Im wondering why I never hear how important or not important it is to have the required resistance of the grounding electrode and not testing for 25 ohms. Is it really that critical,and why 25?what if its alot more or alot less? We have helping balance out the neutral,and hopefully for lightning, what else ? What else is the rod for ? what is the 25 ohmms exactly for,? and if it is so important why is it that i never see the inspector check this? Does anyone know the average resistance of lets say a commercial building with structural grounding or average ground rod readings ? If it is so serious why does the code not require a third ground rod to cut down on resistance if two ground rods still have more than 25 ohms resistance,is more a better thing or worse? Thank you
25 ohms
- Thread starter bond
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- Location
- Bremerton, Washington
- Occupation
- Master Electrician
Re: 25 ohms
The resistance of the groudning system is not important for most applications, perhaps 95%. Keep in mind we don't really know why its 25 ohms, and not 20 or 30. the grounding electrode system is only for lightning. Two ground rods give a 60% reduction over 1. You are free to install more ground rods if you want, but the 2005 NEC will require a conection to a concrete encased electrode (ufer) and in that case no ground rods are required.
The effort and worry about 25 ohms is mostly displaced, we need rather to ensure a low impedance ground fault return path to clear the OCPD in a line to case fault, or as I call it, Bonding, not Grounding.
The resistance of the groudning system is not important for most applications, perhaps 95%. Keep in mind we don't really know why its 25 ohms, and not 20 or 30. the grounding electrode system is only for lightning. Two ground rods give a 60% reduction over 1. You are free to install more ground rods if you want, but the 2005 NEC will require a conection to a concrete encased electrode (ufer) and in that case no ground rods are required.
The effort and worry about 25 ohms is mostly displaced, we need rather to ensure a low impedance ground fault return path to clear the OCPD in a line to case fault, or as I call it, Bonding, not Grounding.
hurk27
Senior Member
- Location
- Portage, Indiana NEC: 2008
Re: 25 ohms
At 25 ohm's, a 120 volt circuit will only draw 4.8 amps through a ground rod. as you can see this would not go very far to balance a neutral if the unbalance current on the neutral is over this amount which in most cases it will be. it have very little effect. But above 600 volts the story changes as at 600 volts a 25 ohm ground rod would pass 24 amps back to the source through the Earth. So the only two things the NEC intends the grounding electrode system to be used for is as Tom has said 1. Lightning protection, 2. For protection from voltages from higher source. as stated in 250.4(A)(1) and (B)(1).
A ground rod or made electrode will have very little to do with the AC power comming in from the utility transformer.
At 25 ohm's, a 120 volt circuit will only draw 4.8 amps through a ground rod. as you can see this would not go very far to balance a neutral if the unbalance current on the neutral is over this amount which in most cases it will be. it have very little effect. But above 600 volts the story changes as at 600 volts a 25 ohm ground rod would pass 24 amps back to the source through the Earth. So the only two things the NEC intends the grounding electrode system to be used for is as Tom has said 1. Lightning protection, 2. For protection from voltages from higher source. as stated in 250.4(A)(1) and (B)(1).
karl riley
Senior Member
Re: 25 ohms
As I remember Bennie tried to chase down the 25 ohm figure and one answer was that it was low enough for grounding a primary distribution line with its higher voltage.
Now it just gets repeated as if it meant something. We like to have a number.
Average utility ground rod resistance measured in 4 midwestern states by a university was about 113 ohms.
Karl
As I remember Bennie tried to chase down the 25 ohm figure and one answer was that it was low enough for grounding a primary distribution line with its higher voltage.
Now it just gets repeated as if it meant something. We like to have a number.
Average utility ground rod resistance measured in 4 midwestern states by a university was about 113 ohms.
Karl
karl riley
Senior Member
Re: 25 ohms
Just found another reference.
1) NESC requires no specific resistance for utility pole grounds.
2) "The resistance of an 8' ground rod for one utility varied from 40 ohms to 1150 ohms."
Reference, Jim Burke.
