earthing. the ECMag article

[don_resqcapt19]

So, are you trying to say that two ground rods do, or don't make a difference?

I am saying that once the system is grounding the addition of grounding electrodes does not make any difference as far as the shock hazard. The impedance will be low enough without any additional grounding electrodes to conduct a fatal amount of current from a 120 volt source. Adding rods does not increase or decrease the hazard.

 

[K8MHZ]

I am saying that once the system is grounding the addition of grounding electrodes does not make any difference as far as the shock hazard. The impedance will be low enough without any additional grounding electrodes to conduct a fatal amount of current from a 120 volt source. Adding rods does not increase or decrease the hazard.

Well, I agree that adding a couple of rods won't decrease the hazard.

I also agree that the addition of two rods, in most cases, makes little difference at that particular premises. But, they DO make SOME difference (good or bad, you decide). That difference should be easier to measure than electrode resistance.

If I get some time I'll go down in the basement and play with electricity. I'll try to see if there is a difference in available fault current and if it is significant or not.

Of course, I will post my methods and results here. Today may not be a good day, though. We are supposed to be getting thunderstorms.

 

[don_resqcapt19]

... I also agree that the addition of two rods, in most cases, makes little difference at that particular premises. But, they DO make SOME difference (good or bad, you decide). That difference should be easier to measure than electrode resistance. ...

It is the electrode resistance that will make the difference. If the current path will carry a fatal amount of current it doesn't make any difference how much more current the path will carry.

 

[K8MHZ]

It is the electrode resistance that will make the difference. If the current path will carry a fatal amount of current it doesn't make any difference how much more current the path will carry.

Indeed. That's why I am so biased against two rods as an electrode. The resistance from one part of the country to the other changes dramatically. There is a chart somewhere we use for grounding radio stuff that shows earth conductivity. The worse the conductivity, the more effort is needed for an effective ground. The lowest number is 1. My area is a 2. I think the best conductivity was 14. Maybe 10, but the only place worse than West Michigan was in solid rock.

Two rods here probably exhibit hundreds of ohms of resistance. I have an older service and am lucky enough to have lots of water pipe, some deep in the ground, as my electrodes.

This is all great food for thought and a reason to do some experiments. Lucky me, my work is play!!

 

[weressl]

Lovely. As I understand, 'earthing' is the British term for what we call 'grounding'. Let's really confuse people.

Not only that it is a term used by the British, but it is the term the IEC adapted and being in use throughout the world.

http://www.electropedia.org/iev/iev.nsf/index?openform&part=195

 

[don_resqcapt19]

Indeed. That's why I am so biased against two rods as an electrode. ...

But my point is that I don't think it makes any difference if you have one rod, two rods, 1000 rods, or a ufer or any other type of grounding electrode. It is my opinion that once the system is grounded at any point, the path back to the system via the earth would almost always have a low enough impedance to cause a fatal amount of current to flow.

 

[K8MHZ]

But my point is that I don't think it makes any difference if you have one rod, two rods, 1000 rods, or a ufer or any other type of grounding electrode. It is my opinion that once the system is grounded at any point, the path back to the system via the earth would almost always have a low enough impedance to cause a fatal amount of current to flow.

Key words highlighted.

And with those an admission of uncertainty?

I'll admit that I have no certainty. I won't until I do some tests and even then I can assure you that all my harbored suspicions will have little chance of fading.

I do think there is a chance that choice and placement of electrodes will affect available current and the effect may be enough to border the fatality threshold.

I performed some tests on several students that showed skin conductivity in the vicinity of around 1000 ohms. Simple math showed that bare 120 conductors in contact with the measured area would equal 120 mA. That may or may not be fatal, depending on duration and the path through the body and other factors. If 120mA were fatal every time, I would have been dead long ago. Increasing the area of skin contact reduced the resistance drastically to around 100 ohms.

To me, that is too wide of range to dismiss any possibility that changes in earth resistance due to the proximity of the electrodes has no influence on available fault current and thus the fatality rate.

