Ground and Neutral bonded at service box

[Mark Nicholet]

I need a little clarification regarding residential equipment grounding system. Any input would be appreciated.

It is my understanding that the neutral and the ground wires are bonded at the service box. Why wouldnt the electricity in the neutral wire (completing the circuit) run down into the ground wire where it is bonded at the service box instead of going out to the transformer. This could thereotically, create the whole grounding system to become energised by back flowing through the grounding circuit. Im not sure I understand what stops the ground wire from becoming energized if they are conducting and bonding to each other at the service box.

As with a sub-panel the ground and neutral are not allowed to be bonded as it may cause the grounding system to become hot. Whats the difference between the sub-panel that isnt allowed to be bonded and the service box that is bondin the neutral and ground. Thank you.

[rbrady]

At the main service entrance the neutral wire is bonded to ground. The neutral wire from the utility is also grounded at the pole and other locations (e.g. the neighbors service). The earth actually does become a parallel path for current, however the impedance (resistance) is so great compared to the neutral circuit wire, that very little current actually flows through the ground.

Whenever you have an auxiliary panel the neutral and ground should not be tied together because the ground wire becomes a parallel path for current with the neutral wire (any current going through the neutral wire will be shared with the ground wire because they have the same connections at both ends). The current traveling through the ground wire causes a voltage drop (any time current travels through a piece of wire there is some resistance - current x resistance = voltage). This can cause a small voltage on the resulting ground circuit. As far as I know the only risk is the possibility of nuisance electrical shocks and/or radio interference due to the voltage drop from the current passing through the ground wire (which is a parallel circuit with the neutral wire).

Very confusing. It took me a long time to really understand what was going on.

[jbettencourt]

You just hepled me understand it a little better now too. Thanks.

[Mark Nicholet]

Originally Posted by rbrady

At the main service entrance the neutral wire is bonded to ground. The neutral wire from the utility is also grounded at the pole and other locations (e.g. the neighbors service). The earth actually does become a parallel path for current, however the impedance (resistance) is so great compared to the neutral circuit wire, that very little current actually flows through the ground.

Whenever you have an auxiliary panel the neutral and ground should not be tied together because the ground wire becomes a parallel path for current with the neutral wire (any current going through the neutral wire will be shared with the ground wire because they have the same connections at both ends).

Thank you very much it is clearer to me as well. The resistance in the grounding system to earth makes sense. When the ground and neutral are bonded at the service box there is 3 ways for the current to travel, isn't there?

1. to the transformer (path of least resistance)
2. to the grounding rod etc (to earth, path of most resistance)
3. back to the grounding equipment, creating a circuit and then back to the transformer (metal box, light fixtures, outlet casings etc)

What prevents option 3, the current from going back to the grounding equipment?

Is electricity on a one way street?

What is the difference in bonding the neutral and ground at the service box as opposed to an auxillary panel, isn't it the same?

Hypothetically, I could understand if there was a one way valve in a water system that would prevent the water from back flowing to other routes.

Thanks again!

[rbrady]

Originally Posted by Mark Nicholet

3. back to the grounding equipment (metal box, light fixtures, outlet casings etc)

What prevents option 3, the current from going back to the grounding equipment?

Thanks again!

The ground conductor to the receptacles, light fixtures, etc. does not have a common path (parallel circuit) with the neutral wire. It is a 1 ended circuit (it is basically a disconnected wire). If you connected the ground wire to the neutral wire at the receptacle or light fixture, it would then share the current going through the neutral wire. It is only there to ground a possible short and normally should not be carrying any current.

I didn't understand exactly what you were asking, I hope this helps.

[Mark Nicholet]

Ok, thanks... it is very confusing. Basically, what I am asking is, why is it okay to bond neutral and ground at service box but not at aux panel. I understand that bonding at aux panel can lead to

"Whenever you have an auxiliary panel the neutral and ground should not be tied together because the ground wire becomes a parallel path for current with the neutral wire (any current going through the neutral wire will be shared with the ground wire because they have the same connections at both ends)."

Why isnt this the same for the service box? ie. why doesnt the ground wire become a parallel path for current with the neutral wire

because of resistance? then why at the aux panel is there not the same resistance as service box bond in the ground conductors?

I really appreciate your help, thanks again.

