Equipment Grounding for Ungrounded System

[bobby ocampo]

Bobby,
Lets start back at the beginning. With a correctly installed solidly grounded system with no faults, exactly how does a connection to earth make that system safer?

If there is an arcing ground fault is still very low the metal enclosure will become energized. If the connection of the bonded EGC to earth is not important then there is a hazard of electric shock if the accidentally energized metal enclosure (due to arcing ground fault) is not connected to earth.

This is the same reason for UNGROUNDED SYSTEM and HRG SYSTEM. The bonded EGC should be connected to ground. so that the accidentally energized metal part will be at zero potential to the ground. This statement does not say that the EGC should not be bonded going to the neutral either in the service equipment or at the neutral of the transformer connected to the gournd because this is important too for the protection agaisnt arc flash hazard brought about by the direct connected single line to ground fault.

If static grounding may be reduced to a safe level by connecting to earth then why can't connecting to earth improve safety against accidentally energized metal enclosure to ground. If in Ungrounded system and HRG bonded egc connected to earth will reduce voltage potential of the energized metal enclosure to earth potential why can it do the same job in SOLIDLY GROUNDED SYSTEM. Arcing ground fault most of the time have a very small current that OCPD may not trip unless the OCPD has Ground fault protection for voltages less than 600 volts.

 

[winnie]

I would like to suggest a thought experiment:

Consider an electrical system in a building where _all_ conduit, enclosures, etc. are correctly bonded together using suitable bonding conductors, but where all such hardware is mounted upon insulating material, with absolutely no galvanic connection between any of the bonded metal and any soil. The electrical system neutral is connected to all of this bonded metal, but again _no_ connection is made to soil.

During normal operation, what is the voltage between the bonded metal and the soil?

During a fault at a piece of equipment distant from the supply transformer, what is the voltage between the bonded metal at the location of the faulting equipment and the soil? Does it make a difference if the fault is solid (bolted) or if it is a high impedance fault? What about if it is a re-striking fault?

Now add a grounding electrode to this system, connected via a suitable conductor to the neutral of the supply transformer. There are _no_ other connections to the soil, other than at the supply transformer.

Do the answers to the above questions change?

My answers below; think about this and then tell me where you think I've gotten it wrong:

1) With no connection between the bonded metal and soil, the voltage between the bonded metal and soil is unstable, and might be quite high (think static charging, eg. by wind), capacitive coupling, etc. Depending upon the size of the system or the level of charging currents, there may be considerable shock potential between the bonded metal and soil; consider for example the shock potential between a hovering helicopter and someone standing on the ground.

2) In a fault condition, current will flow on the bonded metal system, causing a voltage difference between the different parts of the bonded metal system. There may be shock potential between the different parts of the system. Someone coming in contact with the bonded metal at the point of the fault and with soil might experience shock as described in 1), but would experience no shock potential from the fault.

3) A high impedance fault would reduce the current flowing through the bonded metal system, and thus reduce the voltage imposed at the point of the fault.

4) With a earthing connection between the system neutral, the bonded metal, and an earth electrode, then the voltage between the bonded metal and the soil around the earth electrode is reduced to zero. For low current charging sources, the voltage placed on the earth electrode is very slight, and from any point on the bonded metal to any local soil, the voltage is effectively zero.

5) During a fault condition, current is flowing through the bonded metal system, causing a voltage difference between the different parts of the bonded metal system. There may be shock potential between the different parts of the system. Someone coming in contact with the bonded metal at the point of the fault and with soil would experience shock potential because they would form a circuit that includes the voltage drop through the bonded metal system, the earth electrode, and their direct contact with the soil.

6) Impedance in the fault would reduce the current flowing through the bonded metal system, and thus reduce the shock potential between the bonded metal at the point of fault and other bonded metal or the soil.

In all the above cases, what clears the fault is the short circuit current path caused by the intentional connection between bonded metal and transformer neutral. The earth electrode has nothing to do with this fault clearance.

In the above, the earth electrode reduces the shock hazard caused by high impedance external sources, but actually _increases_ the shock hazard associated with faults in the electrical system.

-Jon

 

[don_resqcapt19]

If there is an arcing ground fault is still very low the metal enclosure will become energized. If the connection of the bonded EGC to earth is not important then there is a hazard of electric shock if the accidentally energized metal enclosure (due to arcing ground fault) is not connected to earth.

This is the same reason for UNGROUNDED SYSTEM and HRG SYSTEM. The bonded EGC should be connected to ground. so that the accidentally energized metal part will be at zero potential to the ground. This statement does not say that the EGC should not be bonded going to the neutral either in the service equipment or at the neutral of the transformer connected to the gournd because this is important too for the protection agaisnt arc flash hazard brought about by the direct connected single line to ground fault.

