Grounding at service entrance

Hello, all. As an EE what should be a simple electrical circuit/networks question has me puzzled as to the rationale, and I
can't find it online or in any textbooks.
Assume we are discussing a standard 120/240 V residential service in the U.S. It is required that all ground (green) and neutral (white) wires supplying the inside facility be tied together at the service entrance. That much I understand; if there's a hot (black) wire fault to the metallic part of the device connected to the ground wire, then opening of the breaker/fuse is facilitated (but not guaranteed depending on the amount of the fault current). IOW, the grounded (metal shell, etc) of the device protects the user and ideally blows the fuse.
What I don't get is the need to also connect the neutral/ground wires via a short path (connection to metal water pipe or ground rod(s)) to contact with soil (earth). Other than perhaps lightning protection (at least on the neutral/ground wire side) what other advantages accrue from this earthing? Conversely, what would you give up with no connection to earth? I presume once upon a time power distribution EEs came up with this but I'd like to appreciate their thinking. Thanks for your reply and comment. Sincerely,
--
J. B. Wood e-mail: arl snipped-for-privacy@hotmail.com

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Tue, 14 Jul 2015 06:15:55 -0400, "J.B. Wood"

Were that the only reason, there wouldn't be a need for separate ground and neutrals at all. The reason to have separate grounds and neutrals (so they *have* to be tied together somewhere) is that if the neutral is opened, the device still has the safety ground. The case cannot become "hot".

You're right. It's mainly for lightning protection, though not so much for direct strikes. Think of a cork on the ocean. If lightning strikes close to the house, the entire house will "ride" on the same voltage (wave). You don't want to have two such grounds because you don't want the current from the strike passing through the house. BTW, water pipes are no longer allowed as the building ground (just the opposite, though - they are grounded).
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

The other issue that requires the grounding electrode is to insure that "ground" on the case of your tool is the same potential as the actual ground you are sitting on when you use it. That is why you need an additional ground electrode at separate buildings. There is a chance that the dirt may not be at the same potential if you get very far away. I have measured 30 volts or more between the ground electrodes of buildings that were not that far apart. Dirt is really more of an insulator than a conductor. This prompted Mr Ufer to design the concrete encased electrode. It might not really produce "ground", whatever that means but it does bond the electrical service to the building.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 07/14/2015 01:41 PM, snipped-for-privacy@aol.com wrote:

Hello, and the sounds like we're talking about outdoor use. Is that right? The only other scenario involving earth-grounding I can come up with is a situation in which, say an appliance with a 3-wire cord and an external metallic enclosure is not properly grounded (e.g. someone has clipped off the ground prong to use a 2-wire extension cord). Now assume the hot (black) wire in the appliance faults to the case. In this situation had the ground wire path been intact the circuit breaker/fuse in the residence would open. Again, that has nothing to do with whether the neutrals/ground wires are in contact with the soil.
So we now have a "hot" case with the appliance otherwise operating normally. Assume the user now simultaneously touches the case with one hand and a metallic water pipe or any other conducting path that eventually finds its way back to the soil. Even if there were no earth ground at the service entrance, the neutral of the nearest MV-to-LV distribution transformer would have its neutral grounded. So the person would most likely at the very least receive a shock. Now if said appliance case itself comes into contact with the water pipe then it would be nice to open the applicable residence breaker/fuse. Having the ground/neutrals contacting the soil at the service entrance would promote that action as the electrical path would be shorter than that going all the way back to the grounded neutral at the distribution transformer. This scenario, while promoting the opening of a faulty circuit, doesn't afford user protection while everything is "hot".
An alternate scenario has the neutral (white) wire in the ungrounded appliance faulting to the case. Most likely the person probably wouldn't receive a shock in the touch circumstances previously described. Even with the appliance properly grounded one might never be made aware that such a condition exists as the ground and neutral conductors are now in parallel carrying current. IOW an undetectable fault. (I haven't been considering scenarios when ground fault detectors such as found in bathrooms are employed.) Sincerely,
Sincerely,
--
J. B. Wood e-mail: arl snipped-for-privacy@hotmail.com

