fundamental electricity question

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Designer69

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I have a gap in understanding some very basic electricity flow and was hoping you guys could help me out.

in the picture below if I have a source and a motor say 500 feet away and I run just one single conductor source to load.

Run a ground cable directly into earth at source and load, will the motor actually run? The current will flow thru actual earth and return to the source? the earth is that good of a conductor?

thanks


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petersonra

petersonra

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Northern illinois
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It would work if you had a good enough connection through the dirt.

There are some parts of the world where the distribution system uses the earth as one of the conductors.
 
Jraef

Jraef

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It PROBABLY would run, but no, the earth is NOT a consistently good conductor, hence what you are looking at being a violation of the NEC. What you are showing is referred to as a Single Wire Earth Return (SWER) system and although power utilities can do it under certain conditions, we (end users) cannot do so legally.
 
jumper

jumper

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3 Hr 2 Min from Winged Horses
Not at low voltage. Dirt is not a great conductor and the impedance would prolly be too high.

What you are trying to do is done in rare cases for transmission lines at high voltages.

It is called SWER, single wire earth return. Google it.
 
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GoldDigger

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Designer69 said:
The current will flow thru actual earth and return to the source? the earth is that good of a conductor?

thanks
Click to expand...

The earth itself is a truly excellent conductor, not because it has particularly low resistivity but because there is so much of it. Enormously parallel path network.

The problem, however, is in making the connection to it.
Depending on the nature of the earth, soil chemistry and moisture content a simple driven ground rod could be a low as 5 ohms or less or 100 ohms and more. That is what limits the effectiveness of earth return at utilization voltages for reasonable power equipment. 10 ohms in a 120V circuit will only pass 12A into a short circuit, and the voltage drop will be a problem for any load over .5A.

But if the two end electrodes are identically configured but in locations 2000 miles apart the resistance of that part of the circuit will still be only 10 ohms.
 
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Ingenieur

Senior Member
Location
Earth
Depends on the grounding system
we typically strive for <2 Ohm
it would run fine depending on hp/current

a couple of rods at 25 Ohm?
not so good if at all

the farther away you get the better (lower) the gnd R beyong a certain L
it is illustrated by a fall of potential gnd R test

R = p x L/A
p = resistivity
L = distance
A = area of current gradient, est typ at L^2 / 4

R = p x 4/L
inverse to L, the larger L the lower R
 
Besoeker

Besoeker

Senior Member
Location
UK
Designer69 said:
I have a gap in understanding some very basic electricity flow and was hoping you guys could help me out.

in the picture below if I have a source and a motor say 500 feet away and I run just one single conductor source to load.

Run a ground cable directly into earth at source and load, will the motor actually run? The current will flow thru actual earth and return to the source? the earth is that good of a conductor?

thanks
Click to expand...
Might depend on whether you were in a swamp or a desert.
 
K8MHZ

K8MHZ

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Michigan. It's a beautiful peninsula, I've looked
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Electrician
GoldDigger said:
The earth itself is a truly excellent conductor, not because it has particularly low resistivity but because there is so much of it. Enormously parallel path network.

The problem, however, is in making the connection to it.
Depending on the nature of the earth, soil chemistry and moisture content a simple driven ground rod could be a low as 5 ohms or less or 100 ohms and more. That is what limits the effectiveness of earth return at utilization voltages for reasonable power equipment. 10 ohms in a 120V circuit will only pass 12A into a short circuit, and the voltage drop will be a problem for any load over .5A.

But if the two end electrodes are identically configured but in locations 2000 miles apart the resistance of that part of the circuit will still be only 10 ohms.
Click to expand...

A single ground rod in my area, mostly beach sand, is 1300 ohms impedance. Nearly useless.
 
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GoldDigger

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K8MHZ said:
A single ground rod in my area, mostly beach sand, is 1300 ohms impedance. Nearly useless.
Click to expand...
How far down to the water table?
Mike's video on driving deeper and deeper ground rods by his house in FL is interesting and also provides a good feel for the measurement techniques available. As I recall he got all the way down to 50 feet.
 
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mike_kilroy

Senior Member
Location
United States
I once experimented with a 6ft driven ground rod in out shop to the poco ground wire, on grd rod 100ft away outside building.

Ohm meter showed about 30kohms.

100vdc applied in series with 100 watt incondescent bulb did not light.

120VAC lite it full brilliance.

I don't know if true, but I read ac current changes molecules in dirt touching grd rod and greatly reduce resistance...

Sent from my SM-G900V using Tapatalk
 
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Carultch

Senior Member
Location
Massachusetts
Designer69 said:
I have a gap in understanding some very basic electricity flow and was hoping you guys could help me out.

in the picture below if I have a source and a motor say 500 feet away and I run just one single conductor source to load.

Run a ground cable directly into earth at source and load, will the motor actually run? The current will flow thru actual earth and return to the source? the earth is that good of a conductor?

thanks


View attachment 18538
Click to expand...


Single line / Earth return can work in theory, but in practice, it is difficult to make reliable connections in the Earth for current carrying purposes. Some soils are conductive to grounding electrodes, some have thousands of Ohms. That is why it is standard practice to use wire for the complete round trip path, you expect current to follow. The Earth itself is a great conductor, but the connections to it are difficult to make reliably to use it as a conductor in a circuit. Grounding between the EGC system above grade and everything below grade, is primarily used for voltage reference, without ever expecting it to carry a single ampere of load current.

Another reason why it is a bad idea to rely on the Earth alone for current return, is that a variation of voltage across the ground can cause current to flow through the two legs of people and the four legs of animals. Four leg animals are especially vulnerable to current caused by a ground potential gradient, because such current would flow through their heart. For one simple motor, it wouldn't be too harmful, but if everyone relied on this wiring system, it would be.

Jraef said:
I've had it with this bull!
Click to expand...

Funny you should mention a bull, as I was just talking about four leg animals being vulnerable to voltage gradients.
 
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