Connection to Earth or Ground in Equipment Grounding

[bobby ocampo]

The problem is that the connection to earth does not reduce the electric shock potential of the faulted equipment on solidly grounded systems.

There are different sources of electrocution. One based on the illustration shown which is the step potential and the other one is the energized metal piece. To solve step potential due to the energization of the earth because of a very high fault current, the resistance of the ground should be reduced by installing more ground rods near the energized metal piece or a ground mat connected to earth. This is the reason why I am comparing what is being done in a transformer substation. The reason why there are many ground rods installed in a substation is to reduce step potential if a person is inside the substation at the time of a very high fault current.

The connnection of the energized metal piece to earth is to reduce its potential to ground to reduce electric shock.

It does do that under normal conditions for solidly grounded systems and under both single fault and normal conditions for ungrounded systems.

Reducing the energized metal piece to ground potential is applicable to all types of system (solidly grounded, HRG, Ungrounded System).

Step potential has a different solution to reduce electric shock. If fault current is very low such as in an Arcing ground fault or in HRG, then there will be less step potential. Step potential is equal to the resistance of the ground times value of the fault current.

 

[winnie]

The bonded EGC may or may not open the OCPD during an ARCING GROUND FAULT if he fault current is very low. In this case if the arcing ground fault continues to be present but is still very low that OCPD is not sensing the low level fault for it to operate then the metal piece where this low level arcing ground fault occurs will energized the metal piece. If the metal piece is energized and is not connected to ground the potential of the energized metal piece will be equal to line-to-neutral voltage. Personnel touching this energized metal piece will have electric shock.

I suggest that you consider the following scenarios, and calculate the voltages present. In particular, calculate the voltage between the energized metal and the ground next to that metal.

In all of the following, the source of supply is a 480/277V, 60 Hz system, which is supposed to have a grounded neutral. The fault to the piece of metal is a 25A arcing fault, which will not trip the breaker. The EGC, if present, is a 10AWG copper conductor, 250 feet long, with no other parallel EGC. The grounding electrodes are standard copper rods driven in soil, and have a particularly good 10 Ohm resistance to ground. The grounding electrodes are located at the service, approximately 250 from the fault location. The metal is isolated from the ground, but a person could stand on the ground and touch the metal.

1) The metal piece is properly bonded back to the service neutral, using the EGC specified above. The service neutral is properly connected to grounding electrodes as specified above.

2) The metal piece is properly bonded back to the service neutral, using the EGC specified above. However the service neutral is _not_ properly connected to grounding electrodes.

3) The EGC specified above is not present. The service neutral is properly grounded, as specified above. The metal piece is connected to a separate grounding electrode, meeting the above specifications, but not connected to the neutral grounding electrode system.

Having calculated the voltages present in the different scenarios, you will be able to state for the above specific circumstance, how much or how little the connection to the earthing electrodes soil has changed the touch potential present on the energized bit of metal.

-Jon

 

[bobby ocampo]

Clearly the code requires that all such systems be connected to earth ground. The different impedances present in different voltage systems and different types of system change the benefits to be expected from this connection to earth, and sometimes change the design of the earthing electrodes used. Because of these different impedance conditions, it is useful to consider such different installations separately, and then go on to consider the similarities.

If the code requires connection to ground, why is it less important for safety?

By this I mean to differentiate between the electrical contact with the soil, which must take into account the conductivity of the soil itself, and electrical contact with metal objects which are themselves in contact with soil. A person standing upon a bonded metal plate which it itself resting upon the soil is in a rather different situation than a person standing directly upon the soil next to a rod type grounding electrode.

This is the reason why in certain installations it is a requirement to determine the resistance of the ground. The resistance of the ground is important for step potential. At the instant of the fault regardless if there is EGC or connection to earth, there will be step potential and person standing with spread foot apart will receive an electric shock. It is as important to reduce step potential in installations and installing a ground mat and creating a mesh is one of them.

I am specifically talking about protection of personnel against shock.

This is the crux of our mutual misunderstanding.

