reggieexum1
Member
Hi,
What (in ohms) is the minimum reading to earth at the service ground.
What (in ohms) is the minimum reading to earth at the service ground.
Grounding and bonding
- Thread starter reggieexum1
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bphgravity
Senior Member
- Location
- Florida
Re: Grounding and bonding
Regardless of the fact the code does not seem to require retesting of the resistance to ground, it is best to at least try to obtain the lowest value as possible or at least to 25-ohms.
It is disputed that the grounding electrode system provides very limited protection from external power surges or lightning, but basic Ohm's Law does show reducing ground resitance does help to reduce touch voltage which is lethal at above 30-volts.
Spacing of the second electrode and adding soil treatment can help to ensure a good low impedance path.
Just some food for thought.
Regardless of the fact the code does not seem to require retesting of the resistance to ground, it is best to at least try to obtain the lowest value as possible or at least to 25-ohms.
It is disputed that the grounding electrode system provides very limited protection from external power surges or lightning, but basic Ohm's Law does show reducing ground resitance does help to reduce touch voltage which is lethal at above 30-volts.
Spacing of the second electrode and adding soil treatment can help to ensure a good low impedance path.
Just some food for thought.
- Location
- Illinois
- Occupation
- retired electrician
Re: Grounding and bonding
Bryan,
Don
Bryan,
How? The earth is too poor of a conductor to make any real reduction in touch voltage. If you apply 120 volts to a ground rod and measure the voltage between the rod and the earth a foot away, you will find about 80 volts. A very large portion of the voltage drop occurs very close to the rod.
Don
bphgravity
Senior Member
- Location
- Florida
Re: Grounding and bonding
We have been down this road before, Don.
I have seen all the evidents of Earth rings around a conductor and the estimated VD at each. My statment is that OHM'S LAW doesnt lie.
Simply put, reduce the resistance, raise the current, and drop the voltage. I am not claiming an Earth return path is going clear any faults, I am just saying without a doubt, that touch voltage can be reduced by lowering Earth resistance to the electrode.
We have been down this road before, Don.
I have seen all the evidents of Earth rings around a conductor and the estimated VD at each. My statment is that OHM'S LAW doesnt lie.
Simply put, reduce the resistance, raise the current, and drop the voltage. I am not claiming an Earth return path is going clear any faults, I am just saying without a doubt, that touch voltage can be reduced by lowering Earth resistance to the electrode.
- Location
- Illinois
- Occupation
- retired electrician
Re: Grounding and bonding
Bryan,
Show me some numbers on how the ground rod can reduce the touch potential.
Don
Bryan,
That is a true statement, but it is also the reason why a driven rod has no real effect on the touch voltage.
Show me some numbers on how the ground rod can reduce the touch potential.
Don
karl riley
Senior Member
Re: Grounding and bonding
Bennie, you could also try wearing shoes (rubber soled of course). But if you are trying to reduce touch potential with a good ground rod during a lightning strike, without shoes, good luck!
Karl
Bennie, you could also try wearing shoes (rubber soled of course). But if you are trying to reduce touch potential with a good ground rod during a lightning strike, without shoes, good luck!
Karl
bphgravity
Senior Member
- Location
- Florida
Re: Grounding and bonding
Lets assume that a ground has 100-ohms Earth reistance.(for calculation puposes, not tested) The total return would then be 200-ohms. By Ohm's Law, 120v/200-ohms =.6-amp.
The ground rod Earth resistance at the house is 100-ohm x .6-amp = 60-volt shock hazard.
Add second rod more than six feet from the first and connect in parallel. Now, resistance to Earth has been rduces to 50-ohms at the house. The total return resistance is 150-ohms. By Ohm's Law, 120v/150-ohms =.8-amp.
The ground rod Earth resistance at the house is 50-ohm x .8-amp = 40-volt shock hazard.
Lets assume that a ground has 100-ohms Earth reistance.(for calculation puposes, not tested) The total return would then be 200-ohms. By Ohm's Law, 120v/200-ohms =.6-amp.
The ground rod Earth resistance at the house is 100-ohm x .6-amp = 60-volt shock hazard.
Add second rod more than six feet from the first and connect in parallel. Now, resistance to Earth has been rduces to 50-ohms at the house. The total return resistance is 150-ohms. By Ohm's Law, 120v/150-ohms =.8-amp.
The ground rod Earth resistance at the house is 50-ohm x .8-amp = 40-volt shock hazard.
- Location
- Illinois
- Occupation
- retired electrician
Re: Grounding and bonding
Bryan,
In both cases the voltage at the point where the grounding electrode conductor is connected to the rod(s) will be 120 volts to earth. While you have reduced the voltage drop through the earth, by adding a second rod you have not changed the touch potential. In both cases someone standing on the earth away from the rod and at the same time touching a conductive object that is connected to the grounding system will be subjected to a touch potential of 120 volts.
Don
Bryan,
In both cases the voltage at the point where the grounding electrode conductor is connected to the rod(s) will be 120 volts to earth. While you have reduced the voltage drop through the earth, by adding a second rod you have not changed the touch potential. In both cases someone standing on the earth away from the rod and at the same time touching a conductive object that is connected to the grounding system will be subjected to a touch potential of 120 volts.
