ground resistance value

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vinod

Senior Member
Location
Saudi Arabia
i am working in Saudi Arabia.

We installed a new grounding grid with 4 number of grounding electrode for a new substation. and we are using PVC insulated direct buried ground cables.

our project spec says;
"sufficient grounding electrodes shall be installed to obtain a maximum electrode resistance to earth of 5 ohms. resistance to each equipment like Switch gear, transformer, etc shall be below 1 ohm"

What does it means.

Did we need each electrode <5 ohm or all these 4 electrode connected together?.

if you have any reference for the subject please help me

Vinod:?
 
B

broadgage

Senior Member
Location
London, England
The spec is not entirely clear.
Each part on its own makes sense, but not taken together.

To provide a system of earth electrodes giving a total resistance of 5 ohms is clear enough, simply install sufficient ground rods or other electrodes to achieve this value.

It then states that each item of equipment shall have a ground resistance of 1 ohm, dont see how this can be achieved of the grounding electrode system is 5 ohms ?
It would seem therefore that the grounding electrode system needs to be below 1 ohm, possibly well below to allow for resistance in the ground conductors to each item of equipment.

That would render the first part of the spec redundant.

Or perhaps they mean that the grounding electrode system should be 5 ohms, and that the grounding conductors BETWEEN items of equipment should be 1 ohm.

Achieving either figure is likely to be a challenge in the dry and sandy/rocky conditions that prevail in many parts of Saudi Arabia.
Grounding electrodes may have to be very deep to reach moist soil. This can often be best achieved by well drilling techniques, rather than by digging.

P.S. I hope that no one is trying to achieve a ground connection by burying INSULATED conductors ! I have seen it done !
 
V

vinod

Senior Member
Location
Saudi Arabia
Those TWO values (1 and 5 ohms) also confused me too...

During the testing of individual electrode , we got the following readings
21, 5.8, 3.8 and 2.6 ohms.

after connections all electrodes together with the grid, we got around 1.2 ohms.

Is this reading is a acceptable value?

Do we need to repair the first electrode which i got high value 21 ohms?

please advise
 
B

broadgage

Senior Member
Location
London, England
In practice, 1.2 ohms is probably OK, but if the client has speced 1.0 ohm or less, then that is what you should provide.
Why is one so much higher ? dryer soil ? not deep enough ?
 
zog

zog

Senior Member
Location
Charlotte, NC
vinod said:
Those TWO values (1 and 5 ohms) also confused me too...

During the testing of individual electrode , we got the following readings
21, 5.8, 3.8 and 2.6 ohms.

after connections all electrodes together with the grid, we got around 1.2 ohms.

Is this reading is a acceptable value?

Do we need to repair the first electrode which i got high value 21 ohms?

please advise
Click to expand...

What method are you using to get those readings?
 
K

kenaslan

Senior Member
Location
Billings MT
dry sandy soil is 1000 ohms per meter. Are your rods 10M long? Also without direct copper to soil you may end up with both step voltage as well as touch voltage. What is the voltage of the substation. My guess is you need at least 10 or more grounds. You can always try a chem rod. Are you, or the utility using CP? If so I can see why insulated ground wire.
 
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GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
broadgage said:
The spec is not entirely clear.
Each part on its own makes sense, but not taken together.
Click to expand...
One way to reconcile the language is to interpret it to mean that the parallel resistance of the ground array must be less than 5 ohms while the resistance of the EGC, GEC, or other conductor connecting each piece of equipment to that array must be less than one ohm. That would be satisfied if the actual resistance to ground from each piece of equipment is less than 6 ohms.
And the 4 electrode array would meet the spec if each one were 20 ohms to ground AND they were located outside each other's field of influence. Four identical rods of 20 ohms resistance when measured individually, all pounded in next to each other, will not produce a 5 ohm array resistance!
 
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V

vinod

Senior Member
Location
Saudi Arabia
kenaslan said:
dry sandy soil is 1000 ohms per meter. Are your rods 10M long? Also without direct copper to soil you may end up with both step voltage as well as touch voltage. What is the voltage of the substation. My guess is you need at least 10 or more grounds. You can always try a chem rod. Are you, or the utility using CP? If so I can see why insulated ground wire.
Click to expand...

1.10 ft long electrode

2. max 4.16 Kv

3. As of now according to the design drawing only 4 ground electrodes are installed and this system will tie up with existing ground grid

4. we are using 10 ft Stain less steel rods..

5. CP applied on near by underground pipes.
 
V

vinod

Senior Member
Location
Saudi Arabia
GoldDigger said:
One way to reconcile the language is to interpret it to mean that the parallel resistance of the ground array must be less than 5 ohms while the resistance of the EGC, GEC, or other conductor connecting each piece of equipment to that array must be less than one ohm. That would be satisfied if the actual resistance to ground from each piece of equipment is less than 6 ohms.
And the 4 electrode array would meet the spec if each one were 20 ohms to ground AND they were located outside each other's field of influence. Four identical rods of 20 ohms resistance when measured individually, all pounded in next to each other, will not produce a 5 ohm array resistance!
Click to expand...

Yes Sir, You are right...

Thanks..
 
J

John120/240

Senior Member
Location
Olathe, Kansas
kenaslan said:
dry sandy soil is 1000 ohms per meter. Are your rods 10M long? Also without direct copper to soil you may end up with both step voltage as well as touch voltage. What is the voltage of the substation. My guess is you need at least 10 or more grounds. You can always try a chem rod. Are you, or the utility using CP? If so I can see why insulated ground wire.
Click to expand...

suemarkp said:
What is CP?
Click to expand...

I have wondered also what "CP" refers to ? Some type of chemical treatment ? The above response by kenaslan is the first usage of "CP"

that I have seen in this thread/
 
G

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
John120/240 said:
I have wondered also what "CP" refers to ? Some type of chemical treatment ? The above response by kenaslan is the first usage of "CP"

that I have seen in this thread/
Click to expand...

I think that in this context, CP refers to Cathodic Protection, in which a small DC current is deliberately supplied to the underground pipe system (relative to earth ground) to make sure that any chemical action will be adding material to the pipe rather than corroding it.
 
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