The connection to earth provides protection from certain types of electric shock hazards.

However the soil itself is not a very good conductor. Because of the resistance of the soil itself, dangerous voltages may be easily imposed upon objects which are electrically connected to the soil.

There is a different solution for step potetial. Step potential may be reduce by either reducing the resistance of the earth or reducing the fault current on a single line to ground fault.

To reduce step potetial more than one ground rod are installed and are connected i mesh just like in a transformer substation where resistance of the ground is reduce to reduce the hazard of step potential. Also if the system is from a solidly grounded system is converted to HRG then fault current will be reduced and there wil be less step potential.

The example of the 'grounded lamp post' has already been given, however it is worth repeating. Consider a simple lamp post, with its own rod electrode planted in the soil. The supply to the lamp post is from a common residential grounded electrical system, where the 'neutral' is directly connected to the utility 'multi-earth-neutral', and thus to grounding electrodes all over the surrounding area. Due to a fault in the lamp, the 'hot' conductor comes into contact with the lamp post. Because there is no EGC, insufficient fault current flows to trip the OCPD. The lamp post is thus energized at 120V relative to the 'regional' ground.

Because of the grounding electrode tied to the lamp, current is continuously flowing from the fault, into the grounding electrode, into the soil, thence through the other grounding electrodes in the area, and back to the transformer neutral.

This is another case of hazard due to STEP POTENTIAL. Even with an EGC connected to the source at the instant of the fault there will still be step potential because there is a time delay for the OCPD to operate. There should be additional solution to prevent electric shock hazard from the example. One is to install more ground rods to reduce the resistance of the ground or a ground mat. And even with an EGC if the fault is a low arcing ground fault, the pole will still be energized. In this case the EGC even if it is a low impedance path to operate the OCPD, the OCPD wil not operate because the fault current may be lower than the ampere trip rating of the OCPD. There will be less step potential in this situation, the OCPD will not operate because fault current is still very low but the metal pole will be energized. Therefore to reduce the potential of the energized metal pole is to connect it to earth. This makes connection to earth as important as the other equipment grounding solutions.

Someone standing upon the soil and touching the lamp post would be exposed to a voltage of perhaps 60-100V, most of the 120V source. The ground rod located right at the point of the fault was only slightly reducing the fault voltage.

This is because of the step potential and based on the current flowing in the ground. A different solution for this problem should be done.

 

[bobby ocampo]

I suggest that you consider the following scenarios, and calculate the voltages present. In particular, calculate the voltage between the energized metal and the ground next to that metal.

In all of the following, the source of supply is a 480/277V, 60 Hz system, which is supposed to have a grounded neutral. The fault to the piece of metal is a 25A arcing fault, which will not trip the breaker. The EGC, if present, is a 10AWG copper conductor, 250 feet long, with no other parallel EGC. The grounding electrodes are standard copper rods driven in soil, and have a particularly good 10 Ohm resistance to ground. The grounding electrodes are located at the service, approximately 250 from the fault location. The metal is isolated from the ground, but a person could stand on the ground and touch the metal.

1) The metal piece is properly bonded back to the service neutral, using the EGC specified above. The service neutral is properly connected to grounding electrodes as specified above.

2) The metal piece is properly bonded back to the service neutral, using the EGC specified above. However the service neutral is _not_ properly connected to grounding electrodes.

3) The EGC specified above is not present. The service neutral is properly grounded, as specified above. The metal piece is connected to a separate grounding electrode, meeting the above specifications, but not connected to the neutral grounding electrode system.

Having calculated the voltages present in the different scenarios, you will be able to state for the above specific circumstance, how much or how little the connection to the earthing electrodes soil has changed the touch potential present on the energized bit of metal.

-Jon

Solution is to reduce the step potential by reducing the resistance of the ground. Similar solution done in transformer substation to reduce STEP POTENTIAL plus connection to earth of the energized metal piece.

 

[winnie]

Solution is to reduce the step potential by reducing the resistance of the ground. Similar solution done in transformer substation to reduce STEP POTENTIAL plus connection to earth of the energized metal piece.