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Tue, 14 Jul 2015 14:25:21 -0400, "J.B. Wood"

That may be true if you never work in the garage and live in a stick built house above grade. My first indication that the ground was bad in this house after I moved in was when I touched the range whilst standing barefoot on the terrazzo floor. It lit my ass up! The new GFCI requirements tend to mitigate the pipe problem but you still may have grounded things, like the phone line and TV cable. If you are not providing a single point ground reference, the magic smoke is going to come out of your computers and TVs.

The ground electrode has nothing to do with clearing faults. As you describe the fault path is through the main bonding jumper to the service neutral

A GFCI or now the mandated AFCI would detect that fault and trip.
The main reason is still protection from transients and unusual voltages on the service but that also happens at the pole. These days it probably has more to do with electronics than anything. The foundation of any surge protection scheme is a good grounding electrode system and having everything bonded to that.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 07/14/2015 02:44 PM, snipped-for-privacy@aol.com wrote:

But in the scenario I described it was assumed that a metallic path via the ground wire back to the neutral/ground (green wire) bond at the service panel didn't exist from the appliance. I was considering an alternate path for the fault current. I never assumed that the principle purpose of residential earth grounding was to clear a residential fault. External to the residence, the reason for the widely-used 3-phase, 4-wire multi-earthed neutral medium-voltage (~1 kV to 69 kV) power distribution system is to ensure opening of over-current protection devices (fuses) under fault (overhead wires contacting trees, downed power lines, etc) conditions. The other reason is to provide a convenient grounding point for surge arrestors (when used) on distribution transformers. Sincerely,
--
J. B. Wood e-mail: arl snipped-for-privacy@hotmail.com

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Tue, 14 Jul 2015 15:23:45 -0400, "J.B. Wood"

If you do not have an Equipment Grounding Conductor, you are at the mercy of the GFCI/AFCI to save the person. That is why the GFCI is required in those places where a person is likel;y to be incidently grounded (sinks, concrete floors in unfinished areas, outside etc) but that ignores the tile/terrazzo floor on a slab on grade house. It also does not pick up 240v equipment. In that regard the RCD used in other polaces may be a better idea, even at the 30ma threshold, vs 5 for the GFCI.
The other principle reason for the ground is stated in NEC 250.4(A)(1)
."Electrical systems that are grounded shall be connected to earth in a manner that will limit the voltage imposed by lightning, line surges, or unintentional contact with higher-voltage lines and that will stabilize the voltage to earth during normal operation"
We may be saying the same thing here
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 7/14/2015 2:11 PM, snipped-for-privacy@aol.com wrote:

I have typical US urban overhead power distribution.
If the primary wire falls on the secondary conductors, earthing at my house helps trip the utility overcurrent protection (damage is less than it might otherwise be).
If the neutral from my house to the pole opens, the my earthing gives some (often not much) protection from 120-120V unbalance. Actually we have an all-metal water supply system, so earthing to the water pipe gives a lot of protection, with the neutral current riding on the water pipe to adjacent houses and on their supply neutral back to the utility transformer.
The utility primary wire is 8kV to a primary neutral. The primary neutral is the same wire as the secondary neutral. The neutrals and transformer case are earthed at the transformer pole (and at some other poles). The utility earthing electrodes are similar to a ground rod (which is about the worst electrode). Many of the wires down to the earthing electrodes have been stolen for copper. My earthing electrodes help earth the utility system. If the primary/secondary neutral opens, my earhing electrode helps keep the "ground" at my house from going to 8kV with respect to the earth.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Like I said I am "earthing" about 3 amps from my neighbors in my GEC, just about all the time, Maybe higher in a thunderstorm but it is pretty extensive. I have, four 8' rods scattered around, a Ufer in the house, garage and every significant chunk of concrete I have ever poured including a 20,000 gallon pool., all bonded together with a 2ga ground ring.
The good side of that is my lightning rod has been hit 3 times and I only lost one weather transmitter ... on that pole. Everything else is doing fine.
I do have pretty extensive surge protection tho.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 07/14/2015 04:11 PM, snipped-for-privacy@aol.com wrote:

