I work for a company that sells center pivot irrigation systems. A
is the pivot and electric well motor are powered from a commercial
utility. The electrical service is three phase, 480 volts, and there
will be 1300 feet of quad wire buried from the utility's meter to the
well and pivot.
We sometimes see that one of the underground power wires has gone
bad. Someone will simply substitute the equipment ground wire for the
bad power wire. People think the ground rods and earth will keep them
safe. I'd like to have a short illustration showing that it won't.
I found a chart in an article that shows earth resistance. Farm
ground is 100 ohms/meter. Thirteen hundred feet or 396.24 meters x 100
ohms equals 39,624 ohms resistance in the dirt. I added 50 ohms
resistance for the two ground rods that would be at the utility's power
pole and at the well.
An online Ohm's law calculator put the current flow from the well
motor to the utility's supply at 0.012 amps if there was a short to the
well motor's frame. That wouldn't blow even the smallest fuse in the
equipment. Am I at all on the right track with this?
On Fri, 25 Jan 2019 09:45:51 +0800, Rheilly Phoull
That would prove the earth would carry an amp but it takes at least 15
times that to trip a breaker.
NEC 250.4(A)(5) is pretty clear on that
"The earth shall not be considered as an effective ground-fault
The aquifer here isn't necessarily like a big lake. There will be
water where the
well is, of course, but might not be under the pole where the power
is. One nearby farmer has, I think, three wells supplying a single
pivot. His neighbors a couple miles away get along just fine with a
I'd like to write a short explanation why we need four wires on
and directly relate it to irrigation systems and the wells. A couple
or just a single page is what I'm shooting for. My idea for a title
was Three Wires on
Three Phase Equals Rich Young Widow, (or it could)
How would you do it?
Good title, I think I would dwell on the fact that there would be no
insurance if the wiring was not up to code and that is the first thing
insurance companies would do. Then again perhaps others in the family
like children or wives could be the victims.
The integrity of all 4 of the feed conductors and their connections
should (*MUST*) be maintained at all times. THAT is the safest
manner. That low resistance fault return conductor is essential to
the safety aspect of the design, and is also supposed to allow for
the most likely current level to cause a trip since a short is
usually a high current event anyway. Then it comes down to what
duration the breaker needs at a given overamperage level to trip.
Is it instantaneous or does the trip current level not caue a trip if
it is of too short a duration?
With the service at the center pivot/well I suspect what is happening is
a service phase conductor (meter to pivot service equipment) is failing
and the service neutral is being substituted. The service neutral is
originally earthed at the supply transformer and the pivot service.
The 100 ohms/meter is the "bulk resistivity" of earth. You can't
multiply by length to get a resistance. It is likely the resistance of a
1 ft cube of earth, measured between 1 ft square plates on opposite
sides. If you are measuring between ground rods, the further you are
from the rods the more earth is in the path.
You are using 25 ohms as the resistance-to-earth from a ground rod. It
is the maximum allowed by the NEC. Unless the installer actually
measured it, there is no reason to believe it is that low (or it could
be less). Dry earth would be significantly higher resistance than wet
earth. If 2 connected rods are installed there is no NEC requirement for
resistance, so a common practice is to just install 2 rods. Ground rods
are better than nothing, but not by much.
Your example would be 277 V (phase-to-ground) divided by 50 ohms, which
is 5.5 A, which is likely much less than any branch circuit protection.
As gfretwell wrote, the NEC does not allow the earth to be used as the
path to trip overcurrent protection. That is because an earth path will
not reliably work.
(Around here corner-grounded 480 V might be used, so disconnects are
So imagine one phase of a 60 HP motor gets grounded. The fault current
of 5.5 A may eventually trip the motor overload protection (running
current at 60 HP is about 75 A). Or if the motor is not loaded to 60 HP
- not. Or if the ground is before the motor starter (and overload
protection) - not. Divide the 277 V phase voltage between the 2 rods -
138V at each rod. In general 70% of the voltage drop away from the rod
is in the first 3 ft from the rod. From your "grounded" equipment 3ft
away from the rod there will be at 96 volts from equipment "ground" to
earth. You don't have to be far from the rod to have more like 138 V.
That assumes that the utility earthing is the same as at the service,
which very well may not be true.
If the service is at the pivot/well and, in effect, earthed through the
well, 277V may appear near the meter can and there could be hazardous
voltages on the equipment away from the well.
