Im very cofused, Im learning that many electricians do not know or even care to know the real reasons behind the service neutral and grounding.Please is there someone out there that can explain this to me and convince me that you are right,seeing im getting five different stories.Is the neutral from the transformer on the street to a 240/120 residence really grounded or not? and what is its function? Is it supposed to open circuit breakers in case of a short circuit? Is it supposed to open breakers in case of a ground fault,or both?Whould the breaker trip anyway even if i lost the neutral somewhere along the line?Would the water pipe and groung rod be able to accomplish this if i lost the grounded neutral?and if not why?why? am i learning that earth grounding will not clear a ground fault? what about a short circuit?Is resistance of the earth great or not?Isnt the circuit breaker thermaly protected anyway? Why would i need the neutral to trip it What is the use of a ground rod anyway if its only good for 4.8 amps maximum current at 25 ohms.If a breaker is thermally and magneticly protected how about a bus fuse.? By the way if the grounded neutral is bonded to the grounding electrodein the main disconnect why does,nt some return current flow over the waterpipe and electrode as well as the neutral ? if so isnt this dangerous?why is it so hard for electricians to want to know this. I feel as an apprentice it is imperative to understand all of this but what is frustrating is that Im getting half a dozen different stories even from my teacher or is it not many truly know anyof this.PLEASE HELP
grounding,bonding
- Thread starter bond
- Start date
- Status
- Location
- Illinois
- Occupation
- retired electrician
Re: grounding,bonding
Q1: Is the neutral from the transformer on the street to a 240/120 residence really grounded or not?
A1: The utility connects XO (neutral) at the transformer to their grounding electrode system.
Q2: Is it supposed to open circuit breakers in case of a short circuit?
A2: If the short circuit is between an ungrounded conductor and the grounding conductor, then the grounded conductor is one side of the circuit and will flow enough current to open the OCPD (overcurrent protective device). If the short circuit is between two ungrounded conductors, the grounded conductor will play no part in clearing the fault.
Q3: Is it supposed to open breakers in case of a ground fault
A3: The grounded conductor will be part of the fault clearing path for a ground fault. The path would be; hot to equipment grounding conductor, to main bonding jumper, to grounded conductor and back to the source.
Q4: Would the breaker trip anyway even if I lost the neutral somewhere along the line?
A4: If the grounded conductor if open between the ground fault and the transformer there may not be a low enough impedance fault return path to open the OCPD. An open neutral will have no effect on clearing a fault between two ungrounded conductors.
Q5: Would the water pipe and ground rod be able to accomplish this if I lost the grounded neutral?
A5: It is very very unlikely that a ground rod would be able to open the OCPD under ground fault conditions if the neutral if open. A common under ground metal water piping system may be able to open the OCPD if it is bonded to the grounded conductor at a second building.
Q6: Isn't the circuit breaker thermally protected anyway? Why would i need the neutral to trip it
A6: The breaker reacts to excessive current. If there is a ground fault and the grounded conductor is open, then there will not be excessive current and the breaker will not react.
Q7: What is the use of a ground rod anyway if its only good for 4.8 amps maximum current at 25 ohms.
A7: See 250.4(A)(1).
Q8: By the way if the grounded neutral is bonded to the grounding electrodein the main disconnect why doesn't some return current flow over the waterpipe and electrode as well as the neutral? if so int this dangerous?
A8: Current flows on all parallel paths so some of the grounded conductor current returns to the transformer via the earth and the metal under ground water piping system. In the case of metal under ground water piping system that is common to multiple buildings served by the same utility transformer, it would not be uncommon to find that 15% of more of the grounded conductor current is returning to the transformer via the water piping system. As long as the grounded conductor between the service and the transformer is in good condition, this is not dangerous but it does create excessive electromagnetic fields that some believe to be a hazard. If the grounded conductor is open the current flow could cause excessive heat in the water pipe system and if a plumber opens the pipe to make a repair he will get shocked.
Don
Q1: Is the neutral from the transformer on the street to a 240/120 residence really grounded or not?
A1: The utility connects XO (neutral) at the transformer to their grounding electrode system.
Q2: Is it supposed to open circuit breakers in case of a short circuit?
A2: If the short circuit is between an ungrounded conductor and the grounding conductor, then the grounded conductor is one side of the circuit and will flow enough current to open the OCPD (overcurrent protective device). If the short circuit is between two ungrounded conductors, the grounded conductor will play no part in clearing the fault.
