All I have to say about touch potential is it's impossible to get a perfect connection to the earth. No matter how many ground rods are driven or how deep, you will never get close enough to a low enough resistance to trip a breaker. Therefore, if you are standing near something that is energized and your feet are in conductive contact with the earth, you will be at a different potential and will get shocked when you touch it. If you want a better explanation you have to get it from someone else, I'm only an apprentice with 4 months of experience on the job
Stray Voltage
- Thread starter wclark160
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bobby ocampo
Senior Member
One of the many ways was reducing the earth resistance using additional Rod. Lowering the ground resistance helps discharge the stray voltage. Imperfect bonding creates this touch potential too. EGC should be bonded because in will help lower the return path to the ground and voltage drop. Another way is to create parallel path. Equivalent resistance of parallel conductors reduces the voltage drop. The equivalent resistance of two parallel conductors is always less than the smallest resistance conductor. similarly connecting to earth the metal part of the enclosure will help because the resistance of the metal panel will be lower if in parallel with a low resistance conductor. Principles of paralleling conductors. Current will flow more to the path of least resistance but proportional to the resistance. Less current flowing to the high resistance less voltage drop or less touch potential. This are basic electrical engineering principles. Check IEEE 80
electrofelon
Senior Member
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I'm not sure I'd call that stray voltage, I'd call it installer error
ptonsparky
Tom
- Occupation
- EC - retired
Back when small dairies were common, faults from neighbors were possibilities. They were still called stray voltage even though the real issue is current flow.
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It is certainly an unintended (and unacceptable) earth potential gradient, but as you note, with a more egregious source than commonly seen.
bobby ocampo
Senior Member
Insulation failure of the neutral is the most common cause of stray voltages
electrofelon
Senior Member
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- Cherry Valley NY, Seattle, WA
Can you cite a source or reference for that statement?
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- Illinois
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- retired electrician
Stray voltage most often results from the utility using a multi-grounded neutral system on the distribution system and where there there are no problems with the primary neutral, the stray voltage is the voltage drop on the primary neutral between the point where that conductor is first connected to earth and where you are measuring the stray voltage.
bobby ocampo
Senior Member
This is the reason why they don't allow neutral to be bonded to EGC other than the service equipment. If neutral touches the metal part of a panel. Current will also flow to EGC that is bonded to the metal panel. Ground potential rise will occur in the panel due to the voltage drop of current flowing to the neutral and EGC. Connecting the metal panel to ground bonded with the EGC will help reduce the touch potential Basic electrical engineering
In this case try installing a ground rod and bond egc then measure the voltage of the metal part to ground.
You can try. Best is to find the neutral. Use megger to test the neutral
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bobby ocampo
Senior Member
The requirement of neutral bonded in service equipment will normally have stray voltage because the neutral and the earth are parallel. Requirement of the code to make sure that system is grounded always. If the ground in the utility transformer is disconnected for whatever reason all residential will still be grounded.
Same principle applies when the neutral is connected to a metal panel or equipment other than the service equipment. Current flowing to the neutral will flow to the EGC increasing the ground potential in the metal panel.. Connecting the panel to the earth and bonding the egc to this ground will reduce if not elimated the stray voltage. Try to prove. More parallel conductors with EGC the lower the stray voltage.
im
The equivalent resistance of parallel conductors is always lower than the lowest resistance. Current will flow MORE to the Path of least impedance and with divide to other path. KIRCHOFF'S CURRENT LAW
the lower the equivalent impedance of the parallel lines the lower the GROUND POTENTIAL RISE
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- PE (Retired) - Power Systems
Term stray voltage, not as used in the US. See Don's post #88.
bobby ocampo
Senior Member
You can experiment with UNGROUNDED SYSTEM. When one of the line is connected to ground the potential is zero between the line and the ground
We can reduce the resistance of the ground by paralleling low resistance conductor to the ground. We use this principles in GROUND GRID in substations.
Connection to earth can help and reducing the resistance of the earth will also help. However they ground rod connected to the earth should also be bonded to create an effectively grounded system
Attend engineering seminars to know the truth. These are basic electrical engineering. Ohms law and Kirchoffs current law
@bobby ocampo
I don't have anything to add. Also, I'm not an electrical engineer.
