ECN Forum Forums Technical Discussion Electrical Theory and Applications Does ground rod work??

I do not believe a ground rod would have done anything for you.

Would proper bonding of one service have protected the other?

Hard to say,no reason the bonding for one service would be large enough for the other service. One service might be 100 amps and the other 600 amps.

Also these services came from separate transformers so whatever bond wire between them on the primary side may not be large enough to carry the full secondary current.

JMO hopefully others will jump in.

Bob

The ground rod is my point. I fail to see its need for service bonding. Electricity will not travel miles throught the ground with all the different resistance back to the source, when it will takes its quickest, least reisitive path. Agreed??
I think someone mentioned dissapate. It wont dissapate. Physics proves electrons travel in lets say a loop. What leaves comes back in full. Cannot see current traveling through rock dirt, fill whatever, when a system is properly bonded at the enclosure, it should travel back through the grounded service conductor.

Here is a graphic from the handbook that labels the conductors of a grounding electrode system.




A ground rod by itself can become very important if you have a primary voltage conductor fall across the bare neutral of a triplex heading toward a building.

In that case the voltage might be 7000 volts or more to ground.

A 25 ohm ground rod with a 7000 volt fault to it will result in 280 amps of current flow, (forgetting about the POCOs grounding resistance) well above the primary overcurrent protection device.

In that case the ground rod would aid in the opening of the primary overcurrent device.

Then there is some added protection from lightning strikes.

There are a few cases where a "25 ohms" ground at the service entrance can be useful. Consider a "line cross," or primary-to-secondary fault in a distribution transformer. For a 13.8KV circuit, you may expect to see ~8kV-to-ground on the low-voltage service-entrance wiring. 8000V/25 ohms = 320A, which should be able to operate some upstream 13.8kV overcurrent devices. Distribution-switching transients and indirect lightning hits can force significant current through a 25-ohm "made electrode."

No question that concrete-encased steel or significant underground bare-metal piping can beat out 25 ohms for "better" grounding electrodes, but 25 ohms may be useful in some caes. Always consider properly-sized bonding as a good means of limiting potential difference between exposed conductive electrical-enclosure surfaces.

One plug... IAEI Soares Book on Grounding—even a used copy—is well worth having for code-based grounding concepts and practicalities. [Also, the 1954 R H Kaufmann AIEE paper is excellent for detailing real, high-current tests of AC fault-current propagation that much of present codes are based on.]




[This message has been edited by Bjarney (edited 06-20-2004).]

Bjarney,
Is the resistance of 25 ohms a standard figure in the US?.
It sounds quite high, compared to a maximum Electrode + Ground Lead resistance value of 1 ohm combined, given in our Regulations.
Mind you, our TN-C-S system relies on high Fault currents to disconnect faulty circuits in under 0.4 seconds.

Trumpy, that is a good question. I have searched for an answer in earlier IEEE and AIEE Transactions {the predecessor to IEEE} and found no history, other than the nominal "25 ohms" has been used for many decades. I believe that particular benchmark/default value of resistance is listed in the 1953 National Electrical Code Handbook, and I don’t think it was new then. Mind you, it is supposed to be a absolute maximum, but there is no specific code requirement that every electrode or ground grid be tested after installation.

Nowadays in the NEC, two driven rods suffice for a grounding electrode without testing. In parts of the continent, it is a very hard number to achieve, where in other places, one driven rod tests suitably low. {Often the very desirable “Ufer ground" re-bar with concrete encasement is acceptably and consistently low regardless of weather and soil conditions.}

Other E-C.net members here have reported that some areas of Canada routinely use [nonferrous?] buried-plate electrodes as a alternate to rods.




[This message has been edited by Bjarney (edited 06-20-2004).]

Just as a point of reference, last time I checked the driven galvanized pipe that is my electrode at the service panel [from 1935?] reads about 65 ohms with the plumbing/water-pipe bond isolated, but the steel casing on the water well with static water at around 70 feet measures about 2 ohms using a conventional Biddle “3-lead”/fall-of-potential test set.

There is a 1970 paper ieee-pcic.org/archive/fagan_lee_paper.pdf that is sort of a followup to George Ufer’s earlier work.

At a new public-school building once, a contractor wanted a test on a driven rod, which measured about 22 ohms. He thought that may be ‘too close’ to 25 ohms and likely the inspector would accuse him of “fudging the numbers,” so he used a bucket of water to soak the two test electrodes. Then, he blew a gasket when the retest reading increased to about 23 ohms(!)




[This message has been edited by Bjarney (edited 06-20-2004).]

I only choose 25 ohms for lack of a better figure.

The NEC uses 25 ohms as a cut off point for requiring a second ground rod.

That said once you add a second rod there is no requirement to achieve any amount of resistance.

Looking at the replies so far in this thread, it would be very redundant for me to reply also! (yet here I am replying! ...)

The topic is really interesting, so if I can figure something non-boring, interesting and concise, and most importantly - get some time to do so, I would like to toss in a few additional comments to this thread.

As a side bar, here's a Pirate Joke for everyone's amusement:

Q: Where Do Pirates Take Their Vacations?

A: ARRRRRRRgentina!!!

<add cheezy drum fill>
Tatttttttttattttttttatttttt--- chhhhhing!

Scott35

Just to throw in a couple of alternate ideas from Merrie Olde England:

On TN-C-S supplies, which are effectively like a standard American service where the house grounding system is bonded to the incoming neutral, there is no requirement here to use a local ground rod at all.

In practical terms, there's likely to be a ground connection due to the bonding of water pipes, but as plastic supply pipes are becoming more and more common even that is changing gradually.

In many rural areas of Britain we have a TT system, where there's is no neutral-ground bond, so the ground rod and earth is part of the fault path. The resistance can be considerably more than 25 ohms:
http://www.tlc-direct.co.uk/Book/5.5.2.htm