What is an Arcing Ground fault in Solidly Grounded System?

[bobby ocampo]

What is an arcing ground fault in a solidly grounded system?

 

[Buck Parrish]

If it was a ground fault it should trip a regular breaker. I think the arc fault was designed to protect against an arc that has no ground fault.
Such as a loosely plugged in space heater that is barely making continuity. There fore it is arcing ever so slightly. And heat begins to build. It is my Opinion, that's why the arc fault breakers we're desighned.

 

[brian john]

An arcing ground fault occurs when the impedance of the fault through the arc does not draw enough current to trip the OCP.

I posted an article sometime ago regarding this I will locate and re-post.

Found it.

WHAT IS GROUND FAULT PROTECTION OF DISTRTIBUTION EQUIPMENT:

1. The Ground Fault Protection (GFP) system is designed for equipment protection, NOT PEOPLE PROTECTION as some may think.
2. GFP was first adopted into the NEC in 1971 NEC article 230.95. The reason for this new Article was the increase in sustained arcing ground faults resulting in system burn down that accompanied the increase in the use of 480/277 distribution systems.
3. The basic NEC rule for the mandatory installation of GFP is on Main Line Switches 1000 amps and larger and more than150 volts to ground.
4. Setting for the GFP relay is a maximum of 1200 amps with a maximum of 1.0 second delay.
5. While arc faults occur at all voltage levels sustained arcing ground faults occur at a voltage above 370 VAC. The peak voltage of 208/120 VAC system to ground is 169 VAC, while for a 480/277 VAC system the peak voltage is 391 VAC above the 370 VAC threshold. 120 X 1.414=169 and 277 X`1.414=390 (numbers are rounded off).
6. The nature of this sustained arc is the impedance of the arc is high and the fault does not generate enough current to operate the OCP. But this arc has damaging energy and can burn down switchboards, turn busways into a mass of molten metal and KILL IN the preverbal flash of less that a second.
7. Switchboards AIC ratings are designed for the worse case fault, this is a bolted phase to phase fault, in reality this type of fault, bolted; is rare. Studies have shown most faults start as ground faults and then if the OCP does not clear the fault they may become phase to phase but not bolted.

 

[bobby ocampo]

An arcing ground fault occurs when the impedance of the fault through the arc does not draw enough current to trip the OCP.

Can the bonded EGC help in this situation to trip the OCP if the OCPD has no ground fault protection?

What happens to the metal enclosure where Arcing Ground Fault occurs? Will the Arcing ground fault energized the metal piece of equipment? Will it increase its potential to ground?

I posted an article sometime ago regarding this I will locate and re-post.

Found it.

WHAT IS GROUND FAULT PROTECTION OF DISTRTIBUTION EQUIPMENT:

1. The Ground Fault Protection (GFP) system is designed for equipment protection, NOT PEOPLE PROTECTION as some may think.

Do you mean that even with a ground fault protection and it trips the OCPD it is not a guarantee that People will be protected? How can we protect the people now?

2. GFP was first adopted into the NEC in 1971 NEC article 230.95. The reason for this new Article was the increase in sustained arcing ground faults resulting in system burn down that accompanied the increase in the use of 480/277 distribution systems.
3. The basic NEC rule for the mandatory installation of GFP is on Main Line Switches 1000 amps and larger and more than150 volts to ground.
4. Setting for the GFP relay is a maximum of 1200 amps with a maximum of 1.0 second delay.

If Ground fault protection is for equipment protection then what is for people protection during Arcing Ground fault?

5. While arc faults occur at all voltage levels sustained arcing ground faults occur at a voltage above 370 VAC. The peak voltage of 208/120 VAC system to ground is 169 VAC, while for a 480/277 VAC system the peak voltage is 391 VAC above the 370 VAC threshold. 120 X 1.414=169 and 277 X`1.414=390 (numbers are rounded off).
6. The nature of this sustained arc is the impedance of the arc is high and the fault does not generate enough current to operate the OCP. But this arc has damaging energy and can burn down switchboards, turn busways into a mass of molten metal and KILL IN the preverbal flash of less that a second.
7. Switchboards AIC ratings are designed for the worse case fault, this is a bolted phase to phase fault, in reality this type of fault, bolted; is rare. Studies have shown most faults start as ground faults and then if the OCP does not clear the fault they may become phase to phase but not bolted.

