Dual Ground Rods

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rlundsrud

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chicago, il, USA
Why are dual ground rods not run in parallel when used to alleviate having more than 25 ohms of resistance on a singe rod ? If you ran them in parallel the resistance would always be less than the value of the lowest of the two (1/Rt=1/R1+1/R2), also if one of the cables became disconnect or damaged there would be some redundancy. I am certain there is some perfectly good reason, but it eludes me.


Bob
 
Only way they would not be parrellel would be if grounds were hooked to separate clamps? And the ground rod itself would not add much resistance unless clamps were on opposite ends of rod?

Or are you thinking of the resistance of the wire?
 
I would think ground rods would be one of the only things where parallel paths are good ?? (lightening dissipation) :blink:
 
rlundsrud said:
Why are dual ground rods not run in parallel when used to alleviate having more than 25 ohms of resistance on a singe rod ? If you ran them in parallel the resistance would always be less than the value of the lowest of the two (1/Rt=1/R1+1/R2), also if one of the cables became disconnect or damaged there would be some redundancy. I am certain there is some perfectly good reason, but it eludes me.


Bob
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Lets us know if you get a good answer on this!
 
IMO the general feeling is that rods are pretty poor electrodes anyway so it doesn't really matter how they're connected. Further proof is that as long as you have two rods their resistance could be 1000 ohms and the NEC doesn't care.
 
infinity said:
IMO the general feeling is that rods are pretty poor electrodes anyway so it doesn't really matter how they're connected. Further proof is that as long as you have two rods their resistance could be 1000 ohms and the NEC doesn't care.
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Agreed. The GEC is fantastically low resistance when compared to the rod-to-earth connection, so it really doesn't matter that all electrodes are wired in series on the same conductor because that is a very small part of the overall circuit impedance.

And the electrodes may not be in parallel with the grounding-electrode-conductor, but they are in parallel with the earth in that current does not have to flow through one electrode in order to get to the second.
 
rlundsrud said:
Why are dual ground rods not run in parallel when used to alleviate having more than 25 ohms of resistance on a singe rod ? If you ran them in parallel the resistance would always be less than the value of the lowest of the two (1/Rt=1/R1+1/R2), also if one of the cables became disconnect or damaged there would be some redundancy. I am certain there is some perfectly good reason, but it eludes me.


Bob
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The reason for driving the rods is to establish a connection to earth. The rods are parallel in respect to their connection to earth. We don't care so much whether the connecting conductor is a single conductor or two conductors because the low resistance either way but the rod to earth connections are higher resistance.

In order to truly have a series between rods would mean to incorrectly use a pair of rods and the earth as a substitute for an equipment grounding conductor. Take the many times used example of a metal lighting pole and an attempted use of a ground rod to serve as an equipment grounding conductor. The series is from the pole to the rod, through earth, then back through another rod or other electrode.
 
I am only pointing out that the standard method for installing a second ground rod is to run a conductor from the first to the second (series) rather than running a separate conductor to it from the point that the first one is grounded (parallel). As I previously explained this would allow for a lower total resistance and provide redundancy as you would effectively have two rods that were separately terminated. I have even heard of inspectors making someone change it because they were in parallel.

Bob
 
rlundsrud said:
I am only pointing out that the standard method for installing a second ground rod is to run a conductor from the first to the second (series) rather than running a separate conductor to it from the point that the first one is grounded (parallel). As I previously explained this would allow for a lower total resistance and provide redundancy as you would effectively have two rods that were separately terminated. I have even heard of inspectors making someone change it because they were in parallel.

Bob
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I think you would seldom run into low enough net resistance from two rods that a single 6 AWG conductor wouldn't handle the current that could be dissipated via the two rods.

You need to get series/parallel straight - either connection method is electrically paralleling the rods. The GEC to the first rod is in series with everything beyond it including the first rod though.

It is just like running a cable from outlet to outlet, but the outlets are not connected in series, they are "daisy chained".
 
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