I will admit that I have never made any measurements to support my assertion, but plan to do so.

At least we agree on one thing......NEC required grounding methods do little, if anything, to keep personnel safe from ground fault fatalities. The number of those fatalities is a sturdy testament to that fact.

 

[jwelectric]

First let me take a minute to say how much I have enjoyed this thread. I can?t remember a thread on any of the electrical forums that I have enjoyed as much as I have this one in a long time.

There has been much discussion about grounding or earthing and current in this thread which leads me to believe that there is much confusion about the purpose of connecting our electrical systems to earth.

Article 250.4 tells us why we connect our electrical systems to earth;
(A) Grounded Systems.

1. Electrical System Grounding. Electrical systems that are grounded shall be connected to earth in a MANNER THAT WILL LIMIT THE VOLTAGE imposed by lightning, line surges, or unintentional contact with higher-voltage lines and that will stabilize the voltage to earth during normal operation.

Notice the wording that is in upper case and red lettering above. Where does it say that the connection to earth has anything to do with current or amperage?
We connect to earth to limit voltage not current. We bond our systems to carry current.

I want all exposed non-current carrying metal grounded to limit the amount of voltage on them not to limit the amount of current. If this metal is bonded to the grounded conductor it will not limit the amount of current but will allow enough current to flow to open the overcurrent device. In this case I am not looking to limit the amount of current and want to allow as much current to flow as possible.

The primary purpose of the EGC is to limit the amount of voltage and to carry as much current as possible.

 

[don_resqcapt19]

...
I do think there is a chance that choice and placement of electrodes will affect available current and the effect may be enough to border the fatality threshold.

First the grounding of a system has nothing to do with the available current. That is a function of the power source. There may be some rare cases where the path via to earth back to a grounded system would have such a high impedance that you would not get a fatal shock, but I would fully expect that the earth would not limit the current to a "safe" level in most cases.

...I will admit that I have never made any measurements to support my assertion, but plan to do so.

I haven't made any studies, but have been able to have my wiggy show voltage with on probe on a 120 volt conductor and the second in the earth...nothing other than the wiggy probe itself. My wiggy requires about 15mA to pull in at 120 volts. Maybe not a fatal amount of current but a dangerous level. Remember that the 5mA trip for GFCIs was picked because about 95% of adult females can "let go" at that current. A higher percentage of adult males can let go at that current and a lower percent of children.

At least we agree on one thing......NEC required grounding methods do little, if anything, to keep personnel safe from ground fault fatalities. The number of those fatalities is a sturdy testament to that fact.

And as Mike pointed out, that is not the intended function of grounding.

 

[don_resqcapt19]

First let me take a minute to say how much I have enjoyed this thread. I can’t remember a thread on any of the electrical forums that I have enjoyed as much as I have this one in a long time.

There has been much discussion about grounding or earthing and current in this thread which leads me to believe that there is much confusion about the purpose of connecting our electrical systems to earth.

The discussion we have been having has nothing to do with the purpose of connecting electrical systems to earth...it only has to do with the effects of that connection. It is also my opinion that the code use of the term "equipment grounding conductor" is a big part of the reason for the confusion about grounding and bonding.

I want all exposed non-current carrying metal grounded to limit the amount of voltage on them not to limit the amount of current.

We have to be careful with this. This is the very idea that leads people to think that a grounding electrode can take the place of the required equipment grounding conductor. Yes I know that is not the purpose as stated in the code rule that you cited, but my point is that connecting non-current carrying parts of electrical equipment that is supplied by a grounded system does very little to reduce the voltage from those non-current carrying parts as measured to earth, unless you are very close to the electrode.

If this metal is bonded to the grounded conductor it will not limit the amount of current but will allow enough current to flow to open the overcurrent device. In this case I am not looking to limit the amount of current and want to allow as much current to flow as possible.

The very reason for EGCs and main or system bonding jumpers. There will be a shock hazard at the faulted equipment until the OCPD clears the fault.

The primary purpose of the EGC is to limit the amount of voltage and to carry as much current as possible.