Karl
Karl, I edited out the e-mail address. Charlie
[ November 20, 2004, 06:13 AM: Message edited by: charlie ]
Just found another reference.
1) NESC requires no specific resistance for utility pole grounds.
2) "The resistance of an 8' ground rod for one utility varied from 40 ohms to 1150 ohms."
Reference, Jim Burke.
Karl
Karl, I edited out the e-mail address. Charlie
[ November 20, 2004, 06:13 AM: Message edited by: charlie ]
resistance
Senior Member
- Location
- WA
Re: 25 ohms
>>>>If it is so serious why does the code not require a third ground rod <<<<
Please don't give them any ideas!!
>>>>If it is so serious why does the code not require a third ground rod <<<<
Please don't give them any ideas!!
charlie
Senior Member
- Location
- Indianapolis
Re: 25 ohms
Karl, I have to take exception to that statement but I'll have to wait until I get back to the office to give you the reference. We are required to get 25 ohms but I don't remember where it comes from for sure but I believe it is required in the NESC.
bphgravity
Senior Member
- Location
- Florida
Re: 25 ohms
Section 96B of the NESC states that "individual made electrodes shall, where pratical, have a resistance to ground not exceeeding 25-ohms. If a single electrode resistance exceeds 25-ohms, two electrodes connected in parallel shall be used."
Looks quite similiar to 250.56 of the NEC.
Section 96B of the NESC states that "individual made electrodes shall, where pratical, have a resistance to ground not exceeeding 25-ohms. If a single electrode resistance exceeds 25-ohms, two electrodes connected in parallel shall be used."
Looks quite similiar to 250.56 of the NEC.
petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- engineer
Re: 25 ohms
I'm curious how this is measured. seems to me you can't just stick an ohm meter somewhere and measure it.
I'm curious how this is measured. seems to me you can't just stick an ohm meter somewhere and measure it.
bphgravity
Senior Member
- Location
- Florida
Re: 25 ohms
With one of these:
With one of these:
charlie
Senior Member
- Location
- Indianapolis
Re: 25 ohms
Thanks for the NESC reference Bryan. By the way, that is what our line crews use to measure the resistance of our ground rods. We drive our second rod on top of the first with a coupling followed with a third if we don't get 25 ohms, then we walk.
Thanks for the NESC reference Bryan. By the way, that is what our line crews use to measure the resistance of our ground rods. We drive our second rod on top of the first with a coupling followed with a third if we don't get 25 ohms, then we walk.
petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- engineer
Re: 25 ohms
Just what do you measure between?
Just what do you measure between?
karl riley
Senior Member
Re: 25 ohms
Charlie, thanks for the info. I knew I better identify the source, since I didn't know it myself.
Karl
Charlie, thanks for the info. I knew I better identify the source, since I didn't know it myself.
Karl
Re: 25 ohms
This is the closest answer I have ever found to the 25 ohm resistance for the ground rod has to do with the old high impedence audio eq.
Go to this site for more imformation http://www.equitech.com/articles/rep2.html
MYSTERIES EXPLAINED
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Theoretically, lowering the grounding system's impedance should reduce the voltage present, but in reality, Ohm's law demonstrates that significant improvement is unobtainable. Copper building wire, for example, 12-gauge wire used in branch circuit wiring, has only about 0.l ohms resistance for every 50 feet. Larger sizes have of course, even less, but the difference is insignificant. Regardless of the grounding conductor size and length, Ohm's law, there will still be an unacceptable voltage present in the grounding system.
.
Furthermore, the grounding electrode (ground rod, water pipe, etc.) presents an even greater obstacle. Many have had the experience of driving one or more ground rods with huge copper wires to supplement studio grounding systems and have learned something about futility.
.
Commonly, if one can achieve 25 ohms or less above true earth ground, one is doing quite well. Shaving millivolts off a signal reference grid (grounding system) through grounding techniques is truly a logistical nightmare.
.
