Ground rod resistance

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Dereck,
Zog a ground fault forms a simple voltage divider circuit.
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It is not a voltage divider unless you are in series with the fault path. In the case of a ground fault to the equipment, you are not in series, you are in parallel and the resistance to earth does not change anything.
Don
 
don_resqcapt19 said:
Dereck,

It is not a voltage divider unless you are in series with the fault path. In the case of a ground fault to the equipment, you are not in series, you are in parallel and the resistance to earth does not change anything.
Don
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Don not sure I can agree with that. If you are in series like coming into contact with a live conductors, a divider is formed. With that said that scenerio would leave the body exposed to almost full system voltage because of the high impedance of the body.

However if it is to say the case of the equipment comes into contact, a divider is still formed, and if you hapen to be in parallel it is still a divider circuit, just now the body resistance enters into the equation... An EGC will rougly divide the circuit voltage in half.

Sorry, maybe I don't know what you are driving at. I do agree earth impedance is of no real importance with NEC applications
 
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zog said:
Lets say you have a 4160V (About 2400V to ground) system with a 50 ohm load, and a wire makes contact with the metal casing of the load, if you touch the metal frame (or any other part of the buildings metal) and are standing on ground there would be about a 1200V drop across your body if there ground to earth resistance was 50 Ohms.
Click to expand...

Would this diagram be representative of what we are discussing? If so, we can develop the combination circuit with resitors to determine the voltage drops and currents through each part including the person.

4160shock.jpg
 
Dereck,
What I am saying that if you have a fault to the equipment of one leg of the 4.16kV wye system, you have 2400 volts to ground. The connection of a grounding electrode to the faulted equipment does not change the voltage to ground unless it, 1) flows enough current to open the OCPD, 2) it flows enough current to create a voltage drop on the supply system or 3) you are standing on or very very close to the grounding electrode.
Don
 
I believe this says it all..

neutral earthing
Posted by mukesh on November 29, 2006
Sir,

Why we ground transformer (star side) and Generator?s.
IEEE Std 142-1991 (Green Book) section 1.4.2 states "Numerous advantages are attributed to grounded systems, including greater safety, freedom from excessive system over-voltages that can occur on ungrounded systems during arcing, resonant or near-resonant ground faults, and easier detection and location of ground faults when they do occur." OK, now that we have established why you need to ground the neutral, let's discuss how to ground the neutral. If you effectively ground the neutral, you have just replaced the hazards with ungrounded systems with new hazards in the form of Arc Flash / Blast Hazards. IEEE Std 141-1993 (Red Book)section 7.2.4 states "A safety hazard exists for solidly grounded systems from the severe flash, arc burning, and blast hazard from any phase-to-ground fault." For this reason, IEEE recommends resistance grounding. IEEE Std 142-1991 (Green Book) section 1.4.3 states "The reasons for limiting the current by resistance grounding may be one or more of the following: 1) To reduce burning and melting effects in faulted electric equipment, such as switchgear, transformers, cables, and rotating machines. 2) To reduce mechanical stresses in circuits and appartus carrying fault currents. 3) To reduce electric-shock hazards to personnel caused by stray ground-fault currents in the ground return path. 4) To reduce the arc blast or flash hazard to personnel who may have accidentally caused or who happen to be in close proximity to the ground fault. 5) To reduce the momentary line-voltage dip occasioned by the clearing of a ground fault. 6) To secure control of transient over-voltages while at the same time avoiding the shutdown of a faulty circuit on the occurrence of the first ground fault (high resistance grounding). IEEE Std 141-1993 (Red Book) section 7.2.2 states "There is no arc flash hazard, as there is with solidly grounded systems, since the fault current is limited to approximately 5A." As you can see, it is best to not only ground the neutral, but ground trough High-Resistance (typically 5A) for all systems < 600V, most systems > 600V upto 5kV, and Low-Resistance (typically 200A or 400A) for some systems > 600V upto 5kV, and all systems > 5kV upto 15kV.IEEE Std 142-1991 (Green Book) section 1.4.2 states "Numerous advantages are attributed to grounded systems, including greater safety, freedom from excessive system over-voltages that can occur on ungrounded systems during arcing, resonant or near-resonant ground faults, and easier detection and location of ground faults when they do occur." OK, now that we have established why you need to ground the neutral, let's discuss how to ground the neutral. If you effectively ground the neutral, you have just replaced the hazards with ungrounded systems with new hazards in the form of Arc Flash / Blast Hazards. IEEE Std 141-1993 (Red Book)section 7.2.4 states "A safety hazard exists for solidly grounded systems from the severe flash, arc burning, and blast hazard from any phase-to-ground fault." For this reason, IEEE recommends resistance grounding. IEEE Std 142-1991 (Green Book) section 1.4.3 states "The reasons for limiting the current by resistance grounding may be one or more of the following: 1) To reduce burning and melting effects in faulted electric equipment, such as switchgear, transformers, cables, and rotating machines. 2) To reduce mechanical stresses in circuits and appartus carrying fault currents. 3) To reduce electric-shock hazards to personnel caused by stray ground-fault currents in the ground return path. 4) To reduce the arc blast or flash hazard to personnel who may have accidentally caused or who happen to be in close proximity to the ground fault. 5) To reduce the momentary line-voltage dip occasioned by the clearing of a ground fault. 6) To secure control of transient over-voltages while at the same time avoiding the shutdown of a faulty circuit on the occurrence of the first ground fault (high resistance grounding). IEEE Std 141-1993 (Red Book) section 7.2.2 states "There is no arc flash hazard, as there is with solidly grounded systems, since the fault current is limited to approximately 5A." As you can see, it is best to not only ground the neutral, but ground trough High-Resistance (typically 5A) for all systems < 600V, most systems > 600V upto 5kV, and Low-Resistance (typically 200A or 400A) for some systems > 600V upto 5kV, and all systems > 5kV upto 15kV.
 
