bobby ocampo
Senior Member
don_resqcapt19 said:
The step potential will be reduced therefore step potential will be low. This is the reason why earth resistance is measured to determine the design to lower step potential in a substation.
Connection to Earth or Ground in Equipment Grounding
- Thread starter bobby ocampo
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The step potential will be reduced therefore step potential will be low. This is the reason why earth resistance is measured to determine the design to lower step potential in a substation.
bobby ocampo
Senior Member
coulter said:
The physics is in the vector representation of a single line to ground fault in an UNGROUNDED SYSTEM, SOLIDLY GRUNDED SYSTEM AND HIGH RESISTANCE GROUNDED SYSTEM.
coulter said:
Arcing Ground fault and fault in an HRG system has a low fault current. This fault current are not enough to operate the OCPD.
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Additional connections to the equal potential mesh installed close to the surface will tend to increase the step and touch potential. These additional connections will increase the current flow in the equal potential mesh and any current flow will result in a voltage drop. This voltage drop will increase the step potential within the mesh.bobby ocampo said:
bobby 0 -
Please state your scope, assumptions, and reaining. Remember - your assertions, no matter how often repeated, do not constitute reasoned physics.
Now, show us this "vector representation" you speak of. Pencil sketches are fine. Please show your calculations and reasoning behind assumptions. No scanner - no problem, just use your digital camera on each page. It's works fine.
carl
You're not adding any new information. Raising your voice and repeating yourself does not make your statement any more true.bobby ocampo said:
Please state your scope, assumptions, and reaining. Remember - your assertions, no matter how often repeated, do not constitute reasoned physics.
Now, show us this "vector representation" you speak of. Pencil sketches are fine. Please show your calculations and reasoning behind assumptions. No scanner - no problem, just use your digital camera on each page. It's works fine.
carl
For the three cases listed below, consider that all of the non-current carrying parts are bonded.bobby ocampo said:
HRG ground fault (typical) produces 5A fault current.
Arcing ground fault (solidly grounded):
Here is a typical scenerio I have seen: Consider a solidly grounded system, 480V, 1000kva xfm, fault on an 600A feeder to MCC, 400 feet from main swb.
GF current is on the order of 1000A - 3000A. This will trip the 600A CB - but maybe not as soon as you would like. However most of us that build this sort of stuff usually put in LSIG trip units on the feeders.
Arcing Ground fault (Ungrounded system):
Current is absolutely minimal. The circuit breakers are not supposed to trip. Rather the system waits for the voltage to build to where the xfm or conductor insulation fails.
There is nothing even remotely similar to these three scenerios.
Please note: For any of the three scenerios, driving a ground rod and bonding to the non-current metal parts - changes nothing.
If you are going to answer this, please stay on task, state assumptions and scope.
carl
bobby ocampo
Senior Member
coulter said:
Bonded only and not connected to ground?
coulter said:
With 5 Amps fault current in HRG the OCPD will not operate even with a single line-to-ground fault. If bonded EGC will not be connected to earth then if the current carrying conductor had a ground fault touchng a metal piece and EGC is not grounded or connected to earth then there will be a potential to ground of the energized metal piece equal to line-to-ground potential.
This is shows the importance of connection to earth or connecting to ground EGC. Fault current is so low that voltage drop due to the impedance of the EGC is also low but the metal piece is energized and hazardous if the metal piece is not conncted to earth.
coulter said:
Ground fault current is not always at the range of 1000A to 3000Amps. If the fault is bolted ground fault and based on a 5% impedance for a 1000kVA transformer, approximate ground fault current will be 87% of a 3phase bolted fault. Therefore maximum ground fault current will be approximately 20KA or 20,000 amps.
LSIG means Long time for overload, Short time for low level short circuit and I for Instantaneous, The letter G is for Ground Fault Protection.
Not all ground fault is at the range of 1000A to 3000 amps. Based on NEC MAXIMUM SETTING OF GROUND FAULT PROTECTION IS UP TO 1200amps only. This means that ground fault may be very low such that OCPD will not trip on ARCING GROUND FAULT at lower level.
