Ground Resistance

[Bugman1400]

I really enjoyed the reading from mbrooke and Don and should clarify my last blanket statement about fault current. First of all, the comment about ground rods is not entirely correct. They are not entirely for lightning protection, they do help reduce the step/touch potential for cases where the fault current is extremely high and/or the sub sits on crap soil. Crap soil is the other reason for beefier ground mats. I think the idea to try and make the bonding at the same potential as earth. I've seen ground mats built well outside the sub and well beyond the fence or anything else to touch.

 

[Bugman1400]

I really enjoyed the reading from mbrooke and Don and should clarify my last blanket statement about fault current. First of all, the comment about ground rods is not entirely correct. They are not entirely for lightning protection, they do help reduce the step/touch potential for cases where the fault current is extremely high and/or the sub sits on crap soil. Crap soil is the other reason for beefier ground mats. I think the idea to try and make the bonding at the same potential as earth. I've seen ground mats built well outside the sub and well beyond the fence or anything else to touch.

It sucks not being able to edit your posts.

To add, ground mats beyond 3 to 6 feet of the fence line are uncommon. The biggest impact to reduce resistance between the equipot grid (ground mat) and earth are the ground rods.

 

[don_resqcapt19]

I really enjoyed the reading from mbrooke and Don and should clarify my last blanket statement about fault current. First of all, the comment about ground rods is not entirely correct. They are not entirely for lightning protection, they do help reduce the step/touch potential for cases where the fault current is extremely high and/or the sub sits on crap soil. Crap soil is the other reason for beefier ground mats. I think the idea to try and make the bonding at the same potential as earth. I've seen ground mats built well outside the sub and well beyond the fence or anything else to touch.

I don't see how a lower resistance to earth can reduce the step or touch potential. The lower the resistance to earth, the more current that will flow and the voltage drop on the grounding and bonding system from that current will act to increase the step potential. That is there will be a voltage drop across the grounding mat.

As far as the bonding trying to make everything the same potential as the earth that does not happen. The bonding and grounding mats just raise everything within in the step or touch area to the same voltage. That voltage will be much above the potential of the earth under fault conditions. Raising everything to the same voltage makes the person like the bird on the wire...no hazardous voltage between any two points that can be touched or stepped on at the same time.

Ground mats outside the fenced area can be an extreme hazard under fault conditions if nothing is done to reduce the step potential at the edge of the ground mat. If you just end the mat, then you could have thousands of volts between on foot placed on the earth outside the mat and the other foot placed on the earth above the mat.

 

[Bugman1400]

I don't see how a lower resistance to earth can reduce the step or touch potential. The lower the resistance to earth, the more current that will flow and the voltage drop on the grounding and bonding system from that current will act to increase the step potential. That is there will be a voltage drop across the grounding mat.

As far as the bonding trying to make everything the same potential as the earth that does not happen. The bonding and grounding mats just raise everything within in the step or touch area to the same voltage. That voltage will be much above the potential of the earth under fault conditions. Raising everything to the same voltage makes the person like the bird on the wire...no hazardous voltage between any two points that can be touched or stepped on at the same time.

Ground mats outside the fenced area can be an extreme hazard under fault conditions if nothing is done to reduce the step potential at the edge of the ground mat. If you just end the mat, then you could have thousands of volts between on foot placed on the earth outside the mat and the other foot placed on the earth above the mat.

Don, I see your point about the ground mat if you are restricted to just one connection to earth. You are correct in that the more fault current you have through the resistance to earth, the higher the potential of the mat would be. And, your idea is right if you have no connection to earth. However, if you lower the resistance with more connections to earth, you eventually will get a low resistance between earth and the ground mat to the point that fault current is divided by hundreds of paths to earth. Its like a hundred 1 ohm resistors in parallel between the ground mat and earth.

When the grounding tests are done, the goal is to get the resistance between the ground mat and earth as low as possible. When the resistance is too high, typically more and longer ground rods are used.

 

[don_resqcapt19]

Don, I see your point about the ground mat if you are restricted to just one connection to earth. You are correct in that the more fault current you have through the resistance to earth, the higher the potential of the mat would be. And, your idea is right if you have no connection to earth. However, if you lower the resistance with more connections to earth, you eventually will get a low resistance between earth and the ground mat to the point that fault current is divided by hundreds of paths to earth. Its like a hundred 1 ohm resistors in parallel between the ground mat and earth.

When the grounding tests are done, the goal is to get the resistance between the ground mat and earth as low as possible. When the resistance is too high, typically more and longer ground rods are used.

But as the person in the substation under fault conditions, I really don't care what the voltage is to earth, I only care what the voltage is between the points I can touch.