That is true, but under high fault current conditions, the voltage to earth at the faulted equipment can be dangerously high...until the fault is cleared.

 

[jwelectric]

Lightning is an event between above and earth, therefore we connect our electrical systems that are somewhere between the two to earth. Grounding provides a path for it to travel.

Power surges (not spikes) are very high voltages just like unintentional contact with higher voltage lines. These higher voltages are able to push current through earth better than the lower voltages in our systems clearing the overcurrent devices on the utility side thereby limiting the voltages.

On a system that is not connected to earth the voltages can be very high between any metal and earth so grounding keeps things stable.

Equipment bonding conductors might not do these things therefore we have instead equipment grounding conductors that double as bonding paths.

I fail to see where calling the EGC by any other name helps clear anything.

 

[K8MHZ]

First let me take a minute to say how much I have enjoyed this thread. I can?t remember a thread on any of the electrical forums that I have enjoyed as much as I have this one in a long time.

There has been much discussion about grounding or earthing and current in this thread which leads me to believe that there is much confusion about the purpose of connecting our electrical systems to earth.

Article 250.4 tells us why we connect our electrical systems to earth;
(A) Grounded Systems.

1. Electrical System Grounding. Electrical systems that are grounded shall be connected to earth in a MANNER THAT WILL LIMIT THE VOLTAGE imposed by lightning, line surges, or unintentional contact with higher-voltage lines and that will stabilize the voltage to earth during normal operation.

Notice the wording that is in upper case and red lettering above. Where does it say that the connection to earth has anything to do with current or amperage?
We connect to earth to limit voltage not current. We bond our systems to carry current.

I want all exposed non-current carrying metal grounded to limit the amount of voltage on them not to limit the amount of current. If this metal is bonded to the grounded conductor it will not limit the amount of current but will allow enough current to flow to open the overcurrent device. In this case I am not looking to limit the amount of current and want to allow as much current to flow as possible.

The primary purpose of the EGC is to limit the amount of voltage and to carry as much current as possible.

Excellent point. My question is, how does two ground rods in sand limit voltage? Oh, and voltage must be measured from two points. Which two points are we supposed to limit voltage from when we ground our systems?

 

[K8MHZ]

Lightning is an event between above and earth, therefore we connect our electrical systems that are somewhere between the two to earth. Grounding provides a path for it to travel.

Not true. Lightning is an event between two charge centers. A charge center can be deep in the earth. A few miles from me are sand dunes and when lightning strikes them, the bolt travels through the sand and melts the sand grains together. This is the result:

View attachment 6801

That's a fulgurite.

Power surges (not spikes) are very high voltages just like unintentional contact with higher voltage lines. These higher voltages are able to push current through earth better than the lower voltages in our systems clearing the overcurrent devices on the utility side thereby limiting the voltages.

I have never seen a 'high' voltage line open an OCPD from hitting the ground, but I have seen 7,200 volt lines lay on the ground and arc like an arc welder long enough (a good 5 minutes) to burn itself in two.

On a system that is not connected to earth the voltages can be very high between any metal and earth so grounding keeps things stable.

How can that be if the system is not connected to the earth??? There has to be a connection somewhere to create a difference in potential. The voltage stabilization is essential for a system with many power sources connected together. For a single source, like an ungrounded generator, that is not essential. With no connection to ground at that source, there will be no voltage between the earth and any conductor coming from the generator. Grounding increases the voltage between the earth and the hot conductors yet still does not provide enough continuity to open an OCPD if it hits the ground.

Equipment bonding conductors might not do these things therefore we have instead equipment grounding conductors that double as bonding paths.

It's bonding that does what you are saying, not grounding.

I fail to see where calling the EGC by any other name helps clear anything.

I am with you there, no matter what the effect of grounding is, EGC differentiates beween bonding that is and bonding that is not connected to the earth.