All the knowledge I need concerning stray voltage I can get from Mike Holt's 'Electrical Theory' and Mike Holt's 'Bonding and Grounding' text books and DVDs.
Also, I don't really have the free time to do experiments to try to prove everyone else in the world is wrong.
I don't have anything to add. Also, I'm not an electrical engineer.
All the knowledge I need concerning stray voltage I can get from Mike Holt's 'Electrical Theory' and Mike Holt's 'Bonding and Grounding' text books and DVDs.
Also, I don't really have the free time to do experiments to try to prove everyone else in the world is wrong.
- Location
- Illinois
- Occupation
- retired electrician
But you fail to understand that all a connection to the earth does is to raise the voltage of the earth for a small area around the electrode to match that of the voltage on the conductor that is connected to the electrode. That does nothing to eliminate Neutral to Earth Voltage which is the most common reason for stray voltage. The voltage does not go away when you add electrodes. In all cases, everything connected to the electrical bonding and/or grounding system will have the NEV as measured to the earth that is a few feet remote from the electrode. It really only eliminates the shock potential if you are standing directly on top of the electrode.
bobby ocampo
Senior Member
Sorry to disagree. NOT ALL. This is the reason why we bond and connect all connection to earth to the EGC. We can reduce the voltage around the electrode because we can reduce the resistance of the earth. This principle is used in the installation of a grounding grid in substation.
Try it first. It will provided the ground rod connected to the earth is bonded to the EGC. This will reduce the equivalent impedance between the EGC and the ground. The equivalent impedance of two parallel impedance is always lower than the lowest resistance. Kirchhoff's current law will divide the current to all parallel conductors. Connection to earth will reduce the touch potential to approach zero.
Compare to how an ungrounded conductor in an ungrounded system wil be zero or approaching zero in a single line to ground fault. Same with High Resistance grounding 250.36. Same electrical engineering principle applies.
Best example is a line to ground fault on an ungrounded system or HRG system.
What is the line to ground voltage of a corner grounded delta? Compare if the system is ungrounded
bobby ocampo
Senior Member
You can also consult other experts who knows electrical engineering. No harm in getting a second opinion. Check engineers in IEEE
What is your background or occupation? For example, if someone's profile says they are a master electrician with 35 years of experience then I'm more likely to believe that what they are telling me is true. You have no information at all on your profile, so I don't know if you have any real world experience or if you've just read a lot of books and are just stating theory as fact.
I will take experience over theory every time. For example: in theory you should never have to wash your bath towel. Theoretically since you're clean and you are just drying off the water the towel won't ever get dirty. But experience shows that things work out quite different when put into practice.
Since I don't know your experience, it's hard to believe anything you say. Especially since Master Electricians with decades of real world experience are saying something else.
winnie
Senior Member
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- Springfield, MA, USA
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- Electric motor research
IMHO the problem here is a focus on true statements which are effectively false, and which reinforce a dangerous falsehood. Thus the pushback.
The dangerous falsehood is 'ground rods will effectively dissipate excess electricity and make things safe.' Many people thing that grounding electrodes are a magical cure-all for a host of problems, and will thus misuse ground rods. It is a dangerous false myth that if you connect an enclosure to a ground rod then that enclosure could not shock someone.
For the low voltage ( <600V systems that most electricians on this site work with), a standard ground rod will not pass enough current to trip a breaker, and would only slightly decrease the touch potential at the fault.
Let's take a specific example, a metal lamp post with a line to enclosure fault. The source is a grounded wye system, and the lamp post base is insulated from ground. The post is not bonded to an EGC. A line to pole fault will energize the pole to full line voltage relative to 'remote earth' and present a severe shock hazard.
Now we add a ground rod to the lamp post. With normal ground rod impedance you might see 1 to 10A of current. The impedance of the circuit supplying the fault and the ground rod form a voltage divider which _slightly_ reduces the voltage (to remote earth) of the lamp post and significantly raises the voltage of the soil surrounding the ground rod. Touch potential is somewhat reduced, mainly because a person standing on the soil now has their feet at elevated potential and the potential difference between the two contact points is reduced.