Would High Resistance Grounding be a solution for this Arcing ground fault?

 

[coulter]

What is an arcing ground fault in a solidly grounded system?

One that is not bolted.

In a bolted fault the system voltage is dropped across the source impedance. Very little voltage drop across the fault - thus very little power disipated at the fault location.

In an arcing fault, there is significant Vd at the fault - thus significant power (heat, light, smoke) at the fault

...Would High Resistance Grounding be a solution for this Arcing ground fault?

Yes

According to IEEE 242, which somehow I'm thinking you already know about:
Ieee 242 8.2.2 Solid Grounding
One disadvantage of the solidly grounded 480 V system involves the high magnitude of destructive, arcing ground-fault currents that can occur. However, if these currents are promptly interrupted, the equipment damage is kept to acceptable levels. While low-voltage systems can be resistance-grounded, resistance grounding restricts the use of line-to-neutral loads.

carl

 

[winnie]

Can the bonded EGC help in this situation to trip the OCP if the OCPD has no ground fault protection?

Arcing ground faults occur in situations with a properly bonded EGC. The _key_ characteristic is that the impedance of the fault is so high that the fault current is not sufficient to trip the OCPD. If anything, the bonded EGC makes the arcing ground fault _worse_ because it completes the circuit.

What happens to the metal enclosure where Arcing Ground Fault occurs? Will the Arcing ground fault energized the metal piece of equipment? Will it increase its potential to ground?

During such a fault, current is flowing along the EGC back to the main bonding jumper and thence back to the supply transformer. The current flow on the EGC will result in a voltage drop on the EGC system, thus energizing the piece of the equipment relative to other parts of the EGC system. Since the EGC is bonded to the earth electrode, this means that the equipment will be energized relative to the earth electrode.

Do you mean that even with a ground fault protection and it trips the OCPD it is not a guarantee that People will be protected? How can we protect the people now?

Ground fault protection protects people by detecting fault conditions and shutting down the faulted circuit. No such protection is perfect, and during the ground fault event people may be exposed to hazardous conditions. The expression 'ground fault protection for equipment' is an indication that the permitted fault current levels are high enough to cause injury if they were to actually pass through a person.

GFP with trip levels of about 5mA are available to provide direct protection of people; if a person comes in direct contact with such a 'GFCI' protected circuit than the extremely sensitive ground fault protection would most likely disable the circuit prior to injury. However such extreme sensitive is generally not suitable for a large feeder, and is almost exclusively used for specific branch circuit protection.

Would High Resistance Grounding be a solution for this Arcing ground fault?

Resistance grounding will prevent arcing faults to ground. The grounding resistance acts to reduce current flowing in the EGC system, thus lowering any voltages imposed on the EGC system. The downside is that the grounding resistance _increases_ the voltage imposed upon the neutral conductor during a ground fault. As a result, resistance grounded systems are not permitted to feed line-neutral loads.

-Jon

 

[bobby ocampo]

Arcing ground faults occur in situations with a properly bonded EGC. The _key_ characteristic is that the impedance of the fault is so high that the fault current is not sufficient to trip the OCPD. If anything, the bonded EGC makes the arcing ground fault _worse_ because it completes the circuit.

What do you mean sir by the impedance of the fault is so high?

During such a fault, current is flowing along the EGC back to the main bonding jumper and thence back to the supply transformer. The current flow on the EGC will result in a voltage drop on the EGC system, thus energizing the piece of the equipment relative to other parts of the EGC system. Since the EGC is bonded to the earth electrode, this means that the equipment will be energized relative to the earth electrode.

Do you mean that connecting to ground or earth will reduce the potential of the energized metal part to earth potential on an arcing ground fault in a solidly grounded system during an arcing round fault?

Ground fault protection protects people by detecting fault conditions and shutting down the faulted circuit. No such protection is perfect, and during the ground fault event people may be exposed to hazardous conditions. The expression 'ground fault protection for equipment' is an indication that the permitted fault current levels are high enough to cause injury if they were to actually pass through a person.

