sorry I didn't mean to make it a sticky note
Per Randy's request
What is the purpose of installing the grounding electrodes on a wiring system?
First just what is a grounding electrode? Most people will think of ground rods. A rod is just one of several different grounding electrodes. In 250.52(A) of the NEC is the outline of the different electrodes.
(1) Metal Underground Water Pipe.
(2) Metal Frame of the Building or Structure.
(3) Concrete-Encased Electrode.
(4) Ground Ring.
(5) Rod and Pipe Electrodes.
(6) Other Listed Electrodes.
(7) Plate Electrodes
(8) Other Local Metal Underground Systems or Structures.
The metal water pipe is required to be supplemented by an additional electrode of a type specified in 250.52(A)(2) through (A)(8) and the rod, pipe, or plate shall be augmented by one additional electrode of any of the types specified by 250.52(A)(4) through (A)(8).
250.50 mandates that where any or all of these electrodes are present they must be bonded together to make one grounding electrode system.
What is the purpose of this grounding electrode system? There are four reasons to install this electrode system and four reasons only. The electrode system is not for letting any current flow to earth. The reasons are outlined in 250.4(A) (1) 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.
Lightning can strike a transmission line several miles away and be carried on these lines to our homes. This connection to earth gives lightning a path to travel to where it is trying to go in the first place, earth.
Line surges can and do come from many different places with lightning being one. A short between the primary and the secondary of a transformer could be another.
Unintentional contact with higher voltages can come from the primary over head conductors falling down and hitting the lower voltage lines.
Then we have the stabilization during normal operation which seems to be a great confusion. The phrase “stabilize the voltage to earth during normal operation” has been in the code for many years. In any system the voltage between any two ungrounded (hot) conductors will be regulated by the source. The voltage between any ungrounded (hot) conductor and a conductor that is connected to earth will be regulated by the source. On a system where no connection is made to earth the voltage between the ungrounded conductors and ungrounded metal parts can be from zero to infinite and considered to be unstable. The grounding or connection to earth keeps the voltage stable.
On any circuit there is no need for grounding in order for the circuit to work. When the circuit is working properly all current is being carried on the conductors installed for that circuit.
In the event of an overload all current is being carried on the conductors installed for that circuit. This overload can be due to much of a load or due to voltage drop but grounding plays no role at all.
In the event of a short circuit all the current is being carried by the conductors installed for the circuit and no grounding is needed.
In the event of a ground fault grounding is not needed. All current will be carried back to the main bonding in the service equipment and then to the source via the neutral conductor.
Many proposals have been submitted to have the name of the equipment grounding conductor changed to the equipment bonding conductor. The substantiation for these proposals in most cases had been the confusion about the job of the equipment grounding conductor.
In 250.4(A)(2) we are told that the purpose of connecting noncurrent carrying metal parts to earth is to limit the voltage to ground on these materials.
In 250.4(A)(3) it states; Normally non– current-carrying conductive materials enclosing electrical conductors or equipment, or forming part of such equipment, shall be connected together and to the electrical supply source in a manner that establishes an effective ground fault current path.
This connection of noncurrent carrying metal parts to the source (the neutral in the service equipment) is what is discussed in 250.4(A)(5) to establish a low resistance path back to the source in order to trip the breaker or blow the fuse.
(5) Effective Ground-Fault Current Path. Electrical equipment and wiring and other electrically conductive material likely to become energized shall be installed in a manner that creates a low-impedance circuit facilitating the operation of the overcurrent device or ground detector for high-impedance grounded systems. It shall be capable of safely carrying the maximum ground-fault current likely to be imposed on it from any point on the wiring system where a ground fault may occur to the electrical supply source. The earth shall not be considered as an effective ground-fault current path.
The last sentence of this section tells us that the earth shall not be used as a fault path simply because the resistance of dirt is too high for the voltage to push the current through it back to the source.