Q3: Is it supposed to open breakers in case of a ground fault
A3: The grounded conductor will be part of the fault clearing path for a ground fault. The path would be; hot to equipment grounding conductor, to main bonding jumper, to grounded conductor and back to the source.
Q4: Would the breaker trip anyway even if I lost the neutral somewhere along the line?
A4: If the grounded conductor if open between the ground fault and the transformer there may not be a low enough impedance fault return path to open the OCPD. An open neutral will have no effect on clearing a fault between two ungrounded conductors.
Q5: Would the water pipe and ground rod be able to accomplish this if I lost the grounded neutral?
A5: It is very very unlikely that a ground rod would be able to open the OCPD under ground fault conditions if the neutral if open. A common under ground metal water piping system may be able to open the OCPD if it is bonded to the grounded conductor at a second building.
Q6: Isn't the circuit breaker thermally protected anyway? Why would i need the neutral to trip it
A6: The breaker reacts to excessive current. If there is a ground fault and the grounded conductor is open, then there will not be excessive current and the breaker will not react.
Q7: What is the use of a ground rod anyway if its only good for 4.8 amps maximum current at 25 ohms.
A7: See 250.4(A)(1).
Q8: By the way if the grounded neutral is bonded to the grounding electrodein the main disconnect why doesn't some return current flow over the waterpipe and electrode as well as the neutral? if so int this dangerous?
A8: Current flows on all parallel paths so some of the grounded conductor current returns to the transformer via the earth and the metal under ground water piping system. In the case of metal under ground water piping system that is common to multiple buildings served by the same utility transformer, it would not be uncommon to find that 15% of more of the grounded conductor current is returning to the transformer via the water piping system. As long as the grounded conductor between the service and the transformer is in good condition, this is not dangerous but it does create excessive electromagnetic fields that some believe to be a hazard. If the grounded conductor is open the current flow could cause excessive heat in the water pipe system and if a plumber opens the pipe to make a repair he will get shocked.
Don
russ
Senior Member
- Location
- Burbank IL
Re: grounding,bonding
Don's done a good job of answering your questions.
The biggest things to remember is grounding doesn't clear faults. Grounding is a means to dissipate high voltage, like lightning strikes.
Bonding the metal electrical system to the grounded conductor (NEUTRAL to most guys in the field) is what allows the clearing of faults.
When a ungrounded (HOT) conductor touches ground, (wire or conduit) the current travels back to the panel, across the bond (screw, strap or wire), to the grounded conductor, and then back to the transformer.
[ February 24, 2004, 09:16 PM: Message edited by: russ ]
Don's done a good job of answering your questions.
The biggest things to remember is grounding doesn't clear faults. Grounding is a means to dissipate high voltage, like lightning strikes.
Bonding the metal electrical system to the grounded conductor (NEUTRAL to most guys in the field) is what allows the clearing of faults.
When a ungrounded (HOT) conductor touches ground, (wire or conduit) the current travels back to the panel, across the bond (screw, strap or wire), to the grounded conductor, and then back to the transformer.
[ February 24, 2004, 09:16 PM: Message edited by: russ ]
Re: grounding,bonding
Bond, Don has answered your questions, but you seem to be confused about the dividing line between POCO and end users. POCO's are regulated by the NESC, and end users by the NEC. They have very different rules for different reasons.
The best analogy I have heard used is the NESC rules are developed to keep the power on, and the NEC are developed to turn power off.
Utilities multi-ground their neutral for one main reason. To use earth as a conductor (there are other reasons, but money talks). The NEC forbids earth as a conductor. The reason is simple; voltage levels and distribution size are too small. At the lower voltages used by consumers the impedance of earth is too high to act as a conductor. The size of the GES required by the NEC is too small to produce a low impedance.
Where as a utility the system voltages are much higher, the neutral is grounded every forth pole/tower, and the size of the system makes the impedance much lower. It is basically ohm's law, and parallel circuitry.
For users, or NEC requirements the earth really serves no purpose other than safety. Its main purposes are:
1. To provide a planned discharge path for lightning and accidental contact with high voltages such as contact between primary to secondary distribution.
2. Stabilize system voltages to prevent insulation stress. The magnitude of overvoltages are reduced. When a power supply is grounded, its leakage capacitance to ground is shorted. Since this capacitance is shorted, overvoltage conditions due to resonance between capacitance and distributed inductance are avoided. With the magnitudes of transient overvoltages reduced, the lifetime of all components will increase, improving the overall reliability of the equipment.