crossman said:
Would this diagram be representative of what we are discussing? If so, we can develop the combination circuit with resitors to determine the voltage drops and currents through each part including the person.
Click to expand...
Yes we can say it is an accurate representation of a Case/chassis fault. I would like to point out a couple of things like.

System voltage range can be any voltage like from 120 to 345K or what ever number you want to plug into it. With the diagram you show the case voltage is roughly half the system voltage.

Body resistance is another huge variable. It could be much lower than you show like 300-ohms.

Cable impedance are realistic and good for the discussion.
 
don_resqcapt19 said:
Dereck,
What I am saying that if you have a fault to the equipment of one leg of the 4.16kV wye system, you have 2400 volts to ground. The connection of a grounding electrode to the faulted equipment does not change the voltage to ground unless it, 1) flows enough current to open the OCPD, 2) it flows enough current to create a voltage drop on the supply system or 3) you are standing on or very very close to the grounding electrode.
Don
Click to expand...
I agree with that.
 
cschmid said:
I believe this says it all..

neutral earthing
Posted by mukesh on November 29, 2006
Sir,

Why we ground transformer (star side) and Generator?s.
IEEE Std 142-1991 (Green Book) section 1.4.2 states "Numerous advantages are attributed to grounded systems, including greater safety, freedom from excessive system over-voltages that can occur on ungrounded systems during arcing, resonant or near-resonant ground faults, and easier detection and location of ground faults when they do occur." OK, now that we have established why you need to ground the neutral, let's discuss how to ground the neutral. If you effectively ground the neutral, you have just replaced the hazards with ungrounded systems with new hazards in the form of Arc Flash / Blast Hazards. IEEE Std 141-1993 (Red Book)section 7.2.4 states "A safety hazard exists for solidly grounded systems from the severe flash, arc burning, and blast hazard from any phase-to-ground fault." For this reason, IEEE recommends resistance grounding. IEEE Std 142-1991 (Green Book) section 1.4.3 states "The reasons for limiting the current by resistance grounding may be one or more of the following: 1) To reduce burning and melting effects in faulted electric equipment, such as switchgear, transformers, cables, and rotating machines. 2) To reduce mechanical stresses in circuits and appartus carrying fault currents. 3) To reduce electric-shock hazards to personnel caused by stray ground-fault currents in the ground return path. 4) To reduce the arc blast or flash hazard to personnel who may have accidentally caused or who happen to be in close proximity to the ground fault. 5) To reduce the momentary line-voltage dip occasioned by the clearing of a ground fault. 6) To secure control of transient over-voltages while at the same time avoiding the shutdown of a faulty circuit on the occurrence of the first ground fault (high resistance grounding). IEEE Std 141-1993 (Red Book) section 7.2.2 states "There is no arc flash hazard, as there is with solidly grounded systems, since the fault current is limited to approximately 5A." As you can see, it is best to not only ground the neutral, but ground trough High-Resistance (typically 5A) for all systems < 600V, most systems > 600V upto 5kV, and Low-Resistance (typically 200A or 400A) for some systems > 600V upto 5kV, and all systems > 5kV upto 15kV.IEEE Std 142-1991 (Green Book) section 1.4.2 states "Numerous advantages are attributed to grounded systems, including greater safety, freedom from excessive system over-voltages that can occur on ungrounded systems during arcing, resonant or near-resonant ground faults, and easier detection and location of ground faults when they do occur." OK, now that we have established why you need to ground the neutral, let's discuss how to ground the neutral. If you effectively ground the neutral, you have just replaced the hazards with ungrounded systems with new hazards in the form of Arc Flash / Blast Hazards. IEEE Std 141-1993 (Red Book)section 7.2.4 states "A safety hazard exists for solidly grounded systems from the severe flash, arc burning, and blast hazard from any phase-to-ground fault." For this reason, IEEE recommends resistance grounding. IEEE Std 142-1991 (Green Book) section 1.4.3 states "The reasons for limiting the current by resistance grounding may be one or more of the following: 1) To reduce burning and melting effects in faulted electric equipment, such as switchgear, transformers, cables, and rotating machines. 2) To reduce mechanical stresses in circuits and appartus carrying fault currents. 3) To reduce electric-shock hazards to personnel caused by stray ground-fault currents in the ground return path. 4) To reduce the arc blast or flash hazard to personnel who may have accidentally caused or who happen to be in close proximity to the ground fault. 5) To reduce the momentary line-voltage dip occasioned by the clearing of a ground fault. 6) To secure control of transient over-voltages while at the same time avoiding the shutdown of a faulty circuit on the occurrence of the first ground fault (high resistance grounding). IEEE Std 141-1993 (Red Book) section 7.2.2 states "There is no arc flash hazard, as there is with solidly grounded systems, since the fault current is limited to approximately 5A." As you can see, it is best to not only ground the neutral, but ground trough High-Resistance (typically 5A) for all systems < 600V, most systems > 600V upto 5kV, and Low-Resistance (typically 200A or 400A) for some systems > 600V upto 5kV, and all systems > 5kV upto 15kV.
Click to expand...


