In other words, sticking 2 rods immediately next to each other in the soil would lead to very minimal benefit (kind of like increasing diameter of rod). The two rods shells are influencing each other. The further and further you go out, the less you are in the lets say "saturated" soil influence by the first rod. Placing rods 25ft apart would yield a situation where the shells of influence of each rod are completely not overlapping. You still can't reduce the total resistance by 2, but the math is much better.
Ground Rod Theory
- Thread starter infinity
- Start date
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
But at 6' spacing the two 'spheres' overlap by 3', which can be an issue.
The suggested separation is 2X the length to eliminate all significant overlap.
Yes, but the question is why did they choose 6ft. Because they need to make a rule to govern thousands upon thousands of installations of year. Some of which are on very small lots, requiring spacing of 25ft (or even say 16ft) would start to make a lot of installations difficult.
If you get several rod lengths away then the math does actually get very simple (last sentence of post #61. If you drive 2 and you are say 3 rod lengths away, the resistance can be divided by 2. If you are not greater than several rod lengths, but at least say 1 rod length the Green book gives you some factors to consider, see excerpt below:
Rods being say 8ft usually in the dirt, can take advantage of the factors, if placed 8ft apart. But again the question then is why 6ft and not 8ft.
LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
- Occupation
- Electrical Contractor
I know why. Because I'm 6'3" tall (well, I used to be) and I can use my arm-span as a distancing device.
The Grounding Electrode System is also for protection from line surges, or unintentional contact with higher-voltage lines and to stabilize the voltage to earth during normal operation. Viz.
250.4 General Requirements for Grounding and Bonding.
The following general requirements identify what grounding
and bonding of electrical systems are required to accomplish.
The prescriptive methods contained in Article 250 shall be
followed to comply with the performance requirements of this
section.
(A) Grounded Systems.
(1) Electrical System Grounding. Electrical systems that are
grounded shall be connected to earth in a manner that will
limit the voltage imposed by lightning, line surges, or unintentional
contact with higher-voltage lines and that will stabilize
the voltage to earth during normal operation.
The better the system the better the protection it can provide. As a Volunteer firefighter, with 45 years of active service, I have seen the Grounding Electrode System save several homes from fire ignition and destruction of their wiring when a cross occurred between Distribution and secondary distribution lines. That didn't necessarily save anything that is electronic and uses solid state components from being destroyed but that is better than having insulation damage on every conductor in the building.
Tom Horne
Grounding is seldom an effective safeguard against electric shock. It is the bonding of the conductive non current carrying parts of the electrical system back to the source of supply, which is accomplished by the installation of a bonding conductor erroneously called an Equipment Grounding Conductor in the US National Electric Code (NEC), which provides the first layer of protection. That is backed up by the installation of Ground Fault Circuit Interrupters were required which actually provides protection from electric shock by voltage which originates in the premises wiring system.
Grounding, when installed specifically for the purpose to standards much more stringent than those in the NEC, can reduce the touch and step potential caused by lightning, downed distribution or transmission lines, and accidental equipment contact with distribution lines.
Tom Horne
Research done by the National Institute of Standards and Technology (NIST) and the Massachusetts Institute of Technology (MIT) have shown that the returns on separation diminish at twice the length of longest driven rod apart of 2 rods. Multi rod arrays are much more complex to predict. The other facet of driven rod installations that I've never seen any research on is the effect of longer rods on the twice the length of the longest rod rule of thumb. In actual practice I have driven "stacked" rods to depths up to 50 feet. What I have observed is a sudden drop in the impedance to ground at varying depths. I always believed that was the result of going though a different type of soil strata or by getting below the permanent water table.
I've also done some by guess and by "divine intervention" installations that I found instructive. While working on an Air Force Base's Street Fire Alarm Box telegraphic signal stations' grounding I was finding it very hard to get a low enough resistance to ground to allow the signal boxes to transmit to the fire alarm office using the earth as one conductor of the circuit.