But what if RF filters could be made to operate more cleanly? By design, they are most efficient at doing what they are intended, suppressing stray radio frequencies from entering or exiting equipment chassis. Notice in Figure 1 that they are balanced. However, the voltage supplied to them is unbalanced. Currently, the electrical industry recognizes only one basic type of single-phase, 120-volt circuit. To ensure system safety, one of the two 120-volt supply conductors is always grounded. However, this means that one side of the RF filter has potential relative to the grounding reference; the other is neutral. Consequently, AC leaks through the capacitors unchecked directly into the grounding system.
.
What would happen if the AC supply was balanced? Figure 2 is an example of such a 120-volt application with an RF filter. The potential to ground on each side of the AC line is 60 Volts with each side of the circuit 180 deg. out of phase to the other. Thus, a 120-volt supply is maintained.
.
As a result of this configuration, trace currents flowing 180 deg. out of phase through capacitors on opposite sides of the filter null at the common chassis ground connection. Voltage formerly present in the AC grounding system is thereby eliminated. The difference is dramatic. So pronounced is the effect that conventionally accepted studio grounding methods are exposed as essentially worthless in countering low-frequency noise problems. Harassing EMI ceases. When a symmetrical AC supply is used, the noise floor of any audio or video facility is greatly reduced.
Ronald
This is the closest answer I have ever found to the 25 ohm resistance for the ground rod has to do with the old high impedence audio eq.
Go to this site for more imformation http://www.equitech.com/articles/rep2.html
MYSTERIES EXPLAINED
.
Theoretically, lowering the grounding system's impedance should reduce the voltage present, but in reality, Ohm's law demonstrates that significant improvement is unobtainable. Copper building wire, for example, 12-gauge wire used in branch circuit wiring, has only about 0.l ohms resistance for every 50 feet. Larger sizes have of course, even less, but the difference is insignificant. Regardless of the grounding conductor size and length, Ohm's law, there will still be an unacceptable voltage present in the grounding system.
.
Furthermore, the grounding electrode (ground rod, water pipe, etc.) presents an even greater obstacle. Many have had the experience of driving one or more ground rods with huge copper wires to supplement studio grounding systems and have learned something about futility.
.
Commonly, if one can achieve 25 ohms or less above true earth ground, one is doing quite well. Shaving millivolts off a signal reference grid (grounding system) through grounding techniques is truly a logistical nightmare.
.
But what if RF filters could be made to operate more cleanly? By design, they are most efficient at doing what they are intended, suppressing stray radio frequencies from entering or exiting equipment chassis. Notice in Figure 1 that they are balanced. However, the voltage supplied to them is unbalanced. Currently, the electrical industry recognizes only one basic type of single-phase, 120-volt circuit. To ensure system safety, one of the two 120-volt supply conductors is always grounded. However, this means that one side of the RF filter has potential relative to the grounding reference; the other is neutral. Consequently, AC leaks through the capacitors unchecked directly into the grounding system.
.
What would happen if the AC supply was balanced? Figure 2 is an example of such a 120-volt application with an RF filter. The potential to ground on each side of the AC line is 60 Volts with each side of the circuit 180 deg. out of phase to the other. Thus, a 120-volt supply is maintained.
.
As a result of this configuration, trace currents flowing 180 deg. out of phase through capacitors on opposite sides of the filter null at the common chassis ground connection. Voltage formerly present in the AC grounding system is thereby eliminated. The difference is dramatic. So pronounced is the effect that conventionally accepted studio grounding methods are exposed as essentially worthless in countering low-frequency noise problems. Harassing EMI ceases. When a symmetrical AC supply is used, the noise floor of any audio or video facility is greatly reduced.
Ronald
charlie
Senior Member
- Location
- Indianapolis
Re: 25 ohms
I posted this in another thread but I feel like it bears repeating. Bennie was one of our most loved participants and has passed away. He wrote, "I am getting close to discovering the creation of the 25 Ohm rule, for electrode to earth connection.
I have a friend, from my teenage years, who is now retired as a mathematics professor, and electronic engineer.
