"Not Sufficiently Grounded"

[charlie b]

A local paper had a story this morning about a tragedy.

(The dog owner) had never heard of the term "contact voltage" until her German shorthair pointer was electrocuted . . . when he stepped on a metal plate by a lamppost.

The news article goes on to quote a utility spokesperson as saying,

The lamppost had been put in by a private developer . . . . Once approved, the post became city property. . . . "It wasn't sufficiently grounded," she says. . . . the post is functioning again, and the utility is investigating.

If the utility comment had been, ?It wasn?t wired correctly,? it would have been equally understandable by the public, and it would have been technically accurate. Why does the world think that ensuring something is ?grounded? is the key to preventing shock hazards?

 

[mivey]

Why does the world think that ensuring something is ?grounded? is the key to preventing shock hazards?

Mis-information. And the people who should know better are to blame a lot of the time.

I rebutted an affidavit by an engineer who claimed that a better pole ground at a single location would prevent someone from getting electrocuted because it would cause the fuse/breaker to clear the line if it fell on the ground. The truth is the power company does not rely on a single ground and nobody can predict the impedance of a fault. High impedance faults have been a thorn in the POCO's side for a long time and while there have been many improvements in detection technology, no single item will guarantee that someone won't get hurt.

Electricity is a dangerous commodity and we constantly make choices between the level of safety and how much the safety costs. That is no excuse for someone being negligent and wiring something incorrectly. The truth is that we are human and might mistakenly wire something incorrectly.

If the utility spokesperson made the false statement: shame on them. But it would not be the first time the press mis-quoted the utility. I have found it is better to hand the press a written statement.

 

[don_resqcapt19]

... Why does the world think that ensuring something is ?grounded? is the key to preventing shock hazards?

The code's continued use (misuse) of the term Equipment Grounding Conductor leads many, even in the electrical industry, to think that.

 

[renosteinke]

We can't chide the paper when the NEC itself is packed with bad grammar- especially in Article 250.

 

[Rick Christopherson]

Why does the world think that ensuring something is ?grounded? is the key to preventing shock hazards?

Because in this case, if the plate had been properly grounded, there wouldn't have been a voltage gradient between surfaces for the dog to be electrocuted from, regardless whether the voltage was due to a fault or an induced voltage. It is a dangerous misconception to believe that a grounding system is solely for fault current. I can't even begin to tell you how many times I have come across situations where there was no fault, yet an ungrounded chassis has an elevated voltage, resulting in shock when a person comes into contact with that chassis while touching a nearby ground. The grounding system has many different purposes.

 

[don_resqcapt19]

Grounding would not remove the voltage gradient. The only thing that does that is bonding and having enough current flowing to open the OCPD. The connection to earth (grounding) does not make things safe.

 

[charlie b]

It is a dangerous misconception to believe that a grounding system is solely for fault current.

That is true, but only in that the grounding system is never for fault current. If you have observed an elevated voltage on the chassis of some equipment, it means that a hot conductor has somehow come into contact with the chassis. An equipment grounding conductor should carry that current directly back to the source, and trip the breaker, as Don has already mentioned. The wiring error in the lamp post was not a failure to connect anything to planet Earth, but rather a failure to provide a proper fault path back to the source.

The grounding system has many different purposes.

Clearing a fault is not one of them.

 

[ericsherman37]

Maybe the utility guy said it correctly.... "It was not bonded properly." But the newspaper guy thought, "What the heck is 'bonded'?" and changed it to "grounded." But that would assume that the newspaper guy knew the difference, which I doubt.

I imagine that if the utility guy said, "It was not sufficiently grounded," then he was saying it for the benefit of the general populace ignorant of the subtleties of grounding vs. bonding. But if he had something like, "It was not wired correctly" then I, and the newspaper, and probably the public, would respond with "Well no s**t, Sherlock. How 'bout a little more detail, huh??"

I don't applaud the guy for dumbing down the technical information, but I don't really blame him either. The public has a right to know what's going on, but they don't necessarily need the full technical break-down. 99% of them will be satisfied with hearing "it's not grounded properly" .... and the other 1% (us) can stew over it for a while . But as long as the electrical workers that fix it know what's going on and understand the difference between grounding and bonding and so forth, then I'm satisfied with that.

 

[Rick Christopherson]

If you have observed an elevated voltage on the chassis of some equipment, it means that a hot conductor has somehow come into contact with the chassis.

No. Inductive and capacitive coupling can elevate an isolated chassis' voltage even when no direct fault exists. If the only concern for chassis voltage was the result of fault current, then there would be no need for a grounding conductor, and the chassis could be bonded locally with the grounded conductor.