Once you reference the system to ground via the MBJ the reasons for grounding/bonding change as follows:
1. Grounding and bonding of metallic frames keeps potential differences between frames to a safe value when lightning currents flows or shorts occur. Grounding of power sources prevents system voltages from permanently appearing on frames that personnel can come in contact.
2. Provides a planned low-impedance return path for all power supplies to permit safe operation of OCPD's via EGC's
3. Electrostatic discharges are quickly equalized.
Here is one more thing to think about. Are planes, trains, automobiles, and boats referenced to earth? Absolutely not, there is no reason too, nor is it needed for these types of separately derived systems. They have what is called ground, but it's not, just a fault clearing path and potential difference equalizer.
Bond, Don has answered your questions, but you seem to be confused about the dividing line between POCO and end users. POCO's are regulated by the NESC, and end users by the NEC. They have very different rules for different reasons.
The best analogy I have heard used is the NESC rules are developed to keep the power on, and the NEC are developed to turn power off.
Utilities multi-ground their neutral for one main reason. To use earth as a conductor (there are other reasons, but money talks). The NEC forbids earth as a conductor. The reason is simple; voltage levels and distribution size are too small. At the lower voltages used by consumers the impedance of earth is too high to act as a conductor. The size of the GES required by the NEC is too small to produce a low impedance.
Where as a utility the system voltages are much higher, the neutral is grounded every forth pole/tower, and the size of the system makes the impedance much lower. It is basically ohm's law, and parallel circuitry.
For users, or NEC requirements the earth really serves no purpose other than safety. Its main purposes are:
1. To provide a planned discharge path for lightning and accidental contact with high voltages such as contact between primary to secondary distribution.
2. Stabilize system voltages to prevent insulation stress. The magnitude of overvoltages are reduced. When a power supply is grounded, its leakage capacitance to ground is shorted. Since this capacitance is shorted, overvoltage conditions due to resonance between capacitance and distributed inductance are avoided. With the magnitudes of transient overvoltages reduced, the lifetime of all components will increase, improving the overall reliability of the equipment.
Once you reference the system to ground via the MBJ the reasons for grounding/bonding change as follows:
1. Grounding and bonding of metallic frames keeps potential differences between frames to a safe value when lightning currents flows or shorts occur. Grounding of power sources prevents system voltages from permanently appearing on frames that personnel can come in contact.
2. Provides a planned low-impedance return path for all power supplies to permit safe operation of OCPD's via EGC's
3. Electrostatic discharges are quickly equalized.
Here is one more thing to think about. Are planes, trains, automobiles, and boats referenced to earth? Absolutely not, there is no reason too, nor is it needed for these types of separately derived systems. They have what is called ground, but it's not, just a fault clearing path and potential difference equalizer.
karl riley
Senior Member
Re: grounding,bonding
Bond, your post is a complete statement of the confusion that young people coming into the profession are subjected to because of the lack of understanding that many electricians have about the purpose of grounding and what path actually opens the breaker. It is so well expressed that I am asking your permission to quote your post in the second edition of my book, Tracing EMFs in Building Wiring and Grounding. Do I have your permission?
Karl Riley
Bond, your post is a complete statement of the confusion that young people coming into the profession are subjected to because of the lack of understanding that many electricians have about the purpose of grounding and what path actually opens the breaker. It is so well expressed that I am asking your permission to quote your post in the second edition of my book, Tracing EMFs in Building Wiring and Grounding. Do I have your permission?
Karl Riley
Re: grounding,bonding
I have to make exception to the statement made by dereckbc:
I worked for a utility for 10 years, we would NEVER use the earth for a return path.
The utilities make multiple connections to earth for lightning protection and to establish a neutral ground plane at the equipment for the protection of their employees.
To state that utilities use the earth for a return path is both untrue and unfair. No way could the earth be used, as the impedance would be too high. In order to make the resistance low enough in a connection to the earth, the ground connection would far exceed the cost of simply running another wire along with the "hot" conductors.
To prove my point, refer to newspaper articles that recur every few years about some idiot that steals the copper ground wires from power poles, and eventually electrocutes himself because he gets across the cut wires.
I have to make exception to the statement made by dereckbc:
NO WAY!