Please, do the numbers. Why spend hundreds (or thousands) of dollars to reduce step potential, when you could spend tens of dollars to reduce the source of that step potential?

-Jon

 

[winnie]

If the code requires connection to ground, why is it less important for safety?

The code requires many things, not all of which enhance safety significantly.

It is critical to understand the costs and value of the various safety enhancements; if you have limited funds and want to improve safety, it is critical to understand how to best allocate your funds.

This is the reason why in certain installations it is a requirement to determine the resistance of the ground. The resistance of the ground is important for step potential.

Step potential has little to do with the situation being described. We have a properly grounded and bonded electrical system. During a fault, a potential is imposed upon the metal enclosing the fault and the equipment grounding conductor which carries the fault current back to the source. This shock danger is between this enclosure and the ground, not from one point of the ground and the other.

To reduce step potetial more than one ground rod are installed and are connected i mesh just like in a transformer substation where resistance of the ground is reduce to reduce the hazard of step potential. Also if the system is from a solidly grounded system is converted to HRG then fault current will be reduced and there wil be less step potential.

Or in other words, in situations where step potential is an issue, the solution is to add more interconnected metallic elements. This strongly argues the point that the connection to the soil is of little value for protection from low impedance faults.

This is another case of hazard due to STEP POTENTIAL. Even with an EGC connected to the source at the instant of the fault there will still be step potential because there is a time delay for the OCPD to operate.

I agree that in the 'grounded lamp post' scenario that we are discussing a step potential issue.

By _bonding_ the lamp post, high voltage imposed by a bolted fault is of extremely short duration, and high impedance faults of long duration would only impose a low voltage on the lamp post.

Any NEC required earth electrodes would do very little to change this situation.

As you have suggested, adding an extensive grounding electrode grid would help to reduce the remaining step potential hazard. Such a grounding electrode grid far exceeds what the NEC requires, and is far more costly than is justified by the benefit provided. I would additionally posit that this grounding electrode grid does not improve safety via better connection to the soil, but instead by adding additional metallic conductivity to _bond_ more area together.

-Jon

 

[don_resqcapt19]

There are different sources of electrocution. One based on the illustration shown which is the step potential and the other one is the energized metal piece. To solve step potential due to the energization of the earth because of a very high fault current, the resistance of the ground should be reduced by installing more ground rods near the energized metal piece or a ground mat connected to earth. This is the reason why I am comparing what is being done in a transformer substation. The reason why there are many ground rods installed in a substation is to reduce step potential if a person is inside the substation at the time of a very high fault current.

The additional rods and connection to earth are not what reduces the step potential at a substation grounding mat. It is the closeness of the conductors that make up the mat grid itself. Under fault conditions there may be a step potential of thousands of volts at the edge of the mat.

The connnection of the energized metal piece to earth is to reduce its potential to ground to reduce electric shock.

Please show me how this works. As I said in another post the only way it can do this is to make bring the voltage of the earth up to that of the energized part or bring the voltage of energized part down to that of the earth. Which one happens? Exactly how does this happen?

Reducing the energized metal piece to ground potential is applicable to all types of system (solidly grounded, HRG, Ungrounded System).

Under normal conditions that does work for all of the systems, under fault conditions it does very little to do that in for an solidly grounded system under fault conditions.

 

[bobby ocampo]

The additional rods and connection to earth are not what reduces the step potential at a substation grounding mat. It is the closeness of the conductors that make up the mat grid itself. Under fault conditions there may be a step potential of thousands of volts at the edge of the mat.

What is the purpose of the mesh in the substation? Why is it that you install ground rods other than the ground rod connected to the neutral of the transformer?

Please show me how this works. As I said in another post the only way it can do this is to make bring the voltage of the earth up to that of the energized part or bring the voltage of energized part down to that of the earth. Which one happens? Exactly how does this happen?

This can be compared with what is happening in a single line to ground on an ungrounded system.

Without a fault what is the voltage to ground of all current carrying conductor?