Assuming that the OCPD rating is 600 amps, it means per NEC it is not mandatory to have a ground fault protection at this level. It means that if the fault is an ARCING GROUND FAULT, fault may be very low that it will not be enough to operate the OCPD even with the best EGC. Only protection against hazard of electric shock is to have the energized metal piece, due to ARCING GROUND fault, conneted to earth or grounded.
coulter said:
You miss the point of driving extra grounding rod for reducing the step potential due to a very large ground fault.
First discussion is to show the importance of connecting to earth EGC to lower the potential of the energized metal piece to ground or earth potential.
The addition of the extra grounding rod is to reduce the step potential based on the illustration of the post on a high ground fault.
winnie
Senior Member
- Location
- Springfield, MA, USA
- Occupation
- Electric motor research
Mr. Ocampo,
You present a very confusing picture. On the one hand you show significant understanding of the technology. On the other hand, you keep making statements that appear to be physically incorrect, or which conflate very different situations.
Most of us involved in this discussion are considering conventional building services, including large 480/277V systems, possibly ungrounded or high resistance grounded, probably solidly grounded. If you are making your assessments from the point of view of utility distribution systems, then your very different perspective on very different systems may be part of our mutual misunderstanding. Please clarify the basis from which you are discussing things.
Carl asked you to state your assumptions and reasoning. I've asked you to show your calculations. Your continued pronouncements without stating their basis makes them neither correct nor incorrect, but simply meaningless.
Consider an isolated HRG system, consisting of a generator, building wiring system, equipment grounding conductors, grounding resistance, etc. The grounding resistance connects the generator neutral to the EGC system. If there were absolutely no earth electrode at all, then there is the possibility that the EGC system will be at elevated voltage relative to the earth. Clearly this risk of elevated voltage shows the importance of having some sort of earth electrode. But here is the kicker: without an earth electrode, this elevated voltage may be present without any sort of fault at all, and a low current fault will not in any way change the voltage between this hypothetical un-earthed EGC and the soil. In such a system, a single earth electrode is all that is needed, and there will never be sufficient current injected into that earth electrode to cause a step potential hazard.
Now consider a different HRG system, where the source neutral is connected to an earth electrode via the grounding resistance, but this is _not_ connected to the EGC system. In such a system, a fault to the EGC could place full line-neutral voltage on the EGC relative to soil. Question for you: in such a system, what could be done to reduce the potential on this EGC system?
I repeat to you: show your assumptions and describe the systems in which you are envisioning your examples. In all of the systems that I am considering, proper bonding reduces exposed dangerous voltages to extremely low levels or extremely short durations. In these systems, grounding electrodes beyond the initial required one do very little.
-Jon
You present a very confusing picture. On the one hand you show significant understanding of the technology. On the other hand, you keep making statements that appear to be physically incorrect, or which conflate very different situations.
Most of us involved in this discussion are considering conventional building services, including large 480/277V systems, possibly ungrounded or high resistance grounded, probably solidly grounded. If you are making your assessments from the point of view of utility distribution systems, then your very different perspective on very different systems may be part of our mutual misunderstanding. Please clarify the basis from which you are discussing things.
Carl asked you to state your assumptions and reasoning. I've asked you to show your calculations. Your continued pronouncements without stating their basis makes them neither correct nor incorrect, but simply meaningless.
Consider an isolated HRG system, consisting of a generator, building wiring system, equipment grounding conductors, grounding resistance, etc. The grounding resistance connects the generator neutral to the EGC system. If there were absolutely no earth electrode at all, then there is the possibility that the EGC system will be at elevated voltage relative to the earth. Clearly this risk of elevated voltage shows the importance of having some sort of earth electrode. But here is the kicker: without an earth electrode, this elevated voltage may be present without any sort of fault at all, and a low current fault will not in any way change the voltage between this hypothetical un-earthed EGC and the soil. In such a system, a single earth electrode is all that is needed, and there will never be sufficient current injected into that earth electrode to cause a step potential hazard.
Now consider a different HRG system, where the source neutral is connected to an earth electrode via the grounding resistance, but this is _not_ connected to the EGC system. In such a system, a fault to the EGC could place full line-neutral voltage on the EGC relative to soil. Question for you: in such a system, what could be done to reduce the potential on this EGC system?