Maximum setting of the ground fault protection is 1200 amps and 1sec. At this value and time and based on the illustration of the post and the other illustration with an 18 ohms resistance, at 1200 amps there will be a voltage drop of 21,600 volts on step potential for 1sec using a simple ohms law. This voltage is dangerous based on the fault current. If there is no Ground fault protection then there is a very high value of fault current that may cause step potential but the EGC is not enough to trip the OCPD while the enclosure is still energized to the potential of the current carrying conductor. How should we prevent electric shock in this situation if connection to earth is less important?

GFP with trip levels of about 5mA are available to provide direct protection of people; if a person comes in direct contact with such a 'GFCI' protected circuit than the extremely sensitive ground fault protection would most likely disable the circuit prior to injury. However such extreme sensitive is generally not suitable for a large feeder, and is almost exclusively used for specific branch circuit protection.

I agree sir that GFCI is for people protection class A for 5mA. How do we prevent agains electric shock for high current levels of ground fault but the value is still low enoght to trip the OCPD despite the low impedance EGC to the source?

Resistance grounding will prevent arcing faults to ground. The grounding resistance acts to reduce current flowing in the EGC system, thus lowering any voltages imposed on the EGC system.
-Jon

At this low fault current, if the energized metal piece is not connected to ground, how do we protect people touching the energized metal piece? At this situation the energized metal piece potential is equal to the line-to-neutral voltage of the system. EGC is not used to trip the OCPD because ground fault is reduced to very low level.

Is connection to ground or earth less important?

 

[winnie]

What do you mean sir by the impedance of the fault is so high?

The aspect of the fault that acts to limit the current flowing through the fault.

The current flow on the EGC will result in a voltage drop on the EGC system, thus energizing the piece of the equipment relative.... Since the EGC is bonded to the earth electrode, this means that the equipment will be energized relative to the earth electrode.

Do you mean that connecting to ground or earth will reduce the potential of the energized metal part to earth potential on an arcing ground fault in a solidly grounded system during an arcing round fault?

No, I mean the opposite.

The current flow through the EGC means that different parts of the EGC system will be at different voltages. Because of the fault current on the EGC, the exposed metal at the fault is at a different voltage than the exposed metal at the service. Because the earth electrodes are connected to the service, the metal at the fault will be at elevated voltage relative to the earth.

Maximum setting of the ground fault protection is 1200 amps and 1sec. At this value and time and based on the illustration of the post and the other illustration with an 18 ohms resistance, at 1200 amps there will be a voltage drop of 21,600 volts on step potential for 1sec using a simple ohms law.

How, on a 480/277V system, can you have a fault which causes a 21,600 volt drop in the EGC system?

If you have a metallic connection between the two locations, then you will not have 18 ohms of resistance. If you only have an electrode connection to soil, then the solution is not to install a larger connection to the soil, but instead to provide a metallic connection back to the source.

This voltage is dangerous based on the fault current. If there is no Ground fault protection then there is a very high value of fault current that may cause step potential but the EGC is not enough to trip the OCPD while the enclosure is still energized to the potential of the current carrying conductor. How should we prevent electric shock in this situation if connection to earth is less important?

The dangerous potential that you describe is caused by resistance between the fault and the _source_. The correct solution is to reduce the resistance between the fault and the source, by metallic bonding between the enclosure a the fault and the source.

I agree sir that GFCI is for people protection class A for 5mA. How do we prevent agains electric shock for high current levels of ground fault but the value is still low enoght to trip the OCPD despite the low impedance EGC to the source?

If you have a low impedance EGC back to the source, then a low current fault will not impose a high voltage on the EGC system. This is simple Ohm's law. If you have a low impedance EGC back to the source, and a high voltage imposed upon the EGC system, then you must have a high current fault. Again this is simple Ohm's law.

In a properly installed electrical system, all items connected to an EGC are connected to an earth electrode via that EGC. During a fault, there will be voltage drop across the EGC. The required connection to the earth electrode does nothing to change the voltage drop on the EGC.