If there is line A, B and C, what happens if line C touches the ground? Line A to ground will be equal to Line to line voltage. Line B to ground will be equal to line to line voltage. Line C to ground will be equal to ground potential.

If C becomes ground potential then this proves the point.

Under normal conditions that does work for all of the systems, under fault conditions it does very little to do that in for an solidly grounded system under fault conditions.

In solidly grounded system, just like in any other system, once one of the current carry conductor touches a metal piece it will raise its potetial equal to line to neutral voltage.

 

[winnie]

What is the purpose of the mesh in the substation? Why is it that you install ground rods other than the ground rod connected to the neutral of the transformer?

I do not know enough about substation grounding to answer. Clearly the extensive _metallic_ system will reduce step potentials within its perimeter; I don't know what happens right at the edge of the grounding grid. The grounding electrode array will reduce the impedance to the soil.

This can be compared with what is happening in a single line to ground on an ungrounded system.

....

If C becomes ground potential then this proves the point.

I suggest that what happens in an ungrounded system is not directly applicable to a solidly grounded system.

In the most common solidly grounded systems, the source neutral is connected to local grounding electrodes, bonded metal, and the entire utility grounding electrode system.

In the event of a fault which connects one of the line terminals to an _isolated_ grounding electrode, the overall system voltages to earth will be changed only slightly. The small patch of soil around the isolated grounding electrode will be raised to line voltage, but the vastly larger grounding electrode system connected to the neutral will keep the neutral at the same potential as the bulk of the surrounding soil. The connection to soil does very little to change this voltage.

In the event of a fault which connects one of the line terminals to the source grounding electrode system, then the voltage at the point of the fault will be determined by the voltage drop in the resistance of the metallic equipment grounding and grounding electrode system. The connection to soil does very little to change this voltage.

In solidly grounded system, just like in any other system, once one of the current carry conductor touches a metal piece it will raise its potetial equal to line to neutral voltage.

By 'its' potential, do you mean the potential of the metal piece, or the potential of the soil, or the potential of the earth?

-Jon

 

[bobby ocampo]

I do not know enough about substation grounding to answer. Clearly the extensive _metallic_ system will reduce step potentials within its perimeter; I don't know what happens right at the edge of the grounding grid. The grounding electrode array will reduce the impedance to the soil.

Objective of the mesh in a substation is to reduce the step potential inside the substation in the instant that there is a ground fault and not at the edge of the mesh.

I suggest that what happens in an ungrounded system is not directly applicable to a solidly grounded system.

The energization of the metal piece are the same whenever a current carrying conductor touches the metal piece regardless if it is an ungrounded system, solidly grounded system and High resistance grounded system.

In the most common solidly grounded systems, the source neutral is connected to local grounding electrodes, bonded metal, and the entire utility grounding electrode system.

Same is true with HIGH RESISTANCE GROUNDING. This shows the importance of the connection to earth same importance with the bonded EGC they go hand in hand in the reduction of electric shock to personnel specially if the fault is an arcing ground fault where the current is not high enough to trip the OCPD but the metal piece is already energized.

In the event of a fault which connects one of the line terminals to an _isolated_ grounding electrode, the overall system voltages to earth will be changed only slightly. The small patch of soil around the isolated grounding electrode will be raised to line voltage, but the vastly larger grounding electrode system connected to the neutral will keep the neutral at the same potential as the bulk of the surrounding soil. The connection to soil does very little to change this voltage.

If connection to earth in a High resistance grounded (grounded system) why can't it not do the same in a solidly grounded specially in a low level single line o ground fault such as what is happening in an arcing ground fault?

If this connection to earth can also reduce the potential to ground of an energized metal piece in an ungrounded system, then why can't it do the same in a solidly grounded specially on a low level single-line-to ground fault that the OCPD has not isolated the fault because it wont operate.

In the event of a fault which connects one of the line terminals to the source grounding electrode system, then the voltage at the point of the fault will be determined by the voltage drop in the resistance of the metallic equipment grounding and grounding electrode system. The connection to soil does very little to change this voltage.

