We are always being told that the earth can't be trusted as a good return path for our electrical circuits, even when our ground resistance measurements imply otherwise. I have always assumed that this was due to uncontrollable variables, such as moisture, corrosion, stray currents, loose connections, etc., which could either degrade the paths continuity or unsafely reroute the currents path. Any other opinions or feedback?
Earth Resistance
- Thread starter cosmos
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Re: Earth Resistance
What does earth have to do with a return or fault current path? It is forbidden by the NEC to use earth as a conductor. Suppose your ground electrode had a very low impedance of 5-ohms. That is not low enough to even operate a 20-amp breaker.
LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
- Occupation
- Electrical Contractor
Re: Earth Resistance
If the question is why do we bother with grounding electrodes, it helps to minimize potential differences between the metallic equipment enclosures and the nearby grounded surfaces.
Think "bonding", not "grounding".
If the question is why do we bother with grounding electrodes, it helps to minimize potential differences between the metallic equipment enclosures and the nearby grounded surfaces.
Think "bonding", not "grounding".
- Location
- Lockport, IL
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- Semi-Retired Electrical Engineer
Re: Earth Resistance
But if you are talking about the neutral, then the resistance of that ?return path? would be too high to allow you to operate whatever circuit you had in mind. Let?s use the 5 ohms ground resistance value that Dereck suggested, and place it in series with a load that would draw 10 amps. The resistance of that load is 120/10, or 12 ohms. Add that to the 5 ohms of the ground resistance, and you get 17 ohms. Now divide 120/17, and the load will only get 7 amps. That also means that the load would be operating at 70% voltage.
Summary: The ground resistance is too low to be an effective path for clearing a fault, and too high to be an effective path for returning current from a load.
By the way, the resistance of planet Earth is nearly zero. The 5 ohms discussed above (and the 25 ohms mentioned in the code) is the resistance of the interface between the grounding electrode system and planet Earth.
May I ask what you mean by ?return path?? Are you referring to the ?grounded? conductor (i.e., neutral) or the ?equipment grounding conductor? (i.e., bare or green conductor)? If the later, then Dereck and Larry have answered the question.
But if you are talking about the neutral, then the resistance of that ?return path? would be too high to allow you to operate whatever circuit you had in mind. Let?s use the 5 ohms ground resistance value that Dereck suggested, and place it in series with a load that would draw 10 amps. The resistance of that load is 120/10, or 12 ohms. Add that to the 5 ohms of the ground resistance, and you get 17 ohms. Now divide 120/17, and the load will only get 7 amps. That also means that the load would be operating at 70% voltage.
Summary: The ground resistance is too low to be an effective path for clearing a fault, and too high to be an effective path for returning current from a load.
By the way, the resistance of planet Earth is nearly zero. The 5 ohms discussed above (and the 25 ohms mentioned in the code) is the resistance of the interface between the grounding electrode system and planet Earth.
LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
- Occupation
- Electrical Contractor
Re: Earth Resistance
Let us not forget that this also means that the system ground will have the other 30% of potential present between it and earth.
Re: Earth Resistance
Thanks for the replies! I was confused because many residential overhead lines sucessfully use the earth as the circuit return path. As far as ground resistance, I had assumed that ground currents would seek parallel return paths, and thereby very low resistance, from the ground fault to the ground rod; and I had never seen actual ohmeter readings measuring the ground resistance between two rods seperated by 25 feet. If ground rods do not protect interior wiring, why does the NEC require them, and why is 25 ohms their magic number?
Thanks for the replies! I was confused because many residential overhead lines sucessfully use the earth as the circuit return path. As far as ground resistance, I had assumed that ground currents would seek parallel return paths, and thereby very low resistance, from the ground fault to the ground rod; and I had never seen actual ohmeter readings measuring the ground resistance between two rods seperated by 25 feet. If ground rods do not protect interior wiring, why does the NEC require them, and why is 25 ohms their magic number?
wirenut1980
Senior Member
- Location
- Plainfield, IN
Re: Earth Resistance
We may never know...
We may never know...
B
bthielen
Guest
Re: Earth Resistance
I wonder if the 25 ohms has more to do with providing a low enough impedence path for lightning. With the voltage level involved in these situations, 25 ohms is all but insignificant.
Until I started reading the posts in this website, I thought grounding to earth provided safety for persons that may come into contact with potentially charged metal surfaces. With 25 ohms resistance to ground acceptable, this hardly seems to be the case. Bonding back to the source seems to be the safe way.
Bob
I wonder if the 25 ohms has more to do with providing a low enough impedence path for lightning. With the voltage level involved in these situations, 25 ohms is all but insignificant.
Until I started reading the posts in this website, I thought grounding to earth provided safety for persons that may come into contact with potentially charged metal surfaces. With 25 ohms resistance to ground acceptable, this hardly seems to be the case. Bonding back to the source seems to be the safe way.
Bob
charlie
Senior Member
- Location
- Indianapolis
Re: Earth Resistance
That never happens. The earth is connected to for lightning dissipation and contact of higher voltage lines with lower voltage lines. The circuit return in all cases is the grounded conductor. If that has been broken, you may achieve a return through the water piping system to your neighbor's home but it still will return on the grounded conductor from your neighbor's home to the transformer.
mdshunk
Senior Member
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- Right here.
Re: Earth Resistance
Charlie (or others)... then how do SWER (single wire, earth return) distribution systems in remote areas work?
Charlie (or others)... then how do SWER (single wire, earth return) distribution systems in remote areas work?
charlie
Senior Member
- Location
- Indianapolis
Re: Earth Resistance
Now you are talking about medium voltage not low voltage. Medium voltage is voltage levels that are greater than 1 kV, but less than or equal to 35 kV. The SWER system will use a ground grid to make the connection to the earth on both ends and it works well if you don't mind the "stray voltage" problem. Consider the standard 25 ohms of resistance at 120 volts and plug it into ohms law. Now, do the same thing with 13.2 kV Y/7.62 kV. You will find that current will flow well when medium voltage is involved. Because of stray voltage (stray current, actually), I do not like the SWER system.
Now you are talking about medium voltage not low voltage. Medium voltage is voltage levels that are greater than 1 kV, but less than or equal to 35 kV. The SWER system will use a ground grid to make the connection to the earth on both ends and it works well if you don't mind the "stray voltage" problem. Consider the standard 25 ohms of resistance at 120 volts and plug it into ohms law. Now, do the same thing with 13.2 kV Y/7.62 kV. You will find that current will flow well when medium voltage is involved. Because of stray voltage (stray current, actually), I do not like the SWER system.
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