If static grounding may be reduced to a safe level by connecting to earth then why can't connecting to earth improve safety against accidentally energized metal enclosure to ground. If in Ungrounded system and HRG bonded egc connected to earth will reduce voltage potential of the energized metal enclosure to earth potential why can it do the same job in SOLIDLY GROUNDED SYSTEM. Arcing ground fault most of the time have a very small current that OCPD may not trip unless the OCPD has Ground fault protection for voltages less than 600 volts.

None of that addresses the question that I asked. The question clearly specified no faults. However that being said, the connection to earth does not change the potential of the accidentally energized metal part.

 

[winnie]

Carl
I just assumed that when a few on here are talking about doing away with the earthed ground they are just talking about the earth ground electrode.

No one here is saying that we should get rid of the earthed grounding electrode.

We are just saying that such earth grounding offers only a limited safety benefit.

In my last post, I convinced myself that the earth grounding electrode actually _increases_ the shock risk associated with low voltage faults.

I presume that the safety benefit associated with _high_ voltage faults makes the low voltage risk 'worth it'.

On the possibility of high resistance grounding for residential applications: While this is _not_ allowed by current code, I've often considered the possibility of a resistance grounded system where _all_ circuit breakers include ground fault protection. In the event of a ground fault, the breaker would swiftly open, preventing the problems associated with the neutral hovering at line voltage. Ground fault currents would be greatly diminished. Given that AFCIs include ground fault protection, we may be moving in a direction where high resistance grounding becomes practical in this application.

-Jon

 

[ronaldrc]

Jon

I read these post as some saying the earth ground is not worth a hill of
beans.

Would you stake your life on that GFI kicking in damp location before you
received a fatal shock with the scenario I described ?

 

[don_resqcapt19]

We are required by code to have outlets outside at front and back for outside work. We know the GFI probably want work since
the system is not earth.

That has nothing to do with the GFCI working. It works only on a current difference between the hot and the neutral. The GFCI does not care where the other current is going, it only looks for the difference and opens the circuit when the current is above the set point. If you have an ungrounded system it may very well be that you could grab either conductor of the system while standing on ground and not cause enough current to flow to open the device. The connection to earth would likely increase the current flow to the point that the device will open the circuit. Is that really any safer? Even with a grounded system if the ground fault current is below the trip point you can still get a shock. A long time poster here, now deceased, often said that grounding is one half of a fatal circuit.

 

[don_resqcapt19]

How does a connection to earth get rid of the voltage on the faulted equipment (solidly grounded system)? This is really no different than connecting a load between the hot and the neutral. The voltage on the hot as measured to the earth, the grounding conductor or any other point does not go away when you connect a load. How is a connection to earth any different?

 

[bobby ocampo]

How does a connection to earth get rid of the voltage on the faulted equipment (solidly grounded system)? This is really no different than connecting a load between the hot and the neutral. The voltage on the hot as measured to the earth, the grounding conductor or any other point does not go away when you connect a load. How is a connection to earth any different?

Same answer as how do you get rid of a voltage of a single-line-to-ground fault in an UNGROUNDED SYSTEM and HIGH RESISTANCE GROUNDED SYSTEM. Is connection to earth in this type of system important? Will it reduce the potential of the energized equipment to ground potential? If yes, then it can also reduce the potential of an energized metal piece to ground potential with SOLIDLY GROUNDED. However it is also important to trip the OCPD because of the very high current that may cause arc flash hazard and this is where it is important to have the bonded EGC connected to the neutral as a return path to trip the OCPD. Therefore connection to earth will reduce the potential of the energized metal piece to ground potential at the instant of fault and the bonded EGC will trip the OCPD for the protetion against arc flash hazard and voltage drop brought about by high current.

 

[bobby ocampo]

I would like to suggest a thought experiment:

Consider an electrical system in a building where _all_ conduit, enclosures, etc. are correctly bonded together using suitable bonding conductors, but where all such hardware is mounted upon insulating material, with absolutely no galvanic connection between any of the bonded metal and any soil. The electrical system neutral is connected to all of this bonded metal, but again _no_ connection is made to soil.

During normal operation, what is the voltage between the bonded metal and the soil?

During a fault at a piece of equipment distant from the supply transformer, what is the voltage between the bonded metal at the location of the faulting equipment and the soil? Does it make a difference if the fault is solid (bolted) or if it is a high impedance fault? What about if it is a re-striking fault?

Now add a grounding electrode to this system, connected via a suitable conductor to the neutral of the supply transformer. There are _no_ other connections to the soil, other than at the supply transformer.