Don
- Location
- Illinois
- Occupation
- retired electrician
Re: Grounding and bonding
Bennie,
Don
Bennie,
In the high voltage switch yard, there will be a grid of copper conductors not far below the surface. This is connected to the grounding electrode system for the yard. I though that the intent of this was to reduce the step and touch potential within the yard in the event of a fault. I have read that under fault conditions, the voltage of the yard grounding system can be 5000+ volts above that of the earth a 100' or so away from the yard. If the intent is to create a reduction in the step and touch voltage, why would you want to use a high resistance ground cover?
Don
Re: Grounding and bonding
Don, you are correct that sub-stations and generation plants use grids to minimize touch potentials during a fault condition. Typically utilities will extend the grid 7 feet or more beyond the fence line to protect the public and livestock. Grids are the only cost efficient means I know of to minimize touch potential around ground electrode connections. They more or less represent an equipotential ground plane with holes.
Bennie, utilities use gravel as a cheap ground cover to prevent erosion, vegetative growth, and provide good drainage. I do not believe it is chosen for its electrical properties, just economics.
Don, you are correct that sub-stations and generation plants use grids to minimize touch potentials during a fault condition. Typically utilities will extend the grid 7 feet or more beyond the fence line to protect the public and livestock. Grids are the only cost efficient means I know of to minimize touch potential around ground electrode connections. They more or less represent an equipotential ground plane with holes.
Bennie, utilities use gravel as a cheap ground cover to prevent erosion, vegetative growth, and provide good drainage. I do not believe it is chosen for its electrical properties, just economics.
Re: Grounding and bonding
IEEE Standard No. 80 & 837 Sub-Station Grounding.
The ground fault protection is based on a surface of crushed stone.
A layer of crushed stone in the station and one metre beyond the fence provides further protection by isolating a person walking or standing inside the station from earth.
IEEE Standard No. 80 & 837 Sub-Station Grounding.
The ground fault protection is based on a surface of crushed stone.
A layer of crushed stone in the station and one metre beyond the fence provides further protection by isolating a person walking or standing inside the station from earth.
bphgravity
Senior Member
- Location
- Florida
Re: Grounding and bonding
Don: Your supposing the only fault path is Earth and not the grounded return at the service. This is not the subject of the topic. This topic is how low ground resistance should be on anormal system.
When a conductor or metal equipment is connected to an Earth electrode, it is forced to take the same ZERO potential as the Earth. Any attmept to raise the voltage of a grounded object results in current passing over the connection until the potential of the object and the Earth are equal.
Most of the fault current flows back through the grounded conductor. By reducing the resistance to ground, you ensure voltage wont increase on the metal parts.
Don: Your supposing the only fault path is Earth and not the grounded return at the service. This is not the subject of the topic. This topic is how low ground resistance should be on anormal system.
When a conductor or metal equipment is connected to an Earth electrode, it is forced to take the same ZERO potential as the Earth. Any attmept to raise the voltage of a grounded object results in current passing over the connection until the potential of the object and the Earth are equal.
Most of the fault current flows back through the grounded conductor. By reducing the resistance to ground, you ensure voltage wont increase on the metal parts.
Re: Grounding and bonding
When insulated from ground, the human body will be at the voltage of the conductor. Like working high voltage bare handed. With no difference in potential from the hand to the foot there is no circuit.
The danger with bare handed maintenance on high voltage lines, is a nearby flash over, causing an extremely high current flow on the conductor being worked. The voltage across a small length can be at a lethal level.
When insulated from ground, the human body will be at the voltage of the conductor. Like working high voltage bare handed. With no difference in potential from the hand to the foot there is no circuit.
The danger with bare handed maintenance on high voltage lines, is a nearby flash over, causing an extremely high current flow on the conductor being worked. The voltage across a small length can be at a lethal level.
- Location
- Illinois
- Occupation
- retired electrician
Re: Grounding and bonding
Bryan,
Bryan,
This is one of the most common fallacies about grounding. The earth is just too poor of a conductor to drive the voltage of the electrical system down to ZERO volts to earth. The relative impedance of the grounding electrode system is many 100s of times greater than that of the main fault return path. Changing the grounding electrode impedance from 25 to 5 or even 1 ohm really doesn't make much difference on the voltage that appears on the electrical grounding system under fault conditions. The voltage that will appear on the grounding system under fault conditions will equal that of the voltage drop of the fault return path. Look at a 200 amp service with a 150', 2/0 copper service and a fault in the panel on the load side of the breaker. Based on the time/trip curve, a 4000 amp fault could take as long as one second to clear. The voltage drop on the 150', 2/0 grounded conductor would be 58.02 volts. This 58 volts will appear on everything that is connected to the electrical grounding system until the fault is cleared. A 25 ohm grounding electrode would drop this voltage to 57.98. A 1 ohm grounding electrode system would drop this voltage to 57.19 volts. This voltage will be between any part of the electrical grounding system and the earth at any point more than a few feet from the grounding electrode. The earth connection does not give any increase in electrical safety when ground faults occur within the building electrical system. The most important part of an electrical safety under ground fault conditions is to provide a very low impedance ground fault path back to the power source. This is only accomplished by bonding all nonconductive parts of the electrical system back to the grounded conductor at the service.
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