Are you advocating that an additional, direct connection be made between the equipment and an earth electrode? If so, then there are two situations: 1) where you have an additional isolated earth electrode. As stated, this isolated earth electrode will do little to change the voltage on the EGC at the fault. or 2) where you have an additional earth electrode that is bonded back to the main electrode system. In this latter case you will significantly change the voltage on the faulted equipment, but because of the _metallic_ connection back to the source.

-Jon

 

[bobby ocampo]

Arcing ground faults occur in situations with a properly bonded EGC. The _key_ characteristic is that the impedance of the fault is so high that the fault current is not sufficient to trip the OCPD. If anything, the bonded EGC makes the arcing ground fault _worse_ because it completes the circuit.

The aspect of the fault that acts to limit the current flowing through the fault.

Sorry sir I am confused. EGC is the low impedance path to operate the OCPD. How would you then reduce the impedance of the fault?

No, I mean the opposite.

The current flow through the EGC means that different parts of the EGC system will be at different voltages. Because of the fault current on the EGC, the exposed metal at the fault is at a different voltage than the exposed metal at the service. Because the earth electrodes are connected to the service, the metal at the fault will be at elevated voltage relative to the earth.

So how do you reduce the potential of the energized metal piece if the fault is not cleared even with a low impedance path of the EGC because Arcing Ground fault is very low?

How, on a 480/277V system, can you have a fault which causes a 21,600 volt drop in the EGC system?

Based on the Illustration with an 18 ohms resistance of the soil and using ohms law. If the fault current is 1200 Amps the voltage drop in the ground will be 1200 times 18 which is 21600 volts. The step potential is due to the fault current in the soil and the resistance of the soil. Other wise there will be no measured volltage as shown in the illustration.

If you have a metallic connection between the two locations, then you will not have 18 ohms of resistance. If you only have an electrode connection to soil, then the solution is not to install a larger connection to the soil, but instead to provide a metallic connection back to the source.

But then again if the return path will not operate the OCPD how do you protect the people against hazard of electric shock if the potential of the energized enclosure is still high if you will not connect it to the earth?

The dangerous potential that you describe is caused by resistance between the fault and the _source_. The correct solution is to reduce the resistance between the fault and the source, by metallic bonding between the enclosure a the fault and the source.

What if the system is ungrounded and there is no connection to source but only to a reference ground rod? Isn't the purpose of this connection to earth to reduce electric shock hazard because it will reduce the potential of the energized metal piece to ground potential?

If you have a low impedance EGC back to the source, then a low current fault will not impose a high voltage on the EGC system. This is simple Ohm's law. If you have a low impedance EGC back to the source, and a high voltage imposed upon the EGC system, then you must have a high current fault. Again this is simple Ohm's law.

The energized metal piece is not ONLY dependent on the Fault current. If one of the current carrying conductor touches the metal piece it will increased its potential equivalent to line-to-neutal voltage of the system. If the energized metal piece is not connected to earth then there will be hazards of electric shock.

This is the reason why I am comparing it to what will happen if one of the current carring conductor in an ungrounded system touches a metal piece that is not conneted to earth. The potential of the energized metal piece will be the line-to-neutral voltage of the system to ground because of the system capacitance. Same will happen with HRG.

In a properly installed electrical system, all items connected to an EGC are connected to an earth electrode via that EGC. During a fault, there will be voltage drop across the EGC. The required connection to the earth electrode does nothing to change the voltage drop on the EGC.

It does not change the voltage drop for as long as there is a current flowing in the EGC but connecting the metal piece to earth will reduce its potential to ground.

Are you advocating that an additional, direct connection be made between the equipment and an earth electrode? If so, then there are two situations: 1) where you have an additional isolated earth electrode. As stated, this isolated earth electrode will do little to change the voltage on the EGC at the fault. or 2) where you have an additional earth electrode that is bonded back to the main electrode system. In this latter case you will significantly change the voltage on the faulted equipment, but because of the _metallic_ connection back to the source.

-Jon

If based on what you say that EGC resistance because of its relative distance from the connection to earth will have a voltage drop that will increase the potential of the metal piece to earth then it will help to add additional earth electrode that is bonded to the main electrode system.