Do the answers to the above questions change?

My answers below; think about this and then tell me where you think I've gotten it wrong:

1) With no connection between the bonded metal and soil, the voltage between the bonded metal and soil is unstable, and might be quite high (think static charging, eg. by wind), capacitive coupling, etc. Depending upon the size of the system or the level of charging currents, there may be considerable shock potential between the bonded metal and soil; consider for example the shock potential between a hovering helicopter and someone standing on the ground.

2) In a fault condition, current will flow on the bonded metal system, causing a voltage difference between the different parts of the bonded metal system. There may be shock potential between the different parts of the system. Someone coming in contact with the bonded metal at the point of the fault and with soil might experience shock as described in 1), but would experience no shock potential from the fault.

3) A high impedance fault would reduce the current flowing through the bonded metal system, and thus reduce the voltage imposed at the point of the fault.

4) With a earthing connection between the system neutral, the bonded metal, and an earth electrode, then the voltage between the bonded metal and the soil around the earth electrode is reduced to zero. For low current charging sources, the voltage placed on the earth electrode is very slight, and from any point on the bonded metal to any local soil, the voltage is effectively zero.

5) During a fault condition, current is flowing through the bonded metal system, causing a voltage difference between the different parts of the bonded metal system. There may be shock potential between the different parts of the system. Someone coming in contact with the bonded metal at the point of the fault and with soil would experience shock potential because they would form a circuit that includes the voltage drop through the bonded metal system, the earth electrode, and their direct contact with the soil.

6) Impedance in the fault would reduce the current flowing through the bonded metal system, and thus reduce the shock potential between the bonded metal at the point of fault and other bonded metal or the soil.

In all the above cases, what clears the fault is the short circuit current path caused by the intentional connection between bonded metal and transformer neutral. The earth electrode has nothing to do with this fault clearance.

In the above, the earth electrode reduces the shock hazard caused by high impedance external sources, but actually _increases_ the shock hazard associated with faults in the electrical system.

-Jon

I agree that the bonded EGC will trip the OCPD. I also agree that at the time of fault connection to earth will reduce the potential of the energized metal piece to ground potential. Both are important.

 

[don_resqcapt19]

Same answer as how do you get rid of a voltage of a single-line-to-ground fault in an UNGROUNDED SYSTEM and HIGH RESISTANCE GROUNDED SYSTEM. Is connection to earth in this type of system important? Will it reduce the potential of the energized equipment to ground potential? If yes, then it can also reduce the potential of an energized metal piece to ground potential with SOLIDLY GROUNDED. However it is also important to trip the OCPD because of the very high current that may cause arc flash hazard and this is where it is important to have the bonded EGC connected to the neutral as a return path to trip the OCPD. Therefore connection to earth will reduce the potential of the energized metal piece to ground potential at the instant of fault and the bonded EGC will trip the OCPD for the protetion against arc flash hazard and voltage drop brought about by high current.

It just doesn't do that. The voltage does not go away when you make a connection to earth any more than the voltage on the ungrounded conductor goes away when you connect it to a load.

 

[tryinghard]

How does a connection to earth get rid of the voltage on the faulted equipment (solidly grounded system)?

Same answer as how do you get rid of a voltage of a single-line-to-ground fault in an UNGROUNDED SYSTEM and HIGH RESISTANCE GROUNDED SYSTEM?Will it reduce the potential of the energized equipment to ground potential?

Bobby, an ungrounded system does not have any reference to ground there is no fault path back to source because NONE of the transformers (A, B, or C) are grounded.

A fault will simply cause the conductive items to be exactly the same potential as the phase that is shorting and there will be NO current to earth. A fault on an ungrounded system does not drain to earth; you don?t ?get rid of voltage on a single-line-to-ground fault? it simply remains until a second short happens!

 

[tryinghard]

Notice this graphic, but an ungrounded system does not even have a neutral "N", none of the transformers are grounded there's no path to source.

Notice this graphic, its an incorrectly bonded "grounded system". If it were an ungrounded system there will be NO XO and NO bonding to XO or neutral and the dangerous touch voltage will not be traveling to source because there is no path to any of the source transformers. It will not be traveling to earth either it will simply remain on conductive items until a second short happens.

 

[bobby ocampo]

Bobby, an ungrounded system does not have any reference to ground there is no fault path back to source because NONE of the transformers (A, B, or C) are grounded.

The bonded EGC is connected to an earth reference connected to the metal case of the transformer and then to ground. This is exactly the point where the energized metal piece if not connected to earth will be very hazardous. If the energized metal piece will not be connected to the earth the the potential of the metal piece to ground will be equal to the line-to-neutral equivalent voltage because of the system capacitance.

