||11-07-2008 10:19 PM
Originally Posted by noisyone
So in the diagram, the electrial fault doesn't even travel to the ground rod and the water pipe ground. Is that normal?
Is it a safe and normal practice to attach the ground wire to an empty spot on the neutral bus bar like you mentioned? I understand they seem to bond together anyhow. I just didn't realize that the ground fault would travel all the way back to the transformer. I thought it had to go into the earth. Thanks again for this info.
Yep its safe and the only place you do this is at the service equipment. Service equipment is the first means of disconnect that the service conductors land on. This can be either a stand alone disconnect and then a sub-panel at the dwelling or a main breaker panel. If your service panel is a main breaker panel then the equipment grounds, grounding electrode conductors, and neutrals are all bonded to each other at the neutral bar and the metal of the panel. You will also notice the service neutral is connected to the neutral bar. Current always seeks its source (transformer) and it will take any path available to it to get there. We intentionally construct a low impedance/resistance path back to the transformer with the equipment grounds and bonded metal. Current will take any path available to the source but it will travel the path of least impedance/resistance if one is given for it to use. We give it only one of these low impedance paths.
Current will go to earth but earths resistance is much higher than aluminum or copper wire so the vast majority of your premise wirings neutral current and fault current (if any) returns to the transformer center tap via the lower impedance of the service neutral. So that being the case look at branch circuit neutrals and grounds as two lane highways that end at a one lane highway once it reaches the service panel. Only way to do that is to bond them to the neutral bar(s) which is bonded to the service neutral.
What is confusing you is the grounding electrode system and the equipment ground system. Grounding electrodes are for property protection and do little for human safety. Equipment grounding conductors are for ground fault protection for human safety and facilitate the opening of the circuit breaker to deenergize the faulted circuit.
If the current went to earth it likely would not be at enough amperage to open the breaker due to earths resistance to current flow. A reasonable value for the resistance of earth would be 25 ohms or more. 120 volts/25 ohms is 4.8 amps this wouldn't open a 15 amp breaker. So you see my point.
In contrast the resistance of copper or aluminum is in thousandths of ohms per thousand feet. So just taking a number at random 120/.525 ohms is 129 amps that will definitely open a 15 amp branch circuit breaker because that amperage (current) has to pass thru the breaker on its way to the transformer during a fault to bonded metal like the diagram shows occurring at the receptacle box.