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Thread: Ground Potential Rise

  1. #11
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    Quote Originally Posted by Sahib View Post
    1)Grounding dissipates surge energy lest it should do harm.
    2)In case of Mv to LV accidental contact, grounding on LV side limits ground potential rise.
    Thats a good point- but the ground is only to prevent the voltage of the bonding system as a whole from being overly excessive relative to earth. Ie, reducing the possibility of side flashes and arcing of the bonding system to earth. Even with a good ground it can still rise to thousands of volts when measured to earth just 6 feet away from the ground rod, and as such its the equal potential effect that keeps everything safe.
    I'm in over my head...

  2. #12
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    Quote Originally Posted by Sahib View Post
    1)Grounding dissipates surge energy lest it should do harm.
    If that were true why would anybody spend money on surge protectors?

    2)In case of Mv to LV accidental contact, grounding on LV side limits ground potential rise.
    How does it do that? I know that grounding stabilizes voltages to ground in a normally operating system, but when the wires cross things still go boom.
    If you go and decide to dance with a gorilla the dance ain't over till the gorilla decides it's over.

  3. #13
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    Quote Originally Posted by ActionDave View Post
    If that were true why would anybody spend money on surge protectors?

    Answer : Without ground connection, a surge arrestor does not work.


    How does it do that? I know that grounding stabilizes voltages to ground in a normally operating system, but when the wires cross things still go boom.
    Answer: Suppose ground resistance is zero. Consider 120V, Singe phase line with neutral grounded. Since ground resistance is zero, no matter how large a current flows through neutral to ground, the ground potential does not rise. Now Suppose a MV line contacts the 120V single phase line. A large short circuit current flows via phase, neutral and ground. But since ground resistance is zero, there is no rise in ground potential and the single phase line remains at 120V. In real situation also, there will be only a little rise above 120V if the ground resistance is close to zero.
    Last edited by Sahib; 05-23-18 at 01:38 PM.

  4. #14
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    Quote Originally Posted by Sahib View Post
    Answer: Suppose ground resistance is zero. Consider 120V, Singe phase line with neutral grounded. Since ground resistance is zero, no matter how large a current flows through neutral to ground, the ground potential does not rise. Now Suppose a MV line contacts the 120V single phase line. A large short circuit current flows via phase, neutral and ground. But since ground resistance is zero, there is no rise in ground potential and the single phase line remains at 120V. In real situation also, there will be only a little rise above 120V if the ground resistance is close to zero.
    Ground is ground AKA the earth. Grounding electrodes always have some resistance, so will the GEC. 120 volts applied to GES with 10 ohm resistance carries 12 amps.

    7200 volts applied to 10 ohm GES carries 720 amps.

    Voltage "zones" around the electrode are much larger with 120 volts applied then with 7200 volts applied.

    Few years ago a local rural POCO had an underground 34.5 kVA that had a failure. They repaired it, for whatever reason their splice method failed when they energized it. It sent a surge down a nearby wire fence, arced across to telephone pedestal(s) near fence. There was damages in many buildings within a couple miles or so - all coming into facilities via telephone lines. Ground rod wasn't going to stop that, AFAIK most the telephone pedestals do ordinarily have a ground rod connected to them. I believe there was a couple grass fires associated with that incident as well.
    I live for today, I'm just a day behind.

  5. #15
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    Quote Originally Posted by ActionDave View Post
    If that were true why would anybody spend money on surge protectors?

    How does it do that? I know that grounding stabilizes voltages to ground in a normally operating system, but when the wires cross things still go boom.
    There's a big difference between line-to-line surges and line-to-earth surges.
    Code references based on 2005 NEC
    Larry B. Fine
    Master Electrician
    Electrical Contractor
    Richmond, VA

  6. #16
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    Quote Originally Posted by kwired View Post

    Few years ago a local rural POCO had an underground 34.5 kVA that had a failure. They repaired it, for whatever reason their splice method failed when they energized it. It sent a surge down a nearby wire fence, arced across to telephone pedestal(s) near fence. There was damages in many buildings within a couple miles or so - all coming into facilities via telephone lines. Ground rod wasn't going to stop that, AFAIK most the telephone pedestals do ordinarily have a ground rod connected to them. I believe there was a couple grass fires associated with that incident as well.
    The ground resistance might be too high to prevent operation of upstream fuses to have caused such damages. I do not think any other reason..........

  7. #17
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    kwired: I also bet it happened in rural area; not in densely populated urban area where multigrounded neutral ground resistance is very low.

  8. #18
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    Quote Originally Posted by Sahib View Post
    Answer: Suppose ground resistance is zero. Consider 120V, Singe phase line with neutral grounded. Since ground resistance is zero, no matter how large a current flows through neutral to ground, the ground potential does not rise. Now Suppose a MV line contacts the 120V single phase line. A large short circuit current flows via phase, neutral and ground. But since ground resistance is zero, there is no rise in ground potential and the single phase line remains at 120V. In real situation also, there will be only a little rise above 120V if the ground resistance is close to zero.
    I don't think what you are saying is possible.
    If you go and decide to dance with a gorilla the dance ain't over till the gorilla decides it's over.

  9. #19
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    Quote Originally Posted by mbrooke View Post
    Bonding is more important than grounding. In fact article 250 is mostly around bonding, not grounding. Grounding has little if anything to do with people and property protection at 600 volts and under.
    Exactly the jist of the article's reference to GPR

    >>>>

    So it is perfectly possible for an AC or signal wire to have 5 kV or more between its two ends, for the short time that the lightning current lasts.
    That nanosecond of GPR is far less damaging if the entire residence assumes 1,000,000 volts

    VS. 1,000,000 volts on one end, 500,000 volts on the other, which is FAR more damaging.

    One can even read this theory into past code cycles. Remember when we didn't bond gas lines? The gas guys would have conniptions....then it evolved to 'within 6' , along with a flashover rationale. , these days we either bond the gas lines or fail inspections.


    ~RJ~

  10. #20
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    Quote Originally Posted by Sahib View Post
    The ground resistance might be too high to prevent operation of upstream fuses to have caused such damages. I do not think any other reason..........
    Don't know all details of what happened, would guess overcurrent protection did function maybe just not fast enough.

    Quote Originally Posted by Sahib View Post
    kwired: I also bet it happened in rural area; not in densely populated urban area where multigrounded neutral ground resistance is very low.
    It was in rural area, was also within 1 mile of the substation it was supplied from. After about a mile the line transitions to overhead conductors. This substation is a major hub for the region so to speak. Everyone within 30-50 miles of it is likely being powered through it in one way or another most of the time. The faulted line I mentioned just happened to be just one of the more localized lines leaving the substation.
    I live for today, I'm just a day behind.

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