 

[weressl]

Sorry to jump in the middle, but it seems to me that it would be good to remember during these exchanges, that:

• Grounding conductive, but nonenergized, parts of an elecrical equipment assures that there is minimal(non-harmful) potential difference between it and the surroundings where a person may stand or simultaeously touch.
• (Equipotential) bonding assures that all of the above mentioned components have minimal(non-harmful) potential difference between each other.
• EGC and system grounding assures that in case that a conductive part of an electrical equipment - that is not a current carrying component of the gear - becomes energized, there is a sufficiently low impedance path created so that either the OCP or GFP is activated and removes the voltage from the unintended component.
• Multiple, underground, interconnected grounding electrodes or ground mesh are intended to minimize the step potential difference.

 

[don_resqcapt19]

Sorry to jump in the middle, but it seems to me that it would be good to remember during these exchanges, that:

• Grounding conductive, but nonenergized, parts of an elecrical equipment assures that there is minimal(non-harmful) potential difference between it and the surroundings where a person may stand or simultaeously touch.

• only under non-fault conditions

• (Equipotential) bonding assures that all of the above mentioned components have minimal(non-harmful) potential difference between each other.

• Under fault conditions Equipotential bondind reduces the step and touch potential better than just connecting to an electrode.

• EGC and system grounding assures that in case that a conductive part of an electrical equipment - that is not a current carrying component of the gear - becomes energized, there is a sufficiently low impedance path created so that either the OCP or GFP is activated and removes the voltage from the unintended component.

• Yes it does, but we need to remember that until the fault is cleared there may be hazardous voltage on the faulted equipment

• Multiple, underground, interconnected grounding electrodes or ground mesh are intended to minimize the step potential difference.

• Yes and they do this by raising everything that is interconnected to the same voltage. Under fault conditions this voltage will be much higher than "remote" earth, but since everything within touch or step is at very close to the same voltage there is little hazard.

 

[don_resqcapt19]

...I am with you there, no matter what the effect of grounding is, EGC differentiates between bonding that is and bonding that is not connected to the earth.

The word "grounding" in EGC implies that the connection to earth is important for the EGC to do its job. I don't agree that the connection to earth plays any real part in the function of what the code calls an EGC. It bonds the non-current carrying parts back to the system grounded conductor to provide a low impedance fault clearing path. The fact that it is connected, indirectly, to earth is not important.

The word "grounding" leads many to think that the connection to earth the important function of this conductor. I am sure we have all seen installations where the installer thought that a connection to earth was just as good as installing the code required EGC.

It is interesting to note that when I made the proposals to change the term Equipment Grounding Conductor to Equipment Bonding Conductor, a majority of CMP 5 agreed, but I was one short of the 2/3s majority that is required to accept a proposal, so I know I am not alone with this idea.

 

[weressl]

Lightning is an event between above and earth, ......

The WAST majority of lightning occurs between (charged fields in the) clouds and never discharges to the ground.

 

[K8MHZ]

The WAST majority of lightning occurs between (charged fields in the) clouds and never discharges to the ground.

That is correct. It's what, only about 1 percent of strikes are to the ground?

 

[suemarkp]

I think one other detail here is static electricity dissipation. Some things pickup static -- rotating machinery, even wires in the wind. You don't need a low resistance path to discharge this, as any person who has touched a charged plastic item while wearing tennis shoes on carpet can discover. But you need something or else you can get bit from either wire or chassis. I believe static wristbands for electronic static stations have about a 10K ohm resistance in them.

Connecting the EGC to earth can dissipate any static that has built up on an object -- seems like an easy way to do it as long as that bonding conductor hits the earth at some point. A ground electrode conductor or earthing conductor would be a better term, but implies a separate conductor.

Some cars are prone to this too, and you can see a strap hanging under the car dragging on the ground to dissipate it out. We even have a utility cart at work that was always shocking us (I think it was the tire material used). We hung a 1' piece of braided conductor under it which drags on the ground and now no more shocks.

 

[ritelec]

Lighting....upward strikes.........

http://www.youtube.com/watch?v=RDDfkKEa2ls&feature=related




groovy stuff


Step leader...............


http://www.youtube.com/watch?v=6MUYsIjTKvk&feature=relmfu