I repeat to you: show your assumptions and describe the systems in which you are envisioning your examples. In all of the systems that I am considering, proper bonding reduces exposed dangerous voltages to extremely low levels or extremely short durations. In these systems, grounding electrodes beyond the initial required one do very little.
-Jon
Yes, that's true. It's not supposed to. That's the point of having an HRG system. It appears I correctly assumed you knew that. So, what is your point?bobby ocampo said:
This is an excellect example of why some of your discussions are confusing or, as jon said, meaningless.bobby ocampo said:
My comment was in response to your equating HRG faults with arcing ground faults (in grounded systems - I think. Could have been ungrounded systems - I couldn't tell, you never said) Your response is to discuss Step and touch potential. ???
You lost your focus. I know you have been asked this already - Please stay on point. If you want reasoned responses, you're going to have to do this.
This is an excellent example of why the NEC says it is not a design guide. One does not have to design to NEC minimums. It's okay to design/build a safe, reliable system.bobby ocampo said:
carl
bobby ocampo
Senior Member
winnie said:
Dear Sir Jon,
Please quote what seems to be incorrect from what I have mentioned point by point based your comments.
winnie said:
My comment is based on the statement of other members of this thread that connection to earth is less important. My position is it is as important as the EGC. EGC alone or connection to earth alone is hazardous. Both are important.
winnie said:
I have shown my assumptions. It will be easier to answer if you will quote my posting sir and give your comment based on what I posted so that other members of the thread can follow.
winnie said:
Based on what you said this is the point that will show the importance of connection to earth of the EGC. It is hazardous if EGC is not connected or bonded to earth.
winnie said:
Non current carrying metal piece will not be energized unless there is a fault. The metal piece will have not potential to ground if there is no line to ground fault. Please explain how the metal piece will be energized without a fault.
winnie said:
Even at low current if this current carrying conductor touches the metal piece it will increase its potential to earth equal to line-to-neutral voltage of the system. This is a safe experiment you can measure the voltage to ground if the metal piece is not connected to earth.
winnie said:
This is the comparison I am making for an arcing ground fault with a fault in HRG. Arcing ground fault like single line to ground fault in HRG has low fault current that the EGC despite its return path to the source will not operate the OCPD. Why? Because the fault is also low in an Arcing Ground fault. In line with this the metal piece will already be energized and if the EGC is not connected to earth will have a potential equal to line-to-neutral on a single line-to-ground arcing ground fault.
winnie said:
Dear Sir Jon,
My position is both the EGC and the connection to earth are important. Never in the thread did I say that only one is important. Connection to earth is as important as the EGC for protection against hazard of electric shock. It is very hazardous if somebody in this thread will say that connection to earth is less important.
Dear Sir Jon,winnie said:
Once again the minimum requirement is for the EGC to be connected to earth.
The solution of installing additional grounding electrode is to solve the problem in the illustration of the post showing that there will be step potential from the post despite of the additional grounding electrode installed. At a very high fault current based on the 1000KVA transformer at 5% impedance, the prospective fault current based on short circuit analysis on a single line to ground fault is 87% of the bolted 3phase fault. In this example, single line to ground fault will be 20,000 amps or 20kA. At the instant of the fault there will be a step potential which is hazardous if earth resistance is high. One of the solution to lower step potential is to install additional grounding electrode similar to what is done in a substation.
bobby ocampo
Senior Member
coulter said:
Dear Sir Carl,
It may be better to quote everything that I posted and comment on it. For HRG iwht a single line-to-ground fault touching a metal piece, It will energized the metal piece if the metal piece is not connected to earth. If a person will touch this metal piece (not conneted to earth) there is a big hazard of electric shock because the potential of the metal piece to earth is equal to line-to-neutral voltage or in a 480Y/277 volts HRG system the potential to ground of the energized metal piece is 277 volts.
coulter said:
I have tried to give comments point by point for every comment that you gave so that there will be no confusion. Maybe you should do the same sir.
coulter said:
HRG as a matter of fact considered a grounded system similar to a solidly grounded system except that an impedance is added to reduce the fault current on a single line to ground fault.