If connection to earth can reduce the potential of the enegized metal piece in High Resistance Grounded and Ungrounded system why can't it do the same in a solidly grounded system?

By 'its' potential, do you mean the potential of the metal piece, or the potential of the soil, or the potential of the earth?

-Jon

Potential of the metal piece to ground.

 

[winnie]

If this connection to earth can also reduce the potential to ground of an energized metal piece in an ungrounded system, then why can't it do the same in a solidly grounded specially on a low level single-line-to ground fault that the OCPD has not isolated the fault because it wont operate.


If connection to earth can reduce the potential of the enegized metal piece in High Resistance Grounded and Ungrounded system why can't it do the same in a solidly grounded system?

In all of these cases (ungrounded, high resistance grounded, and solidly grounded), the connection to the soil will reduce the potential of the energized metal.

What is in question is how much the potential is reduced, and how beneficial this reduction is.

Even a simple analysis of this question must include both the impedance of the source that is energizing the piece of metal, and the impedance of the connection to the soil. These two impedances form a simple voltage divider.

If the impedance of the connection to the soil is much less than the source impedance, then the connection to the soil will succeed in 'de-energizing' the piece of metal. There might be current flowing in the connection to the soil, but the voltage of the piece of metal will be low.

If the impedance of the connection to the soil exactly equals the source impedance, then the voltage that remains on the piece of metal will be 1/2 that of the 'ungrounded' metal.

If the impedance of the connection to the soil is much greater than the source impedance, then the connection to the soil will not be able to 'load' the piece of metal, and the voltage of the piece of metal will be essentially unchanged. The voltage will drop slightly, but not in any useful fashion.

When we are talking properly bonded, grounded electrical systems, then the impedance of any fault voltages is very low, measured in fractions of an ohm. In these installations, connections to soil will have much higher impedance than the sources of any fault voltage, and connections to the soil will do very little to reduce any such fault voltage.

-Jon

 

[bobby ocampo]

In all of these cases (ungrounded, high resistance grounded, and solidly grounded), the connection to the soil will reduce the potential of the energized metal.

What is in question is how much the potential is reduced, and how beneficial this reduction is.

It will reduce the potential from line-to-neutral voltage to ground or earth potential which is approximately zero. Is line-to-neutral voltage not significant enough? This shows the importance of connection to ground or earth. Never is connection to earth less important for safety. Otherwise if it is of less important and it will only insignificantly reduce the potential to ground then NEC would have remove it as a minimum requirement for safety.

 

[jim dungar]

Otherwise if it is of less important and it will only insignificantly reduce the potential to ground then NEC would have remove it as a minimum requirement for safety.

The NEC does not arbitrarily remove useless items. Also, there is no argument that the ground rod is needed as part of the lightning abatement system.

The presence of a ground rod is only possibly helpful for fault paths that happen to include dirt, it offers no help for reducing any potential for indoor circuits. But it is bonding possible fault paths together that is most important not simply connecting to dirt.

 

[brian john]

The presence of a ground rod is only possibly helpful for fault paths that happen to include dirt, it offers no help for reducing any potential for indoor circuits. But it is bonding possible fault paths together that is most important not simply connecting to dirt.

This has been repeated time and again in this thread, either someone is missing the point OR.............................................

 

[winnie]

It will reduce the potential from line-to-neutral voltage to ground or earth potential which is approximately zero. Is line-to-neutral voltage not significant enough?

This is a patently incorrect statement.

Connecting a low impedance source to an earth electrode will simply leave the earth electrode energized.

I strongly suggest that you do the math, considering both the source impedance and the earth electrode impedance.

Until such time as you actually do the math, further discussion is useless.

Best Regards,
Jonathan Edelson

 

[bobby ocampo]

In all of these cases (ungrounded, high resistance grounded, and solidly grounded), the connection to the soil will reduce the potential of the energized metal.

What is in question is how much the potential is reduced, and how beneficial this reduction is.

All will reduce the potential of the energized metal piece to ground potential or theoretically zero.