I worked for a utility for 10 years, we would NEVER use the earth for a return path.
The utilities make multiple connections to earth for lightning protection and to establish a neutral ground plane at the equipment for the protection of their employees.
To state that utilities use the earth for a return path is both untrue and unfair. No way could the earth be used, as the impedance would be too high. In order to make the resistance low enough in a connection to the earth, the ground connection would far exceed the cost of simply running another wire along with the "hot" conductors.
To prove my point, refer to newspaper articles that recur every few years about some idiot that steals the copper ground wires from power poles, and eventually electrocutes himself because he gets across the cut wires.
- Location
- Illinois
- Occupation
- retired electrician
Re: grounding,bonding
Earl,
It has been proved in court cases involving dairy aminals that in some case the utility grounded conductor on the primay distribution sytem has been undersized and the earth itself carries part of the grounded conductor current. In the court cases this current was proved to have caused loss of milk production.
Don
Earl,
It has been proved in court cases involving dairy aminals that in some case the utility grounded conductor on the primay distribution sytem has been undersized and the earth itself carries part of the grounded conductor current. In the court cases this current was proved to have caused loss of milk production.
Don
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Re: grounding,bonding
Technically competent people can bring technically accurate information into the courtroom, and non-technical people (e.g., lawyers) can convince other non-technical people (e.g., jurists) to interpret the information in technically inaccurate ways. I like our judicial system better than that of any other country, but I wish I knew a way to remove that weakness.
No it hasn?t. It might have been concluded, or judged, or resolved, but a court can never prove anything. That is not its job.
Technically competent people can bring technically accurate information into the courtroom, and non-technical people (e.g., lawyers) can convince other non-technical people (e.g., jurists) to interpret the information in technically inaccurate ways. I like our judicial system better than that of any other country, but I wish I knew a way to remove that weakness.
Re: grounding,bonding
Earl, I respect your opinion, and it's been 16 years since I worked for a electric utility as a sub-station transmission engineer, but I can firmly say cost is the overriding factor for multi-grounding the utility neutral. It is far less expensive to ground the neutral every 4th or 5th pole/tower (cannot remember exact distances), than to run a full size neutral. At the voltages and distances distribution systems operate at, ground resistance is lower then the conductor resistance. It's really simple ohm's law. Assume each pole/tower ground is 25 ohms, and there are 200 pole grounds in parallel before the circuit arrives too you. The effective impedance of earth is approximate .125 ohms plus the resistance of the neutral cable in parallel. So in effect the neutral impedance in parallel with earth is much lower than any of the phase conductors. This not only save money in cable and installation cost but also saves in transmission power losses. IMO it adds up to big money.
There are other sound technical reasons that I eluded too in my earlier post. Some include to keep the voltage drop minimized between earth and the neutral conductor. If it were only bonded at the transformer in the sub-station, the voltage drop along the neutral would become substantial along the way exposing personnel to greater risk. You hit on this by your statement of people cutting out the pole butt. The neutral can aid in discharging lightning if it is ran above phase conductors, but the move now is to run the neutral below phase conductors. Maybe Charlie E. will sound in on current practices. Static lines ran above the phase conductors serve to protect phase conductors from lightning.
If I have not convinced you yet, how about the SWER system in place which uses earth only as a return conductor?
Earl, I respect your opinion, and it's been 16 years since I worked for a electric utility as a sub-station transmission engineer, but I can firmly say cost is the overriding factor for multi-grounding the utility neutral. It is far less expensive to ground the neutral every 4th or 5th pole/tower (cannot remember exact distances), than to run a full size neutral. At the voltages and distances distribution systems operate at, ground resistance is lower then the conductor resistance. It's really simple ohm's law. Assume each pole/tower ground is 25 ohms, and there are 200 pole grounds in parallel before the circuit arrives too you. The effective impedance of earth is approximate .125 ohms plus the resistance of the neutral cable in parallel. So in effect the neutral impedance in parallel with earth is much lower than any of the phase conductors. This not only save money in cable and installation cost but also saves in transmission power losses. IMO it adds up to big money.
There are other sound technical reasons that I eluded too in my earlier post. Some include to keep the voltage drop minimized between earth and the neutral conductor. If it were only bonded at the transformer in the sub-station, the voltage drop along the neutral would become substantial along the way exposing personnel to greater risk. You hit on this by your statement of people cutting out the pole butt. The neutral can aid in discharging lightning if it is ran above phase conductors, but the move now is to run the neutral below phase conductors. Maybe Charlie E. will sound in on current practices. Static lines ran above the phase conductors serve to protect phase conductors from lightning.