I had the use of a utility vehicle which had a small drilling machine in the side of the utility body of the truck. It was intended for driving ground rods around buildings being hardened against Electromagnetic Pulse (EMP). That work having been done long before my involvement in the fire alarm work the vehicle was available so I was able to get the use of it. I made it through 3 days of driving coupled 10 foot rods to depths up to 50 feet until the resistance to ground got low enough to transmit by ground return before I paused to think why the ground return pathway for a 100 milliampere current was so consistently poor. Thinking about that over lunch I realized that there was a common point to all of those ground return pathways. That was the Grounding Electrode System at the central fire station through which that 100 milliamperes had to reach the receiving apparatus at the fire alarm room. Your right if you think that is not one of the brightest moments anyone ever had.
I returned to the fire station and asked the senior Non Commissioned Officer (NCO) if he would show me how to measure the Grounding Resistance of a grounding electrode. He did and I took the measurement and then asked him to tell me why it was so high. He came out and redid the test and said it was definitely not right. He got the Ground Electronics Engineering and Installation Agency (GEEIA) involved and they reran the test with a meter of their own. They got an even higher impedance than we had although not by much. They examined the entire grounding system and discovered that the World War 2 installed ground rods were 10 foot longs pieces of plain steel reinforcing rods and that the remains of those could be pulled out of the ground quite readily by hand without any tools.
They just said "We would suggest that you repair that" and left. I was now like a dog with a bone. Having been previously detailed to the power production team of the base maintenance unit and already been through basic electronics training I reread all of the available material on Grounding Electrode Systems available at the office. The materials all said that a properly installed Ground Ring was a better electrode than driven rods. I got permission to get a backhoe and operator assigned for 2 separate days. He asked "How deep." I answered "How deep can your machine dig." He ended up about 8 feet deep. The regulation on telegraphic fire alarm maintenance required not less than 4 10 foot driven rods 20 feet apart from any other rod and did not mention Ground Rings at all. I made up the 2/0 AWG bare copper that was what supply had on hand. I got the guys from the power production shop to Cad Weld the rods to the 2/0 copper and drove them into the bottom of the trench using an impact hammer on an extra rod to drive them without going into the trench. As a result each driven rod was 20 feet long and the 2/0 ground ring was 8 feet down. I had read that "Were necessary Bentonite clay was to be used to improve the ground systems conductivity and performance" Several hundred pounds of it were ordered in 100 pound bags. I knew that I would have to do the work but I was more worried about all that work being unproductive after I had asked to do it than I was of the work itself. I didn't want to look like a goof. I put a foot of it in the bottom of the trench and then the Sergeant ordered enough more to get a foot of it on top of the ground ring. When the backhoe operator came back out he tamped the clay by lifting the back end of the machine off the ground with the bucket all the way around the trench. He then back filled while tamping it 2 more times.
When we measure again an got about 4 ohms the sergeant and I though the tester from PMEL had failed. The GEEIA folks came back and reassured us that the reading was only 1 ohm off of their meter and that was within the margin of error of both testers. After that all of the street fire alarm box transmitting stations transmitted fine without any work on their ground rods. I spent several days replacing the ones in the portion of the base that had been built during world war 2 in order to not have to depend on the existing bare steel steel reinforcing bars for their future performance.
Tom Horne
Testing that I did on some of the rods that I have installed confirm that being too close to a basement wall adversely effects performance but not by a great deal unless the basement wall has been coated with waterproofing cement. If the wall is coated the impedance is markedly higher.
Tom Horne
Under the US National Electric Code (NEC) the "Concrete Encased Electrode" must be used were present. In new construction it is always present and our inspectors here are catching it's absence during the ground work inspection of the forms and reinforcing steel.
In most new homes the installation of a true Ufer grounding mat electrode is prevented by the building code requirement for insulation under the slab which insulates the slab from ground. That does not effect the reinforcing steel in the footer though so there is more than enough reinforcing steel to use to comply with the NEC.
Tom Horne
As I wrote above the purpose of system grounding is: "limit the voltage imposed by lightning, line surges, or unintentional contact with higher-voltage lines and that will stabilize the voltage to earth during normal operation." You are correct in stating that System Grounding has little role in human safety.
Tom Horne
One other technique, that is easier to actually use with a hand held radio, is to attach a dribble whip. [A loose wire the same length as a quarter wavelength at the frequency of interest.] That is easily connected to the shield around the antenna connection using a tiny worm screw clamp. It is not useful at frequencies below 140 MHz because the length of a quarter wave whip becomes unmanageable.