[rbrady]

On the ground connection between the main service panel and the pole transformer you have the high resistance of the earth.

On the ground connection between the (improperly wired) auxilliary panel and the the main panel you have the low resistance of the ground conductor.

Clear as mud?

[Mark Nicholet]

lol, it is thanks... I have a pretty good understanding of it now.

[Warren J. Porter]

One other thing to remember is that grounding is more for lightning protection. Very high voltage such as lightning will shunt off to earth because of the high voltage that overcomes the resistance of the earth to ground rod connections. If there is a short somewhere the currant flows mostly to the utility neutral where the breaker then should trip.
Sub panels will do the same thing but only after reaching the main panel by a specific conductor. If the ground is connected to a city water main it is a whole differant story. If some main neutral conductor has a poor conection somewhere then currant can flow back to your neutral or from yours to someone elses, but this is usually under 5 amps. Here is a good link to study, http://ecmweb.com/grounding/electric...uth_grounding/

[Mark Nicholet]

Wow, very interesting. Thanks, I have a whole new outlook on the grounding conductor system now.

[rromoser]

Grounding of receptacles ( or anything electrical) is to provide a return path of minimal resistance to enable overcurrent devices to activate. If a metal cased hedge trimmers had a breakdown in its insulation that energized the case, the grounding wire should provide a return path sufficient to trip the breaker. If not you might go on trimming unaware of the problem until you make contact with something that provides a return path for the circuit (wet shoes on the ground, arm bumping aluminum siding, etc.) it might be your last time to trim the hedge. Yes, a GFCI should be on that circuit. It was just an example.

[rromoser]

Re: lightning protection
Lightning is going to go where ever it wants when it strikes a house. It might follow a #12 awg ground wire, but I doubt it. I've seen where it followed a knotty pine fake beam in a family room and blown out a a strike.ll of the knots (pressure from vaporizing the extra moisture in the knot) My understanding of lightning rods is to help dissipate static in the immediate vicinity and discourage a strike. The large size braided wire typically from a lightning rod is routed the shortest and straighted route to Earth. If it must turn it should be done with a large radius. If the rod does get struck the bolt is more likely to follow a large radius turn on its way Earthbound. One other point. Surge suppressor outlets typically don't have warranty coverage for equipment from a lightning strike. I believe their voltage limitations are around 330v.

[jcundiff]

As explained earlier, this issue is BONDING, not Grounding. Bonding refers to efforts to equalize the electrical potential between grounds and neutrals, gas lines and copper plumbing, building steel between expansion joints etc. I usually explain it to my students by placing two bottles of water on the floor. They are of equal potential. Then I lift one bottle and place it on a table explaining that I am storing energy, and that energy is based on the potential between the table top and the floor. Next I take the second bottle from the floor and place it on a high shelf... again explaining the stored energy and the potential between the high shelf and the floor. I finis by pointing out the potential difference that exists between the high shelf and the table top. Does that help?

John

[rbrady]

Which bottle is a service ground to the water main?

I actually do think of electrical current flow in terms of water flow sometimes. They are certainly different, but there are some basic principals that apply to both - like you could have water pressure (or positive voltage potential), but without a return path to atmosphere (or the negative voltage potential), you get no flow.
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[Rick Barn]

Originally Posted by rbrady

At the main service entrance the neutral wire is bonded to ground. The neutral wire from the utility is also grounded at the pole and other locations (e.g. the neighbors service). The earth actually does become a parallel path for current, however the impedance (resistance) is so great compared to the neutral circuit wire, that very little current actually flows through the ground.

Whenever you have an auxiliary panel the neutral and ground should not be tied together because the ground wire becomes a parallel path for current with the neutral wire (any current going through the neutral wire will be shared with the ground wire because they have the same connections at both ends). The current traveling through the ground wire causes a voltage drop (any time current travels through a piece of wire there is some resistance - current x resistance = voltage). This can cause a small voltage on the resulting ground circuit. As far as I know the only risk is the possibility of nuisance electrical shocks and/or radio interference due to the voltage drop from the current passing through the ground wire (which is a parallel circuit with the neutral wire).

Very confusing. It took me a long time to really understand what was going on.

Clear as mud!

Is the path of least resistance from the transformer pole this way because of the metal rod much deeper in the earth compared to the rod in earth at your service entrance?