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,
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).
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.
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
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
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.
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,
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
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.
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,
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
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
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
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.
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).
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
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.
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,
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.
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
(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
(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
(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
(6) Other Listed Electrodes. Other listed grounding electrodes shall be per
(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.
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.
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).