However, wont this create of what they call a ground loop?

 

[winnie]

Sorry sir I am confused. EGC is the low impedance path to operate the OCPD. How would you then reduce the impedance of the fault?

The issue is that the type of fault that you are asking about, the 'arcing ground fault', itself limits the current flow. The EGC could be a perfect zero resistance conductor, yet the current flow would remain limited by the characteristics of the fault itself.

Consider an artificial fault, caused by someone intentionally connecting a 30KW 277V heater between one line and the EGC. No matter how low the impedance of the EGC, the current of this artificial fault remains limited.

In an arcing ground fault, the current path is through ionized gas. The current is limited by the impedance of this path. You have a _series_ circuit starting from the source, going through the faulted line, through the ionized gas, then through the EGC, and back to the source. The dominant impedance in this circuit is the ionized gas, with little voltage drop on the EGC.

So how do you reduce the potential of the energized metal piece if the fault is not cleared even with a low impedance path of the EGC because Arcing Ground fault is very low?

The potential of the metal piece being hit by the arc is low because of the low impedance EGC. The bulk of the supply voltage is dropped in the ionized gas. Further, the danger of this fault is not electric shock, but instead the many KW of energy being released as heat and radiant energy.

Based on the Illustration with an 18 ohms resistance of the soil and using ohms law. If the fault current is 1200 Amps the voltage drop in the ground will be 1200 times 18 which is 21600 volts.

I quite agree that if you have 1200A flowing through an 18 ohm resistance, you will develop a potential of 21600V. The other side of the coin is that if the supply voltage is less than 21600V, then the current flow _must_ be less than 1200A. If you have a conventional 480/277V source, then you will only get 15A in this fault.

But then again if the return path will not operate the OCPD how do you protect the people against hazard of electric shock if the potential of the energized enclosure is still high if you will not connect it to the earth?

By Ohm's law: if the return path is low impedance, and the fault current is low, then the exposed voltage must be low. Can cannot simultaneously have a low fault current and a high contact voltage if the return path has a proper low impedance.

What if the system is ungrounded and there is no connection to source but only to a reference ground rod? Isn't the purpose of this connection to earth to reduce electric shock hazard because it will reduce the potential of the energized metal piece to ground potential?

That is a different discussion. You are asking about Arcing ground faults in solidly grounded systems.

The energized metal piece is not ONLY dependent on the Fault current. If one of the current carrying conductor touches the metal piece it will increased its potential equivalent to line-to-neutal voltage of the system. If the energized metal piece is not connected to earth then there will be hazards of electric shock.

If the energized metal piece is connected to earth, then there will still be an electric shock hazard. Only if the metal is connected by a low impedance path back to the source will the shock hazard be mitigated.

It does not change the voltage drop for as long as there is a current flowing in the EGC but connecting the metal piece to earth will reduce its potential to ground.

Please consider actually working some numbers, using reasonable values for conductor and ground electrode resistance. I provided a set of examples. A few minutes with a calculator evaluating various scenarios will quickly show where grounding electrodes are useful (and they most certainly are useful), and where they provide very little benefit.

Whenever current flows through the EGC, there will be a voltage drop on that EGC. Consider a 10ga EGC, with a fault current of 100A, and a length of 250 feet. The resistance of the EGC is 0.25 ohms, and the voltage drop 25 volts. Metal at the end of this EGC would be elevated to a voltage of 25V relative to ground, since the other end of the EGC is bonded to a large grounding electrode system.

Now drive a ground rod right at the location of the faulted equipment. This is a very good ground rod with a resistance of 10 ohms to earth. Connect the faulted equipment chassis to this ground rod. Now calculate voltage of the equipment. I will agree that the voltage has been reduced. With the addition of this ground rod, the touch voltage has been reduced from 25V to 24.4V, a grand 1.5% improvement.

Only if you were to install a large grounding grid, including horizontal grounding electrodes for voltage gradient control to prevent step potentials, would you make a significant change in safety at this piece of equipment. With a far smaller investment you could take that EGC and make it larger.

