First let me lay out my situation. I have a remote industrial location, we generate our own power. We seem to have some confusion about what constitutes a ground grid. I have searched the NEC and I am unable to find any references to a ground grid. I have also searched the internet. Most of information I was able to find referred to substation grounding grids. I do not know if that applies since we generate our own power. I would like to access something that gives the definition of a ground grid and some kind of minimum requirements.
Grounding grid
- Thread starter jcblack97
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bphgravity
Senior Member
- Location
- Florida
Your facility is powered by a separately derived alternating current system and only needs to be grounded and bonded per the provisions of 250.30 of the NEC. The 'grounding grid' would be nothing more than all the present electrodes bonded together to form the grounding electrode system...
First up is you are not going to find anything in the NEC about grounding grids because it is outside the scope of the NEC.
Grids are used primarily for two completely different purposes based around equipotential ground planes...
One industry that uses them is as you have observed is electrical generation using high voltage. The purpose of the grid is two fold. The most important is step-potential differences in the event of a ground fault, and the other is POCO uses earth as a conductor, something forbidden by the NEC. The size of the grid varies by the design objectives. I have seen grid distances vary from 10-foot centers up to 20-feet. Just about all utilities use 500 MCM for the cable. The size of the grid depends greatly on the spacing and layout of the yard and equipment spacing. The idea is to have the spacing as such all equipment structures and platforms can easily be bonded to the grid.
The other application for a grid is signal performance, lightning protection and shielding used by military, inelegance, and other forms of communications like telephone, data, and radio towers. In this case the grids spacing is determined by the frequency performance or cut off. Spacing can be from 8-feet where only low frequencies are concerned to millimeters where used as shields to prevent RF from radiating and eves-dropping.
So the answer to your question is there is no ?MAGIC BULLET? size, it is completely a design issue based on the objective and performance.
Grids are used primarily for two completely different purposes based around equipotential ground planes...
One industry that uses them is as you have observed is electrical generation using high voltage. The purpose of the grid is two fold. The most important is step-potential differences in the event of a ground fault, and the other is POCO uses earth as a conductor, something forbidden by the NEC. The size of the grid varies by the design objectives. I have seen grid distances vary from 10-foot centers up to 20-feet. Just about all utilities use 500 MCM for the cable. The size of the grid depends greatly on the spacing and layout of the yard and equipment spacing. The idea is to have the spacing as such all equipment structures and platforms can easily be bonded to the grid.
The other application for a grid is signal performance, lightning protection and shielding used by military, inelegance, and other forms of communications like telephone, data, and radio towers. In this case the grids spacing is determined by the frequency performance or cut off. Spacing can be from 8-feet where only low frequencies are concerned to millimeters where used as shields to prevent RF from radiating and eves-dropping.
So the answer to your question is there is no ?MAGIC BULLET? size, it is completely a design issue based on the objective and performance.
The people that I am dealing with are trying to call a buried 4/0 bare ground wire a grid. At best I would call it a ground ring but I do not think it even qualifies as that. We have two small buildings that are setting on pipe pilings. The 4/0 is only buried about 18 to 20 inches deep. The wire has been bonded to all the pilings and to all the rest of the steel and cable tray through the 4/0 that has been run in the cable tray and it will all eventually end up at the generators. I am not sure what they think is there grounding electrode but I think they think it is the buried 4/0. For some reason they seem to think that if there is a fault between the steel and an ungrounded conductor that the breaker will not trip unless it is directly connected to the so called grid. I am sure this is not the case. I was also just trying to get some information about the depth that a grid needed buried if it was a grid.
- Location
- Massachusetts
As a E&I Q/C Inspector what exactly are you expecting this grid to do or why do you feel a grid is needed?
What you describe could meet the NEC definition of an electrode.
What you describe could meet the NEC definition of an electrode.
jcblack97,
While you can call the 4/0 a ground ring, I believe it's better to consider it as a grounding electrode conductor or an equipment grounding conductor, depending on where it is in the power system. I don't know the details of the structure but based on your description the pipe pilings are most likely serving as the major grounding electrodes. In my estimation there would very little difference in "sink" potential between bonding the 4/0 at the top of the pilings and below grade where they are now.
While you can call the 4/0 a ground ring, I believe it's better to consider it as a grounding electrode conductor or an equipment grounding conductor, depending on where it is in the power system. I don't know the details of the structure but based on your description the pipe pilings are most likely serving as the major grounding electrodes. In my estimation there would very little difference in "sink" potential between bonding the 4/0 at the top of the pilings and below grade where they are now.
Your power system is grounded, correct? The steel is bonded to ground, correct? If so, as long as there's a relatively low impedance path to the system ground, a short to the structural steel will trip the OCPD. How well your system ground is bonded to earth does not matter in this regard.
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