Common Grounding Electrode

[bphgravity]

I have brought this up on other topics, but have not really received a substanial response to my claim. I do not like the idea behind Article 250.58. I understand that the intent of the code is to bring all metallic parts of a premise to the same potential, but isn't this also putting the grounding systems in parallel. My concern is if the grounded conductor were to open on one of the services. With the two separate service electrodes bonded, this provides an additional path of current flow back to the source. This means metallic parts of the system will be energized under normal conditions. Under fault conditions, the service main may never trip due to the higher resistance of the ground path. 250.58 needs to be ammended?

 

[don_resqcapt19]

Re: Common Grounding Electrode

How does bonding electrodes together create a "higher resistance" path?
Don

 

[hurk27]

Re: Common Grounding Electrode

Realy?

The path of current is always back to the source through the return wire. In the case of a hot to ground fault this would be the neutral via the ground to neutral bond at the first means of disconect. Never through earth!

 

[bphgravity]

Re: Common Grounding Electrode

Your still not seeing my point. I know that the return path is the grounded conductor at the services. My concern is that if the neutral were to open on the utility side of one of the services, the fact that the grounding elctrode conductor is bonded to the other services grounding electrode conductor, which is bonded to the grounded conductor at the other service, this provides a parallel path. This way, the metallic structures will become energized. What I mean by higher resistance, is the normal current will always flow back on the grounded conductor, not to the GEC. But, id the nuetral were to open, the path could follow the GEC through the bond conductor, back to the grounded conductor on the other service. Earth is not an issue because the electrodes are required to be bonded together per 250.58.

 

[don_resqcapt19]

Re: Common Grounding Electrode

The parallel path back to the service grounded conductor via the bonded grounding electrodes will be much safer in the case of an open neutral than without the bonding. Without the bonding the only path for the grounded conductor current is the high resistance path through the earth itself. This higher resistance path will cause a greater voltage drop and a higher voltage between any metallic item that is bonded to the electrical system and the earth itself. The voltage on the bonded parts will equal that of the voltage drop on the available return path. The better this path, the less voltage.
Don

 

[bphgravity]

Re: Common Grounding Electrode

Don, what you are describing is how every single service grounding system is connected. If you only have one service and one grounding electrode system and the neutral opens, the circuit opens and no neutral current will be present. In the case of TWO services per 250.58, you have TWO grounded paths to the source. In theory, you could lose the neutral on one service and never no it. The problem is that the grounded path will be the common electrode system. This is a violation and is dangerous. I still say that if the two grounding systems are kept isolated, in the event that the neutral one either service opens, no current can flow. When they are connected, current will use the grounding system to get back to the source.

 

[dereckbc]

Re: Common Grounding Electrode

BPH, if you left the two services isolated as you suggest, there would be no way to clear a cross over fault from one system to the other. That would leave earth as the only path.

The same thing exist with your CATV, telephone, or antenna. What you are suggesting would be very dangerous to personnel and equipment.

Imagine the difference in potential if a lightning strike were to occur with two isolated ground systems like your ac service and CATV.

 

[bphgravity]

Re: Common Grounding Electrode

Okay, I agree. So what is the solution? We want to equilize potential of the two systems, we want to prevent lightining flash over, and we don't want normal operational current flowing on metallic parts. Don't bond, you have the frist two issues, bond, you have the second. Can anyone really say what is the greatest danger? I definitely feel that other grounding electrodes from others wiring systems and antennas need to be bonded together to prevent surge or lightning from using the equipment or personal as ground, but I am concerned with putting two utility power system neutrals paths in parallel.