Hello, and thanks for that NEC quote! I think that sums up the reasons for tying the building white and green wires to earth at the building service entrance. Any other advantages are extra gravy. Sincerely,
--
J. B. Wood e-mail: arl snipped-for-privacy@hotmail.com

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

All grounding electrodes should be bonded together to avoid that fault path you are speaking of. The typical satellite installation has the dish grounded separately from the service. That fault path is through your TV. I went behind the sat box guy and bonded his little 4' rod to an 8' rod I drove that was also bonded to my grid 2 ways. One was an 8ga to the pool grid and the other was a 2ga wire back to the service electrode system. My GES is providing ground for the whole neighborhood. I see 3 amps or so of current in the service neutral with the main breaker off.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wrote in message wrote:

Sounds like you could 'tap' that 3 amp for free to run some stuff. :) Wow, free electricity and legal.
--
Buffalo

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 10/4/2015 3:32 PM, Buffalo wrote:

In your dreams!
--
Don Kelly

remove the 'cross' to reply directly
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 7/14/2015 11:04 AM, krw wrote:

Actually ground and neutral don't have to be tied together. As I understand it, some systems in the UK have the utility transformer neutral earthed at the transformer, then an insulated neutral is supplied to buildings. The building "ground" is earthed at the building, but not bonded to the neutral. The ground/earth is not adequate to carry fault current to trip building breakers, so RCDs are used.

I like the cork analagy. If there is a lighting strike with strong currents earthed at your building the "ground" at your building can rise thousands of volts above nearby 'earth'. Much of the protection at the building is that all wiring rises together.

Depends on what you are saying. Under the US-NEC a water supply pipe that is 10 ft or more metal in contact with the earth is REQUIRED to be used as an earthing electrode, just as it has since time began.
Water pipes used to be allowed to be used as a 'ground' connection inside the building. That is no longer allowed (except within 5 ft of the water service entrance, which is the same location where earthing electrode connections are made).
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Nope you have it backwards. The water piping, assuming 10 or more feet is in direct contact with the Earth, is the main ground. The driven ground rod is referred to as the supplemental ground.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wrote:

There has been a major shift in the thinking about water pipe. Back when it was all metal, that was the best electrode you could get. Now that utilities have transitioned to plastic, not so much. These days the feed, all the way to the wall of the house, might be blue plastic with the copper starting at the basement wall. You still need to size the bonding conductor to water pipe using 250.66 even though 250.122 might be more appropriate.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 7/15/2015 7:26 AM, snipped-for-privacy@aol.com wrote:

My oldest NEC, 1968, says: When there is less than 10 ft of metal in contact with the earth "or there is some likelihood of the piping system being disconnected or isolated through the use of nonmetallic piping or insulated couplings, the piping system shall be supplemented ..."
That is substantially the same as the current NEC, except a supplemental electrode is now always required because a metal water service pipe may be replaced by plastic in the future.
Used to be the "supplemental" electrode was ground rod(s), which are often slightly better than nothing. Now often a "concrete encased electrode" (Ufer ground) is required, and that is also the supplemental electrode. That is a good electrode and loss of a metal water pipe doesn't matter so much.
Far as I know all the water service pipe is Minneapolis - St Paul is metal. Water pipe as an effective earthing electrode is far from dead, and is important here (and elsewhere).
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Florida has decided the Ufer is always going to be "available" on any new house with a poured foundation. It is part of the footer steel inspection. About the only exception will be a piling house without any concrete poured at grade. That is fairly rare since most will have a monoslab poured under the house that is "the garage" and ends up being a bootlegged room, more times than not.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Tuesday, July 14, 2015 at 1:04:12 PM UTC-4, krw wrote:

If you read the totality of the language used in Section 250.50 of the Nati onal Electrical Code (NEC) as published in the United States you will see t hat not only is any underground metal water piping that is Three Meters of more in length permitted to be used as a Grounding Electrode, If it is pres ent it must be used. Shall' as used in the NEC, is the prescriptive rather than the permissive term.
250.50 Grounding Electrode System All grounding electrodes as described in 250.52(A)(1) through (A)(7) that a re present at each building or structure served shall be bonded together to form the grounding electrode system.
250.52 Grounding Electrodes (A) Electrodes Permitted for Grounding. (1) Metal Underground Water Pipe. A metal underground water pipe in direct contact with the earth for 3.0 m (10 ft) or more (including any metal well casing bonded to the pipe) and electrically continuous (or made electricall y continuous by bonding around insulating joints or insulating pipe) to the points of connection of the grounding electrode conductor and the bonding conductor(s) or jumper(s), if installed. (2) Metal Frame of the Building or Structure. The metal frame of the buildi ng or structure that is connected to the earth by one or more of the follow ing methods: (1) At least one structural metal member that is in direct con tact with the earth for 3.0 m (10 ft) or mor e, with or without concrete en casement. (2) Hold-down bolts securing the structural steel column that are connected to a concrete-encased electrode that complies with 250.52(A)(3) and is loc ated in the support footing or foundation. The hold-down bolts shall be con nected to the concrete-encased electrode by welding, exothermic welding, th e usual steel tie wires, or other approved means. (3) Concrete-Encased Electrode. A concrete-encased electrode shall consist of at least 6.0 m (20 ft) of either (1) or (2): (1) One or more bare or zinc galvanized or other electrically conductive co
dia meter, installed in one continuous 6.0 m (20 ft) length, or if in multi ple pieces connected together by the usual steel tie wires, exothermic weld ing, welding, or other effective means to create a 6.0 m (20 ft) or greater length; or (2) Bare copper conductor not smaller than 4 AWG Metallic components shall be encased by at least 50 mm (2 in.) of concrete and shall be located horiz ontally within that portion of a concrete foundation or footing that is in direct contact with the earth or within vertical foundations or structural components or members that are in direct contact with the earth. If multip le concrete- encased electrodes are present at a building or structure, it shall be permissible to bond only one into the grounding electrode system. (4) Ground Ring. A ground ring encircling the building or structure, in dir ect contact with the earth, consisting of at least 6.0 m (20 ft) of bare co pper conductor not smaller than 2 AWG. (5) Rod and Pipe Electrodes. Rod and pipe electrodes shall not be less than 2.44 m (8 ft) i n length and shall consist of the following materials. (a) Grounding electrodes of pipe or conduit shall not be smaller than metri c designator 21 (trade size 3?4) and, where of steel, shall have th e outer surface galvanized or otherwise metal-coated for corrosion protecti on. (b) Rod-type grounding electrodes of stainless steel and copper or zinc coa ted steel shall be at least 15.87 mm (5?8 in.) in diameter, unless listed. (6) Other Listed Electrodes. Other listed grounding electrodes shall be per mitted. (7) Plate Electrodes. Each plate electrode shall expose not less than 0.186 m2 (2 ft2) of surface to exterior soil. Electrodes of bare or conductively coated iron or steel plates shall be at least 6.4 mm (1?4 in.) in thickness. Solid, uncoated electrodes of nonferrous metal shall be at least 1.5 mm (0.06 in.) in thickness.
--
Tom Horne

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Polytechforum.com is a website by engineers for engineers. It is not affiliated with any of manufacturers or vendors discussed here. All logos and trade names are the property of their respective owners.