If I have not convinced you yet, how about the SWER system in place which uses earth only as a return conductor?
hurk27
Senior Member
- Location
- Portage, Indiana NEC: 2008
karl riley
Senior Member
Re: grounding,bonding
I have to get into this. Though I don't have any info on utility motivation for allowing the earth to carry substantial (30% - 50%?) amounts of the neutral current in primary distribution lines, nevertheless, this is what is the case, according to careful studies. Dereck has given the electrical reason that many rods in parallel give a joint resistance which competes with the neutral conductor.
A study I am familiar with, measuring currents on pole grounding conductors along a distribution line showed that current enters the earth in gradually declining amperage until the midpoint of the line, then starts to return to the neutral over the second half, finishing up through the substation ground grid.
But as Hurk posted, the answer to the question about cost sav ing as a motivation for utilities is that some utilities in places like Australia, S. Africa, Alaska and Utah have used the earth as the SOLE return path for neutral curent on single phase distribution lines. (SWER) The justification is saving money on the installation. No mystery about this; noone denies it.
Karl
I have to get into this. Though I don't have any info on utility motivation for allowing the earth to carry substantial (30% - 50%?) amounts of the neutral current in primary distribution lines, nevertheless, this is what is the case, according to careful studies. Dereck has given the electrical reason that many rods in parallel give a joint resistance which competes with the neutral conductor.
A study I am familiar with, measuring currents on pole grounding conductors along a distribution line showed that current enters the earth in gradually declining amperage until the midpoint of the line, then starts to return to the neutral over the second half, finishing up through the substation ground grid.
But as Hurk posted, the answer to the question about cost sav ing as a motivation for utilities is that some utilities in places like Australia, S. Africa, Alaska and Utah have used the earth as the SOLE return path for neutral curent on single phase distribution lines. (SWER) The justification is saving money on the installation. No mystery about this; noone denies it.
Karl
highkvoltage
Senior Member
Re: grounding,bonding
Many of those lawsuits, that were based on loss of milk production, where aimed at the EMF's of transmission lines. These are Delta systems which has a static wire on top of the towers. They claimed that the electric corona field created stray magnetic fields that effected milk production. It was proven at that time that a certain amount of current would flow through ground between substations. I worked on those tower lines for 10 years and bonded to voltages at 765kV. On those lines we could draw a 6' to 8' arc when connecting the bonding jumper to the bucket truck, so the corona field on a 765kV line was at least 12' wide. We would also get shocked if a truck was just park under those lines so we would ground them to drain the truck of the magnetic field much like static. I have never seen a utility get suit for a distribution system. Most of your distribution system now are a Wye configuration and are grounded as the other poster stated.
Many of those lawsuits, that were based on loss of milk production, where aimed at the EMF's of transmission lines. These are Delta systems which has a static wire on top of the towers. They claimed that the electric corona field created stray magnetic fields that effected milk production. It was proven at that time that a certain amount of current would flow through ground between substations. I worked on those tower lines for 10 years and bonded to voltages at 765kV. On those lines we could draw a 6' to 8' arc when connecting the bonding jumper to the bucket truck, so the corona field on a 765kV line was at least 12' wide. We would also get shocked if a truck was just park under those lines so we would ground them to drain the truck of the magnetic field much like static. I have never seen a utility get suit for a distribution system. Most of your distribution system now are a Wye configuration and are grounded as the other poster stated.
hurk27
Senior Member
- Location
- Portage, Indiana NEC: 2008
Re: grounding,bonding
I didn't know a bucket truck can go that high?
We use to get set down on the lines by helicopter. but had to bond on just before. We were replacing the spacers and I think it was a 3meg line that ran out of Florida to other states.
I didn't know a bucket truck can go that high?
We use to get set down on the lines by helicopter. but had to bond on just before. We were replacing the spacers and I think it was a 3meg line that ran out of Florida to other states.
karl riley
Senior Member
Re: grounding,bonding
We don't want to confuse magnetic fields with electric fields. You can't ground out a magnetic field. The electric field creates the corona and the voltage effects.
Karl
We don't want to confuse magnetic fields with electric fields. You can't ground out a magnetic field. The electric field creates the corona and the voltage effects.
Karl
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