Even a simple analysis of this question must include both the impedance of the source that is energizing the piece of metal, and the impedance of the connection to the soil. These two impedances form a simple voltage divider.

While there is a voltage drop based on the impedance of the connection to the soil which I never said is not important, at a very blow arcing ground fault which may energized the metal piece will increase its potential if the EGC is not connected to ground.

This can be proven in HRG where fault current is lower than 5amps. There is a single line to ground fault but because the fault is so low the voltage drop of the low impedance path to earth is negligible. However if the energized metal part is not conneted to earth, the potential of the energized metal piece will be equal to the line to neutral voltage of th system. This proves the connection to earth IS NOT LESS important than the EGC.

If the impedance of the connection to the soil is much less than the source impedance, then the connection to the soil will succeed in 'de-energizing' the piece of metal. There might be current flowing in the connection to the soil, but the voltage of the piece of metal will be low.

This shows how important the cnnection to the ground of the EGC. Connection to Earth is as important because not all ground fault may be big enough to operate the OCPD even how good your EGC is unless the OCPD has Ground Fault protection and has been set at a low level.

If the impedance of the connection to the soil exactly equals the source impedance, then the voltage that remains on the piece of metal will be 1/2 that of the 'ungrounded' metal.

EGC is a low impedance path. Soil resistivity is important to know and this is the reason why resistance of the soil is measured.

If the impedance of the connection to the soil is much greater than the source impedance, then the connection to the soil will not be able to 'load' the piece of metal, and the voltage of the piece of metal will be essentially unchanged. The voltage will drop slightly, but not in any useful fashion.

This is not always the case of ground fault. Most groundfault starts with an arcing ground fault. Unless it is an accident that one of the current carrying conductro accidentally touched the metal piece.

When we are talking properly bonded, grounded electrical systems, then the impedance of any fault voltages is very low, measured in fractions of an ohm. In these installations, connections to soil will have much higher impedance than the sources of any fault voltage, and connections to the soil will do very little to reduce any such fault voltage
-Jon

Purpose of connection to ground in equipment grounding is to reduce the potential of the energized meal piece to ground potential as can b shown during a single line-to-ground fault in a HIGH RESISTANCE GROUNDED.

 

[bobby ocampo]

This is a patently incorrect statement.

How to you reduce the potential of the energized metal piece in a HIGH RESISTANCE GROUNDED SYSTEM?

Connecting a low impedance source to an earth electrode will simply leave the earth electrode energized.

I strongly suggest that you do the math, considering both the source impedance and the earth electrode impedance.

Until such time as you actually do the math, further discussion is useless.

Best Regards,
Jonathan Edelson

The math can be seen on the vector representation on what happens to a HIGH RESISTANCE GROUNDING during a ground fault. HRG will be the representation of an arcing ground fault where fault current is still very low but the fault will already energized themetal piece if the equipment is not connected to earth.

 

[don_resqcapt19]

Objective of the mesh in a substation is to reduce the step potential inside the substation in the instant that there is a ground fault and not at the edge of the mesh.

Yes that is exactly the purpose of the mesh, however additional connections to the earth increase the amount of current flowing in the mesh and increase the step potential.

 

[don_resqcapt19]

Originally Posted by winnie
In all of these cases (ungrounded, high resistance grounded, and solidly grounded), the connection to the soil will reduce the potential of the energized metal.

What is in question is how much the potential is reduced, and how beneficial this reduction is.

All will reduce the potential of the energized metal piece to ground potential or theoretically zero.

That just does not happen with faults on solidly grounded systems.

 

[coulter]

bobby 0 -

All will reduce the potential of the energized metal piece to ground potential or theoretically zero. ...

This statement does not fit any physics known to me.

... HRG will be the representation of an arcing ground fault where fault current is still very low but the fault will already energized themetal piece if the equipment is not connected to earth. ...

I have no clue what this means. "HRG and "arcing ground fault" are somehow equated:-? :-? :-? :-?

Bobby - are you like joking or teasing. This is pretty close to nonsense.

carl