There is no practical safety benefit to adding this ground rod.
Now let's say we add a second ground rod. This second ground rod reduces the ground electrode impedance. The changed voltage divider means the voltage of the lamp post goes down slightly, and the increased extent of the grounding electrode spreads out the voltage gradient in the soil. This reduces the touch potential slightly, and thus makes the situation slightly safer, but does not in any practical way make the situation safe.
Mr. Ocampo is _correct_ that adding better grounding reduces the danger of shock from exposed metal. We are pushing back because in a practical sense this reduction in hazard is negligible for our purposes.
But the physics is correct, and in different circumstances additional grounding does improve safety. Consider higher impedance sources and more extensive grounding.
Mr. Ocampo brings up the example of a grounding grid at a substation. In this case the source impedance is much higher (either from the transmission line or from lightning, and the ground electrode system is very extensive). This grounding could reasonably be expected to significantly reduce touch and step potentials, and will likely trip the protective relaying.
Exact same physics applies in both cases.
And if you were to install a huge grounding grid at your lamppost it would reduce touch potential and trip the breaker. Or you could simply install the EGC.
During fault cleaning time the circuit conductor-fault-EGC impedances form a voltage divider, and energize the pole creating a touch potential. Adding a ground rod will reduce this touch potential. Determining if this reduction in touch potential has any practical safety benefit is an engineering exercise.
Jon
The dangerous falsehood is 'ground rods will effectively dissipate excess electricity and make things safe.' Many people thing that grounding electrodes are a magical cure-all for a host of problems, and will thus misuse ground rods. It is a dangerous false myth that if you connect an enclosure to a ground rod then that enclosure could not shock someone.
For the low voltage ( <600V systems that most electricians on this site work with), a standard ground rod will not pass enough current to trip a breaker, and would only slightly decrease the touch potential at the fault.
Let's take a specific example, a metal lamp post with a line to enclosure fault. The source is a grounded wye system, and the lamp post base is insulated from ground. The post is not bonded to an EGC. A line to pole fault will energize the pole to full line voltage relative to 'remote earth' and present a severe shock hazard.
Now we add a ground rod to the lamp post. With normal ground rod impedance you might see 1 to 10A of current. The impedance of the circuit supplying the fault and the ground rod form a voltage divider which _slightly_ reduces the voltage (to remote earth) of the lamp post and significantly raises the voltage of the soil surrounding the ground rod. Touch potential is somewhat reduced, mainly because a person standing on the soil now has their feet at elevated potential and the potential difference between the two contact points is reduced.
There is no practical safety benefit to adding this ground rod.
Now let's say we add a second ground rod. This second ground rod reduces the ground electrode impedance. The changed voltage divider means the voltage of the lamp post goes down slightly, and the increased extent of the grounding electrode spreads out the voltage gradient in the soil. This reduces the touch potential slightly, and thus makes the situation slightly safer, but does not in any practical way make the situation safe.
Mr. Ocampo is _correct_ that adding better grounding reduces the danger of shock from exposed metal. We are pushing back because in a practical sense this reduction in hazard is negligible for our purposes.
But the physics is correct, and in different circumstances additional grounding does improve safety. Consider higher impedance sources and more extensive grounding.
Mr. Ocampo brings up the example of a grounding grid at a substation. In this case the source impedance is much higher (either from the transmission line or from lightning, and the ground electrode system is very extensive). This grounding could reasonably be expected to significantly reduce touch and step potentials, and will likely trip the protective relaying.
Exact same physics applies in both cases.
And if you were to install a huge grounding grid at your lamppost it would reduce touch potential and trip the breaker. Or you could simply install the EGC.
During fault cleaning time the circuit conductor-fault-EGC impedances form a voltage divider, and energize the pole creating a touch potential. Adding a ground rod will reduce this touch potential. Determining if this reduction in touch potential has any practical safety benefit is an engineering exercise.
Jon
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
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