-Jon

 

[bobby ocampo]

The issue is that the type of fault that you are asking about, the 'arcing ground fault', itself limits the current flow. The EGC could be a perfect zero resistance conductor, yet the current flow would remain limited by the characteristics of the fault itself.

Consider an artificial fault, caused by someone intentionally connecting a 30KW 277V heater between one line and the EGC. No matter how low the impedance of the EGC, the current of this artificial fault remains limited.

In an arcing ground fault, the current path is through ionized gas. The current is limited by the impedance of this path. You have a _series_ circuit starting from the source, going through the faulted line, through the ionized gas, then through the EGC, and back to the source. The dominant impedance in this circuit is the ionized gas, with little voltage drop on the EGC.

I this case do you agree that the metal piece may be energized if the metal piece is not conneted to earth just like what is happening to HRG and ungrounded system?

The potential of the metal piece being hit by the arc is low because of the low impedance EGC. The bulk of the supply voltage is dropped in the ionized gas. Further, the danger of this fault is not electric shock, but instead the many KW of energy being released as heat and radiant energy.

The low impedance EGC connected to earth makes the energized metal piece to ground potetial similar to what happens on a single line to ground fault in an ungrounded system. If EGC is not connected to earth because it is less important the energized metal piece will have potential to ground.

I quite agree that if you have 1200A flowing through an 18 ohm resistance, you will develop a potential of 21600V. The other side of the coin is that if the supply voltage is less than 21600V, then the current flow _must_ be less than 1200A. If you have a conventional 480/277V source, then you will only get 15A in this fault.

Prospective fault current is thousand of amps compared with the rated current. the step voltage is dangerous and this is the reason why in substation where there is very high fault current more grounding rods are installed and a mesh is created to reduce the step potential.

Please give the computation why did you arrive at 15amps.

By Ohm's law: if the return path is low impedance, and the fault current is low, then the exposed voltage must be low. Can cannot simultaneously have a low fault current and a high contact voltage if the return path has a proper low impedance.

This is the reason why I am comparing it to HRG. There will be high contact voltage if the EGC is not connected to ground.

That is a different discussion. You are asking about Arcing ground faults in solidly grounded systems.

Why will it be different if in HRG which is a grounded system may have the same effect if the EGC is not conneted to earth?

If the energized metal piece is connected to earth, then there will still be an electric shock hazard. Only if the metal is connected by a low impedance path back to the source will the shock hazard be mitigated.

Not just connected to the source but connected to earth. Both are important. You can compare it with HRG where there is a ground fault but limited current flow but should be connected to earth. It is unsafe to say that connection to earth is less important.

Please consider actually working some numbers, using reasonable values for conductor and ground electrode resistance. I provided a set of examples. A few minutes with a calculator evaluating various scenarios will quickly show where grounding electrodes are useful (and they most certainly are useful), and where they provide very little benefit.

Whenever current flows through the EGC, there will be a voltage drop on that EGC. Consider a 10ga EGC, with a fault current of 100A, and a length of 250 feet. The resistance of the EGC is 0.25 ohms, and the voltage drop 25 volts. Metal at the end of this EGC would be elevated to a voltage of 25V relative to ground, since the other end of the EGC is bonded to a large grounding electrode system.

Now drive a ground rod right at the location of the faulted equipment. This is a very good ground rod with a resistance of 10 ohms to earth. Connect the faulted equipment chassis to this ground rod. Now calculate voltage of the equipment. I will agree that the voltage has been reduced. With the addition of this ground rod, the touch voltage has been reduced from 25V to 24.4V, a grand 1.5% improvement.

The value of the potential of the metal piece will be coming from the current carrying conductor touching the metal piece not from the voltage drop of the EGC. Again an example will be an HRG where there is a very small current flowing through the EGC. If the EGC is not connected to earth the potential of this energized metal enclosure is equal to the line to neutral voltage of the system. This is the danger of not connecting to earth and it is of no difference during an arcing ground fault except the resistance is coming from the arc and not from the resistor in HRG system.

Only if you were to install a large grounding grid, including horizontal grounding electrodes for voltage gradient control to prevent step potentials, would you make a significant change in safety at this piece of equipment. With a far smaller investment you could take that EGC and make it larger.

