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.
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