Tom Horne
The fault between a distribution line and a secondary distribution line most often occurs between the conductors due to storm damage rather than a transformer "core puncture." Internal faults in secondary distribution transformers are not that common an occurrence.
Tom Horne
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Regardless what you see or hear about on TV weather coverage.
LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
- Occupation
- Electrical Contractor
I believe that only footings qualify for Ufer electrodes.
- Location
- New Jersey
- Occupation
- Journeyman Electrician
Not every new structure will have a footing that qualifies as a CEE. Some footings are poured on top of a material that keeps the concrete from being in contact with the earth.
LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
- Occupation
- Electrical Contractor
I investigated and wrote a report on a fire in the utility room of a home. It occurred during a storm which caused tree damage which caused a POCO primary to hit secondary and service conductors.
Someone had done a non-permitted-or-inspected service upgrade, and connected absolutely no grounding electrode connections, not even the original ones. They also used no cable clamps, etc.
The EGC in the water-heater cable was the only connection between the service and the plumbing system. The fire was caused by the EGC melting and igniting the filler, sheath, then nearby wood.
Someone had done a non-permitted-or-inspected service upgrade, and connected absolutely no grounding electrode connections, not even the original ones. They also used no cable clamps, etc.
The EGC in the water-heater cable was the only connection between the service and the plumbing system. The fire was caused by the EGC melting and igniting the filler, sheath, then nearby wood.
I'm not sure what you are saying here. The code requires that a piece of bare #4 AWG or larger copper wire 20 feet or more in length or a piece of 1/2 inch rebar 20 or more feet in length encased in not less than 2 inches of concrete on all sides shall be used as the Concrete Encased Electrode. If the reinforcing steel is not 1/2 inch or larger; which would be a violation of all of the model building codes; then you can install a 20 foot long bare 4 AWG or larger copper conductor in its place. Viz.
"250.50 Grounding Electrode System. All grounding electrodes
as described in 250.52(A)(1) through (A)(7) that are
present at each building or structure served shall be bonded
together to form the grounding electrode system. Where none
of these grounding electrodes exist, one or more of the
grounding electrodes specified in 250.52(A)(4) through
(A)(8) shall be installed and used.
Exception: Concrete-encased electrodes of existing buildings or structures
shall not be required to be part of the grounding electrode system where
the steel reinforcing bars or rods are not accessible for use without
disturbing the concrete.
250.52 Grounding Electrodes.
(A) Electrodes Permitted for Grounding.
(3) Concrete-Encased Electrode. A concrete-encased electrode
shall consist of at least 6.0 m (20 ft) of either (1) or (2):
(1) One or more bare or zinc galvanized or other electrically
conductive coated steel reinforcing bars or rods of not
less than 13 mm (1∕2 in.) in diameter, installed in one
continuous 6.0 m (20 ft) length, or if in multiple pieces
connected together by the usual steel tie wires, exothermic
welding, welding, or other effective means to create a
6.0 m (20 ft) or greater length; or
(2) Bare copper conductor not smaller than 4 AWG
Metallic components shall be encased by at least 50 mm
(2 in.) of concrete and shall be located horizontally within that
portion of a concrete foundation or footing that is in direct
contact with the earth or within vertical foundations or structural
components or members that are in direct contact with
the earth. If multiple concrete-encased electrodes are present
at a building or structure, it shall be permissible to bond only
one into the grounding electrode system."
Tom Horne
mtnelect
HVAC & Electrical Contractor
- Location
- Southern California
- Occupation
- Contractor, C10 & C20 - Semi Retired
I love this forum ... Where else could you go to express your opinion ?
LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
- Occupation
- Electrical Contractor
Agreed. I'm saying that even a slab on grade with no plastic would not qualify under any circumstances.
That material is only there to prevent any concrete from infiltrating the soil thus requiring slightly more concrete to poor the footings. They are only a cost avoidance technique. At least one of the model building codes forbids that practice and the inspector can require that a listed material be used for that purpose. The additional cost of a listed material will usually deter the use of such footer trench linings.
Tom Horne