A fault will simply cause the conductive items to be exactly the same potential as the phase that is shorting and there will be NO current to earth. A fault on an ungrounded system does not drain to earth; you don?t ?get rid of voltage on a single-line-to-ground fault? it simply remains until a second short happens!

Yes you are right and unless this bonded EGC is connected to earth the personnel that may accidentally touch this energized metal part will receive an electric shock. This shows why connecting to earth is important.

 

[bobby ocampo]

Notice this graphic, but an ungrounded system does not even have a neutral "N", none of the transformers are grounded there's no path to source.

In this case the bonded EGCs purpose is not a return path to the source but a return path to the earth to reduce the potential of the energized metal part to ground potential to reduce electric shock. This is also one reason why the EGC of the Solidly grounded system should also be connected to the earth specially if the single line-to-ground fault is an arcing ground fault. Arcing ground fault may not trip the OCPD BUT MAY ENERGIZED THE METAL PIECE of the equipment if not connected to earth.

Notice this graphic, its an incorrectly bonded "grounded system". If it were an ungrounded system there will be NO XO and NO bonding to XO or neutral and the dangerous touch voltage will not be traveling to source because there is no path to any of the source transformers. It will not be traveling to earth either it will simply remain on conductive items until a second short happens.

The point is it is realy important that neutral is bonded to the EGC and connected to earth is very important for safety in solidly grounded system specially in arcing ground fault.

 

[winnie]

Mr. Ocampo,

You quoted my entire though experiment, but did not comment on my claim that the connection to the earth ground electrode would actually increase the shock potential at a fault location that is on the system but _distant_ from the ground electrode.

-Jon

 

[bobby ocampo]

Mr. Ocampo,

You quoted my entire though experiment, but did not comment on my claim that the connection to the earth ground electrode would actually increase the shock potential at a fault location that is on the system but _distant_ from the ground electrode.

-Jon

This problem is step potential which may be solved by a grounding mat or interconnecting ground rod in a mesh similar to the solution done in substation to reduce step potential. The number of rods to be buried will depend on the ground resistance measurement. Fault at the main circuit breaker of substation is very high that there is a danger of step potential on a single line to ground fault because of th IR voltage drop of the soil. In either case the ground rod is still installed.

What you are saying is the reason why EGC should be connected back to the source or the service equipment to trip the OCPD which is as important as connection of the bonded EGC to earth.

 

[tryinghard]

In this case the bonded EGCs purpose is not a return path to the source but a return path to the earth to reduce the potential of the energized metal part to ground potential to reduce electric shock?

There must be a path from and to source for current to flow. An ungrounded system is a delta system, the ?C? transformer is ungrounded and there is no XO tap or bonding jumper to the transformer ?C? (C1 & C2).

In your claim why does current want to return to earth?

 

[winnie]

In your claim why does current want to return to earth?

On this point I quite agree with Mr. Ocampo.

There is no _true_ ungrounded system. What we call 'ungrounded' is really 'capacitively grounded'.

It seems plausible to me that in appropriate fault conditions, the bonded metal that is in proximity to an ungrounded system could have its voltage 'pumped' relative to ground. This 'pumping' would of necessity be some high impedance source, and a simple ground electrode would be enough to eliminate any significant voltage on the entire bonded metal system.

-Jon

 

[winnie]

This problem is step potential which may be solved by a grounding mat or interconnecting ground rod in a mesh similar to the solution done in substation to reduce step potential. The number of rods to be buried will depend on the ground resistance measurement.

This problem is not step potential in the soil, but voltage drop in the EGC connected to the faulted apparatus.

A grounding grid would only serve to provide a lower impedance reference from which to experience this voltage drop, unless the faulted apparatus was itself directly connected to the grounding grid.

In the latter case, you've essentially created a large metallic system that acts to keep all points near a fault at the same potential, thus eliminating the chance of shock. This large metallic system is sitting on the soil, but it seems to me that in the situation described the soil contact is incidental to the metallic conductivity.

-Jon

 

[tryinghard]

On this point I quite agree with Mr. Ocampo.

There is no _true_ ungrounded system. What we call 'ungrounded' is really 'capacitively grounded'.

It seems plausible to me that in appropriate fault conditions, the bonded metal that is in proximity to an ungrounded system could have its voltage 'pumped' relative to ground. This 'pumping' would of necessity be some high impedance source, and a simple ground electrode would be enough to eliminate any significant voltage on the entire bonded metal system.

-Jon

Sure, and with this finding it would be best to replace the ungrounded system with grounded.