If the resistance-to-earth is lower there is a higher probability of
tripping overload protection (but not likely the branch circuit
protection). But the contact-voltage hazard does not change.
IMHO there is a significant shock or electrocution hazard. In addition
to lawsuits from victims/survivors, OSHA could be very unhappy, which
means companies/individuals become very unhappy. It is, in any case, a
For the 70% drop away from a rod:
Imagine concentric 1" thick cylinders of earth, the length of a rod,
away from the rod. They are in series, and the resistance-to-earth will
be the sum of the resistance of each cylinder. Compared to the cylinder
at 1 ft, the cylinder at 2 ft has 2 x as much earth, thus 1/2 the
resistance. The cylinder at 4 ft has 4 x the earth, thus 1/4 the
resistance as the cylinder at 1 ft. The resistance rapidly decreases
away from the rod, hence most of the voltage drop occurs near the rod.
For your amusement, approximate 3-ph currents @480 V:
60 HP 77 A
100 HP 124 A
1/2 HP 1.1 A
2 HP 3.4 A
And hiking a corn field with a wire locator in July or August isn't
any fun. The temptation to cheat "temporarily" gets the best of people.
A local mechanic claims there is nothing more permanent than temporary.
That directly answers my question. An ungrounded well 50 feet
from the electric supplier's meter isn't really any safer than an
ungrounded well 1300 hundred feet from the electric supplier's meter.
We started using ufer grounds a few years ago but there are bunches
of wells and pivots with just ground rods. The company has been
furnishing 10 foot ground rods which might help a bit.
Some of the surrounding utilities are using those. One is
apparently switching to a wye connection. Variable speed drives on
a well motor can save $3/hour according to the University of Nebraska.
I'm hoping those catch on. A vfd sees corner ground as single phase
power so it has to be over sized on those. More dollars, of course.
I hadn't thought about that. A shorted motor on the pivot could
put dangerous voltage on the well motor or panel.
I like that.
Center pivot irrigation seems to be spreading like weeds.
Ufer is concrete encased electrode (20 ft) - in pivot base??
You can also get sectional ground rods. You drive a rod, attach a
coupling, add a rod on top, and drive it. Can do multiple rods.
Well casing might be good.
The argument I have heard for corner-grounded is cheaper 2-pole
disconnects. Single phase VFD should negate that, and I wouldn't think a
utility would like the unbalanced load.
Interesting what backgrounds (like you) are around a.h.r
I did know one or two guys who claimed they had the gift. One used
the old farmer CeeTee pliers if I remember correctly. Digging a new well
generally isn't done during growing season unless the outer casing
collapses. Guys just make do if the issue is the the well losing water.
The well pumps some air which escapes through the sprinklers nearest the
center point. They might be able to have column added to lower the
bowls depending on the static water level.
I grew up on a farm in eastern Nebraska during the 60s and 70s.
Irrigation was mainly furrow irrigation. Land levelers were busy guys
making it possible for water to run from one end of the field to the
other. I don't remember any pivots in my immediate area back then.
Pivots gradually moved into eastern and central Nebraska as farms got
bigger. Furrow irrigation requires a lot of labor and isn't very
efficient for water use. Guys can control and monitor their pivots from
their cell phones now. Furrow irrigation involved hiking the field to
see if the water had made it to the lower end of the field. No more
opening and closing gates on pipe to irrigate another section of the field.
Yes. I must've learned Mr. Ufer's name from a newsgroup or maybe
from Mike Holt's site. There are still bomb storage facilities east of
Casing in the old days was concrete. Well drillers are using pvc now.
Our electrical suppliers don't sell single phase disconnects for 480
volt. We have to put dummy fuses in the grounded leg. The power
suppliers want the grounded leg switched even though they put in single
phase meters. The grounded leg and equipment ground are joined in the
power company's meter.
It beats talking to my coworkers. All of the different experiences
are a big help. I hope to maybe make something like a diagram on Mike
Holt's site. It shows a power pole with a goofball kneeling next to it.
120 volts. It shows him touching the pole and the voltage rings around
the pole. My thought is to use a picture of an irrigation well and
adjust the voltages to show what a farmer might encounter if a well or
pivot isn't grounded correctly.
Something like this: http://preview.alturl.com/o9ckv
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