 

[don_resqcapt19]

Re: Common Grounding Electrode

bp,
What do you propose to do in areas served by a common underground water piping system? All of the buildings are required by the NEC to use this pipe as a grounding electrode. This places the water pipe in parallel with all of the grounded conductors and it will always be carrying some of current that should be on the grounded conductor. The same thing happens with a cable TV distribution system. The cable shield is conneced to the service grounding system at every house and they are all tied together at the street. This shield is also in parallel with the grounded conductor.
If you have a serivce where the only grounding electode is at the building and the neutral is opened on the line side of the service disconnect, then all metallic parts that in any way connected to the building grounding system will be raised to a voltlage above earth by the amount of the voltage drop in the return path through the grounding electrode system. In my opinion, it will always be safer to have the grounding systems bonded together.
Don

 

[dereckbc]

Re: Common Grounding Electrode

BPH, the solution I use for industrial applications is to establish a principle ground point (PGP). The PGP is a copper bus bar that radially connects all electrodes (water pipe, UFER, ground rod arrays, ground rings, building steel, etc.) to form a single point ground (SPG). Then from the PGP you extend radially to the individual services with a dedicated GEC, SDS included. This will prevent fault currents from flowing through all systems and go directly to the source.

This may not seem practical for residential services but it can with a little imagination. Just dedicate a small section of a bare GEC and connect all bonding conductors (CATV, Telephone, ground rods, etc.) limited to a 1-foot section. It will accomplish the same principle.

If you will give me your e-mail address I can send you some details.

Hope that helps.

Dereck

[ March 18, 2003, 01:02 PM: Message edited by: dereckbc ]

 

[bphgravity]

Re: Common Grounding Electrode

This is a published statement made by Mike Holt himeself: "Potentially dangerous neutral current can flow on the grounding electrode conductor when multiple services are grounded to the same single electrode. This occurs because meutral current from each service has multiple parallel return paths back to the electric utility power supply. This is of particular concern if the grounded (neutral) conductor from one of the services is opened." This is an issue for me on a power quality/ power repair job I am currently consulting on. The facilty at which this exact event has happening needs a solution to stop it. The facilty has several services coming to one very large building for manufacturing purposes. A recent storm broke the grounded conductor from the service transformer that was connected to two separate services. The machinery never shut down because the grounding electrodes at these two service points were bonded to the concrete-encased electrode that is bonded to the other three services at the building. the neutral currents used this bond to flow back to the source by means of the other services. Somehow, no one in the facilty got shocked from touching any of the metallic parts. What happened was one of the services taking on the additional neutral loads had a completely isolated event that caused a shutdown and transfer to generator backup. When a service tech went to the transfer panel to read genrator times, he received a near fatal shock and produced an extremely large arc flash when the panel door swung open and came into contact with an isolated support pole that was not apart of the grounding electrode system. In short, I believe if the GEC were never bonded together, when the neutrals opened to start with, the problem would have been immediatly noticed and corrective action could take place. However, due to the bonding of the electrodes, the entire metallic structure took on the 480/277 potential. I due want to be code complaint, I do want to ensure all sytems at equal potential, but what do I do to prevent this odd, but possible reoccuring event from happening again? All my data and communication systems are properly bonded and I see no hazard with these. My problem is with 250.58 alone.

 

[don_resqcapt19]

Re: Common Grounding Electrode

bp,
Until the bonding of the grounded conductor occurs only at the supplying transformer, there is no way that I know of to solve your problem.
I really think that the problem in your facility would have been worse without the bonding of the electrode systems. It is my opinion that even without the intentional bonding of the grounding electrode systems, there would be inadvertent bonding of the grounding systems for the two services via the equipment grounding conductors that are run to the equipment. This may occur vie the metal building structure, process piping, and/or other items that have a physical connection to EGCs that originate in more than one service. This inadvertent bonding would be of a higher resistance than the what you have now, but it may have been low enough to not immediately cause noticeable problems. The higher resistance of this accidental path would result in a higher voltage to ground on all of the metallic items connected to the electrical grounding system. The ampacity of this accidental path may be so low that the grounded conductor current flowing in the path would produce enough heat to cause a fire.

Don

 

[bphgravity]

Re: Common Grounding Electrode

Don, you make great point! Thank you and everyone for all your input. Your ideas and comments on this and other posts has been worth paying for.