The first comparison is in the fault current. Single line-to-ground fault in HRG and Arcing Ground fault are LOW. In a solidly grounded system this means that if Arcing Ground Fault is very low, even with the best EGC the OCPD will not operate. With the same very low fault current in an HRG due to the resistor installed in the neutral to ground, OCPD in this case is allowed not to operate in a single-line-to-ground fault.
However like in HRG, if the metal piece is not conneted to earth the metal piece on a single-line-to-ground fault will be energized and will have a potential to earth of 277 volts. Therefore in an arcing ground fault which has a very low fault current similar to HRG, it will also energized the metal piece and again similarly if the metal piece is not conneted to earth then there is a hazard of electric shock if the metal piece is not conneted to earth.
coulter said:
Time and again I try hard to answer the post point by point based on your comments.
coulter said:
The ground fault protection setting in NEC is an example that shows that fault current may be very low as compared with the OCPD rating. This also shows that if the OCPD has no ground fault protection, OCPD may not operate if the level of ground fault is lower than the OCPD rating.
Using the assumption that ground fault current may be lower than the OCPD RATING (without ground fault protection), OCPD inspite of the EGC will not operate. And while there is a ground fault the metal piece is energized to a level equal to 277 volts if the metal piece is not conneted to earth. Therefore, to reduce the potential to earth of the energized metal piece, CONNECTION TO EARTH IS IMPORTANT.
weressl
Esteemed Member
coulter said:
OK. so you don't know physics.
Round and rotating earth was also nonsense to many at one point in time.
I2t.
weressl
Esteemed Member
coulter said:
An arcing ground or phase fault on a SGR system may NOT trip the circuit breaker after repeated restrike and considerable damage UNLESS. Look at the DURATION of the arc and the MAGNITUDE.
Arcing ground faults seldom develop on RGS or HRGS since the potential difference is limited as due to the resistance introduced via the star point. A bolted fault will not trip because of the current limitation.
Phase-to-phase faults behave identically in all grounding schemes.
Aside: Since 80% of phase faults originate with grounding faults, RGS and HRGS are sigbificantly reduce the potential of such faults as long as the single ground faults are identified and removed.
winnie
Senior Member
- Location
- Springfield, MA, USA
- Occupation
- Electric motor research
bobby ocampo said:
If you have a piece of metal that is entirely isolated from the earth, then it may hold a charge relative to the earth and thus be at an elevated potential relative to the earth. No current flow is required; simply a charge on the metal.
A single connection to an earth electrode will eliminate this 'static' potential.
bobby ocampo said:
If a current limited fault to bonded metal occurs, then the potential of the metal will be raised, but only slightly. The voltage of the metal will not become the 'line to neutral' voltage, but will instead be raised by the voltage drop in the EGC system. Most of the line-neutral voltage will be dissipated in whatever is acting to limit the fault current. You have a series circuit, which includes the source (transformer or generator), the circuit conductors, the EGC system, and whatever limits the fault current. It doesn't matter if the current flow is limited by a grounding impedance or by something else, the net result is that the voltage of the bonded metal will be raised by the fault current * EGC resistance.
bobby ocampo said:
This is not correct. The limited current flowing in the EGC system will cause one portion of the system to be at slightly elevated potential relative to other portions, in proportion to the current flow and the resistance. If any single portion of the EGC system is connected to an earth electrode, all other portions of the EGC system will be at low potential relative to the earth.
bobby ocampo said:
That understanding of the lamp post illustration is misguided. It is quite true that an extensive grounding electrode system would eliminate the step potential hazard around the energized lamp post. However simply adding the code required bonding EGC would be more effecting and less expensive for eliminating this hazard. With the required EGC, then no additional grounding electrode is needed.
bobby ocampo said:
It is quite true that a bolted fault in a solidly grounded system can cause so much current flow that the potential of bonded metal will be raised significantly. However such a fault will quickly open OCPD. Additional grounding electrodes will not significantly reduce the potential of the bonded metal during such a fault condition, however sufficient grounding electrodes might serve to form an equipotential system and eliminate contact potentials. In this latter case, I contend that the safety benefit is the result of the metallic bonding of the equipotential grid, and not a result of the contact with soil.
-Jon
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