-Jon

The comparison of the post illustration on the step potential is why in substation ground rods are installed and are interconnected together to lower the step potential incase of high fault.

 

[winnie]

I this case do you agree that the metal piece may be energized if the metal piece is not conneted to earth just like what is happening to HRG and ungrounded system?

We are discussing a fault in a solidly grounded system. In this discussion, the metal piece is connected to earth, via the EGC. Despite the EGC, the metal piece may still become energized, because of fault current flowing through the EGC.

If this bonded and grounded metal piece becomes energized because of fault current, then additional connections to the soil will reduce its voltage only slightly.

If the metal piece is not at all connected to earth, then it could most certainly be energized, even without a fault. I do not argue that we should not have the connection to the earth. But you seem to believe that additional connections can significantly alter low impedance fault voltages, and seem to be arguing for additional earth electrodes. I am arguing that these additional electrodes offer very little benefit.

The low impedance EGC connected to earth makes the energized metal piece to ground potetial similar to what happens on a single line to ground fault in an ungrounded system. If EGC is not connected to earth because it is less important the energized metal piece will have potential to ground.

If you have no connection to earth at all, then the voltage of the entire bonded metal system is undefined, and the bonded metal may be at elevated voltage without any fault at all. I have never said that grounding electrodes should be eliminated. I have simple said that additional grounding electrodes will not offer significant benefit.

Please give the computation why did you arrive at 15amps.

Given:
a grounded neutral source, 480/277V with 277V line to neutral.
neutral grounding via local electrodes as well as power company multi-earth-neutral
an isolated grounding electrode with 18ohm resistance to earth
a bolted connection from a 277V leg to the isolated grounding electrode

We have a series circuit where the supply voltage is 277V and the dominant impedance is an 18Ohm resistance. Via Ohm's law E = I * R
277 = I * 18
I = 15.4A

The value of the potential of the metal piece will be coming from the current carrying conductor touching the metal piece not from the voltage drop of the EGC.

Remember, you are asking about faults in a solidly grounded system. At the service point the EGC is connected to earth ground. Therefor the only thing that can change the potential of the metal piece relative to earth ground is the voltage drop in the EGC.

Again an example will be an HRG where there is a very small current flowing through the EGC. If the EGC is not connected to earth the potential of this energized metal enclosure is equal to the line to neutral voltage of the system. This is the danger of not connecting to earth and it is of no difference during an arcing ground fault except the resistance is coming from the arc and not from the resistor in HRG system.

If the EGC system is not connected to earth somewhere, then the metal may be energized, even without any sort of fault. But we are talking about grounded systems where there is a required connection to earth, and we are asking about voltages present on another part of the system.

If you have a properly bonded, but not grounded electrical system, then a fault will cause a potential difference between one part of the EGC system and another. But that fault will not cause a potential difference to earth. Without any sort of earth connection, the potential of the EGC system is entirely undefined.

-Jon

 

[bobby ocampo]

We are discussing a fault in a solidly grounded system. In this discussion, the metal piece is connected to earth, via the EGC. Despite the EGC, the metal piece may still become energized, because of fault current flowing through the EGC.

If this bonded and grounded metal piece becomes energized because of fault current, then additional connections to the soil will reduce its voltage only slightly.

If the metal piece is not at all connected to earth, then it could most certainly be energized, even without a fault. I do not argue that we should not have the connection to the earth.

This is what I am driving at. Connection to ground is as important as the EGC. Both EGC and connection to earth or ground is important.

But you seem to believe that additional connections can significantly alter low impedance fault voltages, and seem to be arguing for additional earth electrodes. I am arguing that these additional electrodes offer very little benefit.

The installation and bonding of additional Ground Electrode is for the purpose of reducing step potential as show in the illustration of the POLE.

If you have no connection to earth at all, then the voltage of the entire bonded metal system is undefined, and the bonded metal may be at elevated voltage without any fault at all. I have never said that grounding electrodes should be eliminated. I have simple said that additional grounding electrodes will not offer significant benefit.