thats not what the discussion is about (Unless I missed something in there) , we are discussing ground rod resistance
 
I believe when I posted my response we are talking why they are rquiring 25 ohms to ground or less..I believe the article I posted from EC&M explains the reason for the grounding..Now how we achieve the grounding is another story..I am curious about grounding as well and am always looking to learn something..I am curious what resources you use in your EE areas..I believe you, dereck and Don are talking about how to achieve the 25ohms and the affects if you dont..
 
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Yes we were, but we are talking about ground to earth values, not system to ground like your post discussed.

BTW, good post about grounded vs ungrounded vs resistance grounded systems. I like when people quote IEEE documents instead of saying "I was always told...."
 
Man there I go apples and oranges..I used to do ground testing for the portable mining equipment and we would use plate electrodes..they always have pay loaders available to move stuff so it was not an issue and in MN we also have thoughs standards..but our soil type around here is different..I also have more knowledge and experience since then and can see some of the issues around plate electrodes but they are easy to use..but all the portable generators that were used here while i was doing it were 480 volt..
 
zog said:
Lets say you have a 4160V (About 2400V to ground) system with a 50 ohm load, and a wire makes contact with the metal casing of the load, if you touch the metal frame (or any other part of the buildings metal) and are standing on ground there would be about a 1200V drop across your body if there ground to earth resistance was 50 Ohms.

Same senraio, with a 1 ohm resistamce to ground and the voltage drop across your body would be about 47V drop across your body.

So, you are saying there is no difference between a 1200V shock and a 47V shock?
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If the metal frame was bonded to a ground electrode and you were standing on something that was bonded to the same ground electrode, the voltage between your hands and feet would be the fault current times the resistance of the bonding conductor. It wouldn't matter what the resistance of the ground rod was.

If you are standing on earth instead of something bonded to the ground electrode, then the matter is more complicated. It depends on how much of the current returns through the EGC and how much through the earth. It also depends on how the earth current flows and the resistivity of the soil.
 
Well I assumed we were based on the application of the OP, but he never got back to us on that. The example of lower values providing for safety only applies to ungrounded systems as I stated several times in this thread, I also stated if this is grounded then it dosent matter.
 
crossman said:
Would this diagram be representative of what we are discussing? If so, we can develop the combination circuit with resitors to determine the voltage drops and currents through each part including the person.
Click to expand...
It could be representative if the enclosure was insulated from the earth where the poor guy is standing. Otherwise, there will be another parallel path from the enclosure through the earth without going through the guy. But let's look at your case.

First, use 1000 ohm for body resistance per IEEE Std 80, IEEE Guide for Safety in AC Substation Grounding. Use 150 ohms for the parallel combination of two feet (IEEE Std 80 for rho=100 ohm-m).

Resistance to neutral through guy and earth: RE=1000+150+50=1200ohm
Resistance to neutral through EGC: RG=0.5 ohm
Parallel resistance: RP=0.5?1200/(0.5+1200)=0.49979
Total fault resistance: RT=0.5+0.49979=0.99979 close enough to 1
Total fault current: I=2400/1=2400A
Voltage of enclosure: VE=2400-2400?0.5=1200V
Current through unfortunate guy and earth=1200/1200=1A
Current needed for fibrillation (seriously messing with the unfortunate guys heart rythm, leading most probably to his imminent demise): IB=0.116/sqrt(t) where t is the fault clearing time (IEEE Std 80)
For a 6 cycle fault (0.1 sec), IB=0.367A Guy in serious trouble with 1A

It would help if the guy wore insulated sole boots. It would help if there were a ground loop around the equipment, bonded to the equipment, so that the guy's feet were closer to the voltage of the enclosure. If the guy were standing on a steel floor and the equipment was bonded to the floor, then the voltage across him would be limited to the small resistance of the enclosure and bonding conductor times the part of the current going through it.
 
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