In every case outlined above the earth grounding plays no role what so ever in the circuit. In every case outlined above the circuit over current device be it fuse or breaker will protect the circuit and people.
Earth grounding is for four reasons and four reasons are outlined in 250.4(A)(1) and has no role in how the circuits works in any type of premises wiring system.
The grounding electrode system or the equipment grounding conductor does not take current from the system and dump it into earth. In the event of a hot coming in contact with a metal object that is bonded to the equipment grounding conductor that current is carried back to the service equipment where it then travels back to the transformer via the grounded neutral conductor which will draw a high current that trips the breaker or blows the fuse.
Ground rods installed at swimming pools, outdoor hot tubs, ornamental pools, fountains, wharves, docks, boat houses, and piers are installed for the same four reasons outlined in 250.4(A)(1). The installations or grounding electrodes at any remote building or structure be it a storage building, work shop, detached garage, barn, or dock is for the same four reasons.
There is a big misconception that grounding electrodes installed at places around water is to relieve stray voltages. These grounding electrodes are installed for the same four reasons as any other grounding electrode as outlined in 250.4(A)(1).
If these rods removed stray voltages at buildings and structures around water would they not remove the stray voltages found around our out building and structures as well as our homes?
Are not plumbers troubled by stray voltages doing work on metallic piping systems all the time? This proves that the grounding electrodes installed at the service does not mitigate stray voltages.
Most stray voltages come from the utility which uses the earth as a fault path. The distribution lines are of high enough voltage to push the current through earth to clear any fault currents. In most cases across America the primary voltages at the transformer supplying our homes is 7200 volts and at these voltages is enough to drive over 200 amps through earth and open the fuse supplying the transformer. Most of the fuses supplying our transformers at our homes will be 15 amps or less.
On water where boats and ships cannot earth ground the fault current path must be established through the water craft. This fault current path sometimes is the water therefore giving stray voltages to the water. In these cases the earth grounding at our piers and docks will not remove these voltage gradients. This is accomplished by equal bonding grids and planes which brings the water and anything we come into contact with to the same potential therefore no current flow.
If you find this post informative and useful then please post and let me know that my time was not wasted.
Also if you disagree please post and list the information which disproves anything posted.
Or just post you comment
Last edited by jwelectric; 03-30-2011 at 04:01 PM.
sorry I didn't mean to make it a sticky note
The one thing I do not see addressed here is the voltage drop in wires to earth ground or the differences in remote earth ground potential voltage. I'll be thinking about the rest of it...
Perhaps he means if the grounding conductor is undersized, and it momentarily carries a lightening strike, it will likely melt in a nano second, diverting the flow path to the ground rod.
Last edited by ballvalve; 03-31-2011 at 01:53 PM.
This site is for lightning rods that go on top of a building.
I don’t see the equipment grounding conductor melting during a lightning strike any more than I see the conductors in the link you posted melting.
You may be aware that most ground rods often cannot be driven to their full depth due to rocks. In this area its a given that you might get 4 or 6 feet, then its cut it off and beat it with a hammer to make it look like it went in. A very few might add a second rod to obtain the length.
Would it be permissable or better to lay the rod horizontally in a trench 2 or 3 feet deep and bend it up at the pole?
First you must try up and down, if this doesn’t work then lean it to 45 degree angle and if it still won’t go then bury it 30 inches deep. No need to bend it up
Way back when, when I was in the army, the communications chief at Ft. Bliss, El Paso, Tx said that when they went out on a field excercise to the desert, in order to get a good ground (necessary for some radio communications) they had to prepare long in advance with help of the cooks. They had them order lots of extra salt. Then, when they needed to go out on the exercise, they drove their ground rods/array then mixed up lots of salt water, and then poured it on the ground periodically for the duration of the exercise. This gave them a decent ground plane for the antennae and operation of the radios. Now, how does this apply to a conventional home - not much except to realize that grounds, returns, neutrals, and power is a lot more complicated than most people realize. There are lots of smart people out there that have spent lots of time trying to figure out how to make the things we take for granted work properly. Not following the defined rules can make the whole thing start to fall apart, giving unexpected results, some of which can kill you.
