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Old 01-14-2014, 06:02 AM   #1
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Help make less confussing grounding electrode and equipment grounding?


There seems to be quite a lot of confusion among non-professionals about grounding electrode and equipment grounding. I am not a licensed or professional electrician and am humbled by the level of experience I see in this forum and I've read a lot of posts. I was wondering if some of the more experienced members could critique the following to help both me and other non-professionals become better electricians? It is written for the non-professional but experienced electrician. If you have time, please feel free to comment even if it's harsh.

Thanks,
Jack


Below, "bonding" means a physical electrical connection between parts of an electrical system usually by wire or bonding screws.

There must be a safe, low-impedance path of system current and fault current back to the transformer via the system neutral cable, and there must be a safe, relatively low-impedance path of high-voltage current from a lightening strike or transformer short back to the earth. These two paths are implemented via the equipment grounding in the first case, and the system grounding in the second. Equipment grounding is implemented via a system of bare copper wires connected as needed throughout the home. System grounding is implemented via a grounding electrode driven into the earth usually near the meter pan and connected to both system neutral and equipment grounding.

Equipment grounding:

All current-carrying material in the home such as metal pipes, metal enclosures such as meter pan, mast, conduits carrying electrical conductors including metal junction boxes, metal parts of current carrying devices like stoves and dryers, should be connected to the equipment grounding system of the home. This is usually identified by the bare copper wiring in the home. Metal parts of the home are connected to this wiring system, and this system is in turn, "bonded" to the system neutral in the meter pan either by a green bonding screw or bare copper wire connecting the equipment grounding bar to the system neutral bar in the panel. In this way, if there is an electrical short, a hot wire touching the metal part, the (majority of the) current will be diverted through the equipment grounding system, back to system neutral and back to the transformer by this low-impedance path, quickly tripping the circuit breaker, rather than a relatively high impedance path of a human body thereby resulting in minimum shock hazard to life. In the case of doubly-insulated equipment, the metal parts are insulated and not in direct contact with humans and so no electrical path exists to cause harm.

System grounding:

Likewise, in the case of a lightening strike or transformer short which could energizes the metal systems of the house or place a high voltage on the metal parts of a system (I think), this high-voltage will be diverted to the earth via the grounding rod which is in turn "bonded" to the system neutral usually in the meter pan, as well as bonded to the equipment grounding system, thereby providing a relatively low-impedance path of electricity to flow back to earth rather than through humans if the path back to the transformer has been disrupted, again reducing the possibility of a serious shock hazard.

Sub-panel bonding and grounding:

In the case of a sub-panel, we do not bond the equipment grounding system, the bare copper wires, to the sub-panel neutral because if the equipment grounding (bare copper wires) and system neutral are connected, the bare copper wires leading to the subpanel would carry (zero-voltage) current back to the main panel under normal operating conditions like using a light bulb. Rather, we wish the bare copper wires leading to a sub-panel, and the equipment grounding wires in the subpanel to be bonded to each other but isolated from the subpanel neutral. In that way, the equipment grounding will be free of current except in the case of a fault thereby providing an unobstructed, free path back to the main panel, completing the electric circuit and tripping a breaker quickly. However, the subpanel, and equipment grounding, as well as all metal parts of this subpanel are likewise "bonded" back to earth via a separate grounding rod for the reason explained above.

Last edited by jackmel; 01-14-2014 at 06:22 AM. Reason: clarity
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Old 01-14-2014, 09:50 AM   #2
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Quote:
Sub-panel bonding and grounding:

In the case of a sub-panel, we do not bond the equipment grounding system, the bare copper wires, to the sub-panel neutral because if the equipment grounding (bare copper wires) and system neutral are connected, the bare copper wires leading to the subpanel would carry (zero-voltage) current back to the main panel under normal operating conditions like using a light bulb. Rather, we wish the bare copper wires leading to a sub-panel, and the equipment grounding wires in the subpanel to be bonded to each other but isolated from the subpanel neutral. In that way, the equipment grounding will be free of current except in the case of a fault thereby providing an unobstructed, free path back to the main panel, completing the electric circuit and tripping a breaker quickly. However, the subpanel, and equipment grounding, as well as all metal parts of this subpanel are likewise "bonded" back to earth via a separate grounding rod for the reason explained above
You seem to have a good understanding. The only things I see is in your Sub-panel descriptions you should explain that sub-panels or panels that are not the service equipment panel are fed with 4 wires (H-H-N-Grd) because of that if we bond neutral and ground at the sub-panel then neutral current will have parallel paths to the service panel and current will split on that parallel path. No current should be on equipment ground wiring unless a ground fault exists and then very briefly as the breaker should trip.
Next there are sub panels that are in detached structures fed from the service equipment that require grounding electrode systems. A sub-panel that is in the same structure/home with the service equipment does not require ground rods as the SE panel grounding electrode serves that purpose.
Lastly before the NEC changed code that requires all 'sub-panels' to be fed with 4 wires many were allowed to have 3 wire feeders under the correct circumstances and those will be bonded neutral and ground at the panel as there is no equipment ground ran with the feeder.
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Last edited by Stubbie; 01-14-2014 at 09:53 AM.
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