I asked him about the 25 Ohm resistance. He acted surprised when I asked; like saying " you mean you don't know?"
The answer is complex due to the math involved, which is beyond my capability.
A simple answer is; A static charge, on metallic objects, produced by a common high voltage surge,
Will reach equilibrium with the earth within 10 seconds when connected by not more than 25 Ohms resistance in the path to the earth.
The initial surge will have a time domain of 2 to 10 ms. The resultant electrostatic charge will not equalize in a short time, if there is a high resistance path between the negative and positive charges.
The math is from Charles Coulomb and Johann Gauss."
The previous information is found in this thread.
I posted this in another thread but I feel like it bears repeating. Bennie was one of our most loved participants and has passed away. He wrote, "I am getting close to discovering the creation of the 25 Ohm rule, for electrode to earth connection.
I have a friend, from my teenage years, who is now retired as a mathematics professor, and electronic engineer.
I asked him about the 25 Ohm resistance. He acted surprised when I asked; like saying " you mean you don't know?"
The answer is complex due to the math involved, which is beyond my capability.
A simple answer is; A static charge, on metallic objects, produced by a common high voltage surge,
Will reach equilibrium with the earth within 10 seconds when connected by not more than 25 Ohms resistance in the path to the earth.
The initial surge will have a time domain of 2 to 10 ms. The resultant electrostatic charge will not equalize in a short time, if there is a high resistance path between the negative and positive charges.
The math is from Charles Coulomb and Johann Gauss."
The previous information is found in this thread.
- Location
- Bremerton, Washington
- Occupation
- Master Electrician
Re: 25 ohms
petersonra
The clamp on meter can only be used on a rod that is connected to the electrical system. It injects a pulse and comes up with the resistance somehow.
petersonra
The clamp on meter can only be used on a rod that is connected to the electrical system. It injects a pulse and comes up with the resistance somehow.
charlie
Senior Member
- Location
- Indianapolis
Re: 25 ohms
Tom, since I no longer work in the field, I am not sure about this statement. I don't believe the rod can be connected to anything or you will get a false reading.
Tom, since I no longer work in the field, I am not sure about this statement. I don't believe the rod can be connected to anything or you will get a false reading.
- Location
- Massachusetts
Re: 25 ohms
I was on a job where we had both this tester...
I was on a job where we had both this tester...
pierre
Senior Member
- Location
- Westchester County, New York
Re: 25 ohms
Bob
What may have happened is the clamp type tester requires the conductor to be connected to the system. The 3 point system requires the conductor to be disconnected. (as I understand it). maybe the conductor was disconnected when the clamp type was connected, creating a high resistance?
Bob
What may have happened is the clamp type tester requires the conductor to be connected to the system. The 3 point system requires the conductor to be disconnected. (as I understand it). maybe the conductor was disconnected when the clamp type was connected, creating a high resistance?
Re: 25 ohms
Bob, I have one of AEMC meters, and it does have a couple of drawbacks. In order for it to work, the GEC has to be terminated to the neutral and GEC, plus the transformer gound must be made up.
The other thing is the meter will always read a higher impedance than it acutally is. This is because the meter reads the UUT impedance plus the series impedance of the utility ground rod. The meter is reading a loop.
So if the neutral is open it will not take a reading. On the other side of the coin if the meter reads something extremely low, you are probable reading a loop inside the building. So you have to exercise some care with it. I use it for spot inspection, not actual impedance.
Bob, I have one of AEMC meters, and it does have a couple of drawbacks. In order for it to work, the GEC has to be terminated to the neutral and GEC, plus the transformer gound must be made up.
The other thing is the meter will always read a higher impedance than it acutally is. This is because the meter reads the UUT impedance plus the series impedance of the utility ground rod. The meter is reading a loop.
So if the neutral is open it will not take a reading. On the other side of the coin if the meter reads something extremely low, you are probable reading a loop inside the building. So you have to exercise some care with it. I use it for spot inspection, not actual impedance.
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