Then I agree sir that connection to earth is as important as the EGC.

The additional grounding electrodes is for a different solution. Step potetential is hazardous too. Solution for step potential is to reduce the earth resistance to a low level to prevent voltage drop if a man is standing on a fault current.

Given:
a grounded neutral source, 480/277V with 277V line to neutral.
neutral grounding via local electrodes as well as power company multi-earth-neutral
an isolated grounding electrode with 18ohm resistance to earth
a bolted connection from a 277V leg to the isolated grounding electrode

We have a series circuit where the supply voltage is 277V and the dominant impedance is an 18Ohm resistance. Via Ohm's law E = I * R
277 = I * 18
I = 15.4A

Math for fault current is different sir? It is not just computing for the rated current of the system.

Remember, you are asking about faults in a solidly grounded system. At the service point the EGC is connected to earth ground. Therefor the only thing that can change the potential of the metal piece relative to earth ground is the voltage drop in the EGC.

The potential difference because of the voltage drop on the EGC is still important. However, statements saying that connection to ground is less important is dangerous and hazardous.

If the EGC system is not connected to earth somewhere, then the metal may be energized, even without any sort of fault. But we are talking about grounded systems where there is a required connection to earth, and we are asking about voltages present on another part of the system.

Based on your statement, connection to earth or ground is as important as the bonded EGC.

High Resistance Grounded system is considered a grounded system.

If you have a properly bonded, but not grounded electrical system, then a fault will cause a potential difference between one part of the EGC system and another. But that fault will not cause a potential difference to earth. Without any sort of earth connection, the potential of the EGC system is entirely undefined.

-Jon

There will be a potential difference to the earth. This can be proven based on the vector representation of a grounded system called HIGH RESISTANCE GROUNDED SYSTEM on a single-line-to-ground fault.

 

[winnie]

If the metal piece is not at all connected to earth, then it could most certainly be energized, even without a fault. I do not argue that we should not have the connection to the earth.

This is what I am driving at. Connection to ground is as important as the EGC. Both EGC and connection to earth or ground is important.

I have said over and over again that the connection to the earth electrode _is_ important to protect from shock from high impedance sources.

I stand by my statement that additional connections to additional earth electrodes are essentially meaningless and irrelevant for low impedance faults.

But you seem to believe that additional connections can significantly alter low impedance fault voltages, and seem to be arguing for additional earth electrodes. I am arguing that these additional electrodes offer very little benefit.

The installation and bonding of additional Ground Electrode is for the purpose of reducing step potential as show in the illustration of the POLE.

The 'pole' illustration shows the importance of bonding via the EGC. If the pole were properly bonded, then the OCPD would trip, and a high voltage would be present for only an instant. While it is true that adding a large and expensive array of grounding electrodes at pole would reduce step and contact potentials to the pole, adding an inexpensive EGC would prevent the pole from being energized in the first place.

Math for fault current is different sir? It is not just computing for the rated current of the system.

The maximum available fault current in the system assumes a bolted fault with no impedance.

Impedance in the fault path reduces the current flowing through the fault.

In the case of a fault to an isolated earth electrode, the fault current is limited by the impedance of this earth electrode, and the impedance of the source grounding electrodes.

If you have a properly bonded, but not grounded electrical system, then a fault will cause a potential difference between one part of the EGC system and another. But that fault will not cause a potential difference to earth. Without any sort of earth connection, the potential of the EGC system is entirely undefined.

There will be a potential difference to the earth. This can be proven based on the vector representation of a grounded system called HIGH RESISTANCE GROUNDED SYSTEM on a single-line-to-ground fault.

I would appreciate it if you could draw a picture of this.

Draw a properly bonded and grounded system (if you wish, make it a HRG system). Draw all of the required EGC conductors, and the bare minimum required earth electrodes. Draw in the fault. Now calculate out all of the various touch voltages. Make sure you include the impedance of the EGC and the grounding electrodes.

Now add in your desired additional grounding electrodes, and calculate out the various touch voltages.

I am quite certain that you will find that after the addition of the bare minimum required grounding electrodes, adding additional electrodes will do next to nothing to improve safety.

-Jon