Important note - I'm not a pro
Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013
in the area where I live the dirt is so full of minerals that it is no problem to get a current to travel through the ground. As far as voltage, I get as much voltage through my ground rod as I do through my neutral.... and it's 200 ft to the transformer. A magnet will pick up all kinds of small rocks in my yard. Of course, we're not all that far from Birmingham which is known for it's iron ore and this is coal country too.
Ja, a volt meter deliberately has a high impedance so as not to affect the circuit under test. Decades ago I paid hundreds of dollars for my B&K VTVM for that very reason.
If Randyj knew his Ohm's law, he wouldn't make such a statement unless he were simply trolling.
Here are some inserts from a study done by Institute of Electrical and Electronics Engineers over a five year period between 1988 and 1993 for driven rods for utility power stations and communication companies where low resistive earth grounding is imperative.
What is a transient voltage? As defined; 2: (physics) a short-lived oscillation in a system caused by a sudden change of voltage or current or loadThis paper presents the advantages of deep driven electrodes over shallow (10 feet or less) electrodes
This paper utilizes field data taken from over 140 deep driven electrodes installed over a 5-year period in several states.
According to the IEEE Green Book,3 the grounding electrode resistance of large electrical substations should be I Ohm or less. For commercial and industrial substations the recommended ground resistance is 2-5 Ohms or less. This low resistance is required due to the high potential to earth of the electrical system. Many equipment vendors and communication companies require ground systems of less than 3 Ohms resistance.
This paper evaluates the field data taken from 140 deep-driven ground rods installed between May 1988 and July 1993. Ground rod depths ranged from 15 to 90 feet. All resistance measurements were done with the three point fall-of-potential method using a Biddle Megger, Model No. 250220-1, Null-Balance Earth Tester.
Note that the average 5-foot ground rod measured 66 Ohms and at 10 feet is 29.8 Ohms, by interpolation an 8-foot ground rod would average approximately 40 Ohms. The average 8 and 10-foot ground rod failed to meet the NEC minimum of 25 Ohms or less. Depths of 30 feet are required for 5 Ohms or less. The first 20 feet of depth represented the greatest change in earth resistance.
(According to my math two 8 foot rods in parallel would equal 20 ohms and a current draw at 120 volts of 6 amps not enough to trip a 15 amp breaker or to blow a 15 amp fuse -Mike-)
The majority of the rods ranged in resistance of 0.9 -2.0 Ohms at a depth of 40-60 feet.
As shown by the data presented, the average 8 to 10-foot ground rod will not meet minimum NEC code requirements for earth resistance. The resistance of a shallow (10 feet or less) electrode will vary greatly as seasonal conditions change. Due to high earth resistance, the typical shallow electrode is unable to maintain an electrical system at earth potential during transient voltage conditions and lightning surges. Where stable resistance values of less than 5 Ohms are required, electrode depths of 30-60 feet are necessary.
Transient voltages can be introduced by the magnetic field between the high voltage transmission lines and earth.
Solar flares which flex earth’s magnetic field can introduce transient voltages. (one weber [100 million lines of flux] for one second equals one volt)
As I mentioned before in another thread, I worked on a project for Ontario Hyydro where we were improving the grounding of their lattice towers for lightning protection. In some cases we had as many as 50 ground rods on a single tower.
The company I work for now is a NUG and has hydro generation as well as biomass cogeneration facilities. We also have communication towers and a tall smokestack that frequently gets hit by lightning. Equipment can be affected by induced voltages from the EMP even if not hit directly. We have a truck scaling operation where we needed to take extraordinary measures with bonding due to the efficiency of the grounding with the amount of buried metal.