Grounding would not remove the voltage gradient. The only thing that does that is bonding and having enough current flowing to open the OCPD. The connection to earth (grounding) does not make things safe.

No, it is quite the opposite. The dog was walking when it encountered a voltage difference between the localized earth and an isolated plate. If the plate was not isolated (not grounded) then the only voltage difference that could exist would be limited by the voltage drop across the grounding conductor to the localized earth.

Even if the condition was the result of a direct short on an unbonded system, the net effect would have been to elevate the voltage of the entire localized earth (physical earth, i.e. dirt) and the voltage gradient would have been dispersed radially outward without the abrupt change in voltage that was present within the distance of the dog's stride.

The one piece of information that everyone here is assuming, and was not specifically stated in the article, is that there was a short from a live conductor to this plate. Think about how people communicate. If there was a short, the person (interviewee) would most likely have said that, and the public would have understood it. The fact that this was not stated in the article tends to suggest that a fault condition was not responsible for the elevated voltage.

If you are having a hard time believing that a voltage can exist without a direct fault, put an ammeter on your grounding conductor downstream from the bonding location. If your system has no faults or improper ground loops, then you should never see current at this location. But the fact is, it is common to have current on the grounding conductor downstream from the bond. Current doesn't flow without a voltage difference driving it.

 

[Dennis Alwon]

The code's continued use (misuse) of the term Equipment Grounding Conductor leads many, even in the electrical industry, to think that.

The term grounding conductor has been eliminated from the 2011 NEC definitions as well as the entire code.

 

[jumper]

The term grounding conductor has been eliminated from the 2011 NEC definitions as well as the entire code.

Er, Dennis:

250.8 Connection of Grounding and Bonding Equipment.
(A) Permitted Methods. Equipment grounding conductors,
grounding electrode conductors, and bonding jumpers
......

 

[don_resqcapt19]

The term grounding conductor has been eliminated from the 2011 NEC definitions as well as the entire code.

But not the term Equipment Grounding Conductor, and that term is the root of most of the misunderstanding.

 

[don_resqcapt19]

...

No, it is quite the opposite. The dog was walking when it encountered a voltage difference between the localized earth and an isolated plate. If the plate was not isolated (not grounded) then the only voltage difference that could exist would be limited by the voltage drop across the grounding conductor to the localized earth.

Even if the condition was the result of a direct short on an unbonded system, the net effect would have been to elevate the voltage of the entire localized earth (physical earth, i.e. dirt) and the voltage gradient would have been dispersed radially outward without the abrupt change in voltage that was present within the distance of the dog's stride.

The problem is that the voltage gradient will still exist, even with a local grounding electrode. At about 3' from the electrode you will have 85% of the voltage that is on the item being grounded. The only way to get rid of this voltage is by bonding back to the source so the OCPD will clear the fault.

The one piece of information that everyone here is assuming, and was not specifically stated in the article, is that there was a short from a live conductor to this plate. ...

Making my point that a fault clearing EGC is required.

If you are having a hard time believing that a voltage can exist without a direct fault, put an ammeter on your grounding conductor downstream from the bonding location. If your system has no faults or improper ground loops, then you should never see current at this location. But the fact is, it is common to have current on the grounding conductor downstream from the bond. Current doesn't flow without a voltage difference driving it.

With a utility supplied system there are multiple points where the system is connected to the earth. There will always be some current on the Grounding Electrode Conductor because there is a parallel path, via the grounding electrode and the earth, for the grounded conductor current. The voltage that drives this current is the voltage drop on the service grounded conductor.

 

[Dennis Alwon]

Er, Dennis:

250.8 Connection of Grounding and Bonding Equipment.
(A) Permitted Methods. Equipment grounding conductors,
grounding electrode conductors, and bonding jumpers
......

Yeah, the term egc is there but not just grounding conductor as it was in 2008.

But not the term Equipment Grounding Conductor, and that term is the root of most of the misunderstanding.

Yes, I know I guess I was saying the code seems to be moving in the direction of trying to clean things up-- let's see what happens next cycle-

It seems every year this grounding thing gets changed somewhat.

 

[Rick Christopherson]

The problem is that the voltage gradient will still exist, even with a local grounding electrode. At about 3' from the electrode you will have 85% of the voltage that is on the item being grounded. The only way to get rid of this voltage is by bonding back to the source so the OCPD will clear the fault.

Well, given this, then you understand my point, but just don't realize yet that you agree with it. If you have 85% voltage (15% difference) at 3-feet, then you have not killed your dog, whose stride is only 18 to 24 inches.

