extend the grounding grid using as much area as possible because in that way the whole resistance value tends to be reduce.
Also take a look at Ufer Grounding systems, the explanation could be usefull to you

Regards

 

[krisys]

The Bentonite vendor should be able to give the exact quantity of material to be used to achieve a certain desired resistivity.

However, to improve the earth resistance, the better method is to go for the deep-well electrodes. This is a permanent solution. We have been using this in the deserts, where the soil resistivity was 3400Ω-m.

 

[Power0020]

well, the bad news is that it is about 12 m of compacted industrial (platics) rubbish, deeper than that will be a bedrock that may be slightly better. The SS buildings will be constructed on piles resting on the bedrock, I thought about using the piles but can't risk the corrosion of heavily loaded concrete for earthing.

Since the station is connected by UG cables with sheaths connected to ground at remote ends, I am sure that the ground return fault current won't exceed few hundred Amperes and the total resistance will be very small (about 10 remote ends, all connected in parallel), would this be acceptable? what about sensitive equipment?

the touch voltage with 12 ohm went crazy exceeding 450 kV!!

 

[ScottyUK]

Touch voltage can't ever be higher than the line-line voltage regardless of what fault you manage to contrive. It also has to obey Ohms Law. That should bring things down to a more realistic level.

 

[lume7006]

One question:

Is your facility based on NEC code?
If it is, then, what short-circuit current are you taking into account for your calculations?
The one in low voltage or the one in medium?

This could be easier if you have a very clear picture of your grounding system and real needs

 

[krisys]

We should know what is the system voltage you are talking about and what is the maximum earth fault current. Also please note that if it is an earth mesh, the value looks to be ridicules.

My experience is that for the switchyards or substation having earth mesh, the touch potential and step potentials are not a big issue. By increasing the quantity of grid earthing conductor you can bring it down to the acceptable value. The main problem is with the "Ground Potential Rise GPR)"

SO in my opinion you have to change the way you look at the problem.

 

[krisys]

 well, the bad news is that it is about 12 m of compacted industrial (platics) rubbish, deeper than that will be a bedrock that may be slightly better. The SS buildings will be constructed on piles resting on the bedrock, I thought about using the piles but can't risk the corrosion of heavily loaded concrete for earthing.]

Please note that the deep well earthing station need not be in the same substation. It can be made in a relatively friendly terrain. Then you should connect the deep well earth station to the new substation ground grid with adequately sized earthig conductor. Sorry, that I am not using the precise terminologies from NEC, as I am now not using the NEC!!!

 

[HamburgerHelper]

Some utilities up north use at a minimum 20 ft grounding rods to get below the frost depth to get a good ground.

With respect to corrosion, concrete is sometimes used to encase a grounding rod to prevent deterioration of your ground due to corrosion. Maybe, you can use the piles for grounding if you choose the right concrete.

 

[karls2]

If you can't improve easily the ground resistor you can try:
increase the value of the touch voltage by add surface material like washed granite similar to pea gravel,
to take in account the split factor in order to reduce the value of the current in the ground resistor who will allow you to decrease GPR.

 

[karls2]

Concerning the Bentonite I have heard that in time we are not sure we keep the improvment.