Why can I not use multiple earth grounds in house circuit?

If you ground in any other place than the service then you are using the ground as a return path to the service. That is a against the code, because it is dangerous. The idea is to open the effected overcurrent breaker. Using the ground as a return path may or may not open the overcurrent device. Lets not get into high impedance grounds on this thread.

Supplemental grounding is allowed with the proper installation. I.E. you use the same size wire as your service. Soars Book on Grounding at your local library. Explains this well. Section 250 of the NEC is hard to understand for some people.

I was working at a University and someone bought a ion beam splitter something like $3 million dollars and 600 amps at 480v. The company would not warranty it unless there was a "clean ground" ( god I hate those words). I refuse to connect the equipment based on the manufacturers recommendations. The engineer from the manufacture came out and we sat in a conference room for 2 hours while I explained to him why he was wrong. He relented and changed the documentation for the machine. ( I have no idea why you want to split ions so do not ask me.)

It really is rather simple. Contrary to popular belief electricity is not trying to return to earth! It is trying to return to the source of the current. For most homes that is a utility company transformer. If the Equipment Grounding Conductor pin of the receptacle is connected to the earth without a low resistance path back to the source the voltage on all exposed metallic parts of the wiring system that are connected to that receptacles green terminal will rise to dangerous levels and stay there until someone is injured or the circuit is deenergized. If the green terminal of the receptacle is bonded back to the neutral of the utility supply then the fault will draw excessive current the moment it occurs and the circuits Over Current Protective Device will open thus deenergizing the circuit. The language from the code makes the objective of bonding clear. Notice that using the earth as the return path is specifically forbidden. The purpose of grounding electrical systems is to limit the voltage to earth to a value that does not exceed the rating of the insulation on the conductors. The purpose of bonding on the other hand is to provide "an effective ground-fault current path."

-- Tom H

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. (2) Grounding of Electrical Equipment. Non?current-carrying conductive materials enclosing electrical conductors or equipment, or forming part of such equipment, shall be connected to earth so as to limit the voltage to ground on these materials. (3) Bonding of Electrical Equipment. 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. (4) Bonding of Electrically Conductive Materials and Other Equipment. Electrically conductive materials that are likely to become energized shall be connected together and to the electrical supply source in a manner that establishes an effective ground-fault current path. (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 permanent, low-impedance circuit 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 used as the sole equipment grounding conductor or effective ground-fault current path.

M.M. wrote:

Maybe an explanation from a lot lower on the education ladder will help. The equipment ground wiring and ground rods serve different purposes. The purpose of the ground rod is mainly for lightning protection. A lightning bolt is looking for a path to the earth. The metal in structures and and electrical components make a handy target. Lightning rods and such, along with ground rods are an attempt to divert the lightning to a path that won't harm our stuff. The purpose of the equipment ground wiring is to provide a low resistance path for fault current back to the the source of the power. AC power is looking for a path back to its source, not to earth. It will take all available paths, not just the one with the least resistance. Let's take a motor with a short circuit to its frame. The installer connected a ground rod to the motor, but did not install an equipment ground. The utility company has installed a ground rod at its transformer. The fault current from the motor frame will have to travel through the motor ground rod, through the earth and then through the utility's ground rod to get back to its source. Soil isn't a very good conductor. The soil's high resistance will mean there isn't enough current flow to trip a breaker or blow the fuse. The motor frame will still be energized. This situation could exist indefinitely. The power there will still be looking for any path back to its source. A person touching the frame would create another path. Let's take the same scenario but with an equipment ground installed. The fault current from the motor frame will have an easy path back to its source through the equipment ground wiring. It will be enough to trip the breaker and deenergize the motor frame.

Dean

| I mentioned using another ground rod and wiring to the ground prong on | a standard 110 volt house plug to establish a ground citcuit for a | computer on a non grounded (1959 built) house. | I got blazed by someone from this group. After thinking about it | for awhile, I still am having problems figuring out why my suggestion | would be wrong. Can someone explain this to me in undwerstandable | terms please?.... M.M.

No matter what you want to do, you can always find someone on the net that will tell you not to do it, and tell you it is wrong. The thing to look for is someone who includes a description of why it is wrong. If they don't include the description, use the "delete" key.

To clarify further, there are actually TWO separate return paths involved. One is the neutral and it is the return path for the current during normal usage. The other is the grounding wire, which among other purposes, also serves as a low impedance path for a FAULT (short circuit between line wire and the metal frame of equipment, outlet box, conduits, etc) so that the circuit breaker will see the situation and open the circuit.

If the ground at one outlet is connected ONLY to earth, and not back to the power source, then a short circuit from line to ground will likely not draw enough current to trip the breaker. However, a ground fault breaker should be able to detect this, so if you have such a circuit, put a ground fault breaker in to be at least safer than before if it is not practical to correct the ground wiring right then.

As long as the protection on the circuit is strictly overcurrent protection, then you need to have a low impedance return path for every piece of metal anywhere close to a live wire that could possibly short circuit to that metal under any adverse condition. Even so, some kinds of faults, such as an arc, may provide their own impedance and prevent quick breaker reaction (seconds, but plenty long enough to ignite a fire). This is one reason we now have arc-fault protection devices, and require them in places people may be sleeping where they would not react quickly enough to the problem.

| Lets not get into high impedance grounds on this thread.

That would definitely make the thread longer since it can work, but it is more complicated. It needs its own thread, which will likely show up some day (when I get more time).

| Supplemental grounding is allowed with the proper installation. I.E. you | use the same size wire as your service.

There are risks to weigh in doing this. For example if you have 2 points to ground at significant distance (a situation you would have with a feed to a detached building) you can earth flow currents (frequent but usually low with passing storms, and occaisional high with a lightning strike) going through your grounding wire.

| I was working at a University and someone bought a ion beam splitter | something like $3 million dollars and 600 amps at 480v. The company would | not warranty it unless there was a "clean ground" ( god I hate those words). | I refuse to connect the equipment based on the manufacturers | recommendations. The engineer from the manufacture came out and we sat in a | conference room for 2 hours while I explained to him why he was wrong. He | relented and changed the documentation for the machine.

Did he ever define "clean ground"?

From: snipped-for-privacy@WaltDisney.com (M.M.) I mentioned using another ground rod and wiring to the ground prong on a standard 110 volt house plug to establish a ground citcuit for a computer on a non grounded (1959 built) house. =A0=A0=A0=A0=A0=A0=A0=A0I got blazed by someone from this group. After thinking about it for awhile, I still am having problems figuring out why my suggestion would be wrong. Can someone explain this to me in undwerstandable terms please?.... M.M.

It is rather simple, the specific location of supply & load ciruits has to be thoroughly analized. You see; if you have a bonded neutral at the supply or disconnect, and your circuit breaker panel feeder cables include a ground wire neutral & hot wires, if you re-bonded the neutral at the circuit breaker panel or further into the household system you will have multiple neutral paths and that could be dangerous and not recommended by the nec.

You say you have a no grounded system, but i think what you mean is you don't have any bonded receptacles which is probaby not 100% so, since the armoured cable & metal conduit is an effective ground for such receptacles.

Concerning the extra ground rod, it is uncalled for unless your current grounding rod & electrode are so corroded you have poor continuity or no ground present at the meter panel/disconnect., in that case you should consult with a license contractor and lay your worries to rest.

Roy