If you have done "hot work", then you know that you can touch an elevated voltage as long as you don't simultaneously touch a non-elevated voltage. What killed the dog was that he had 100% voltage gradient (for example only) within his stride. If you have 85% voltage at a radius of 3 feet, then you have only 15% voltage difference across this distance. In some cases, possibly this one, that is the difference between being lethal and an irritating tingle.

In the non-grounded scenario, you will have 100% voltage difference across mere inches. In the grounded scenario, you will spread this voltage difference out across several feet--depending on the conductivity of the soil.

The farther you spread this voltage gradient, the less lethal it will be, simply by preventing a person's body from being capable of spanning 100% of the entire gradient.

Consider the example of a nearby (not direct) lightning strike. During a nearby lightning strike, the person with their feet close together will survive, while the person with a wide stance will be killed. That's because the close feet position will have a lesser voltage difference than the wider foot stance.

If you are having a hard time believing that a voltage can exist without a direct fault, put an ammeter on your grounding conductor downstream from the bonding location. If your system has no faults or improper ground loops, then you should never see current at this location. But the fact is, it is common to have current on the grounding conductor downstream from the bond. Current doesn't flow without a voltage difference driving it.

With a utility supplied system there are multiple points where the system is connected to the earth. There will always be some current on the Grounding Electrode Conductor because there is a parallel path, via the grounding electrode and the earth, for the grounded conductor current. The voltage that drives this current is the voltage drop on the service grounded conductor.

Re-read what I said more carefully. I stated "downstream from the main bonding point". What you are describing is the result of multiple ground loops created as the result of how the POCO installs their system.

Downstream from the main bonding point, there should not be any ground loops, and therefore, there should be zero current on the grounding conductor (by your assertion, not mine) unless there is a fault.

In reality, there is current on that conductor (conductors) because there will be induced voltages within the system that are not attributed to direct faults.

 

[hurk27]

Well, given this, then you understand my point, but just don't realize yet that you agree with it. If you have 85% voltage (15% difference) at 3-feet, then you have not killed your dog, whose stride is only 18 to 24 inches.

If you have done "hot work", then you know that you can touch an elevated voltage as long as you don't simultaneously touch a non-elevated voltage. What killed the dog was that he had 100% voltage gradient (for example only) within his stride. If you have 85% voltage at a radius of 3 feet, then you have only 15% voltage difference across this distance. In some cases, possibly this one, that is the difference between being lethal and an irritating tingle.

In the non-grounded scenario, you will have 100% voltage difference across mere inches. In the grounded scenario, you will spread this voltage difference out across several feet--depending on the conductivity of the soil.

The farther you spread this voltage gradient, the less lethal it will be, simply by preventing a person's body from being capable of spanning 100% of the entire gradient.

Consider the example of a nearby (not direct) lightning strike. During a nearby lightning strike, the person with their feet close together will survive, while the person with a wide stance will be killed. That's because the close feet position will have a lesser voltage difference than the wider foot stance.
Re-read what I said more carefully. I stated "downstream from the main bonding point". What you are describing is the result of multiple ground loops created as the result of how the POCO installs their system.

Downstream from the main bonding point, there should not be any ground loops, and therefore, there should be zero current on the grounding conductor (by your assertion, not mine) unless there is a fault.

In reality, there is current on that conductor (conductors) because there will be induced voltages within the system that are not attributed to direct faults.

Rich look at this graphic:

What Don was saying is there will be a 85% voltage drop across the earth in 3' and as close as 1' you will drop 82 volts, so the Dog would still receive at least an 82 volt shock.

And here is a very good thread that details experiments that prove the above graphic is real.

Time To Eat Crow

Ground rods do not provide against touch hazard!


as far as the lightning goes, if your standing next to a ground rod and it happens to be struck by lightning, I doubt you will survive no matter where your feet are placed.

 

[don_resqcapt19]

...
Re-read what I said more carefully. I stated "downstream from the main bonding point". What you are describing is the result of multiple ground loops created as the result of how the POCO installs their system.
Downstream from the main bonding point, there should not be any ground loops, and therefore, there should be zero current on the grounding conductor (by your assertion, not mine) unless there is a fault.

It is the same issue with EGCs that are connected to anything that is in contact with the earth or some conductive path to the earth. It is a parallel path for grounded conductor current.

Both the parallel grounded conductor current and the induced currents you talk about are driven by small amounts of voltage and would not, in normal conditions, create any type of hazard. I don't see how the addition of a local "grounding" (earthing) connection to something that is bonded back to the service grounded electrode system will change anything. The currents flowing on the EGC from the parallel paths or induction will never be high enough to create a measurable voltage drop on the EGC. Unless you have an elevated voltage at the load end of the EGC you do not have a safety hazard.

 

[don_resqcapt19]

Thanks Wayne, I did state the voltage backwards.