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Thread: efficacy of system grounding (earthing)

  1. #1
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    efficacy of system grounding (earthing)

    I am hoping to clarify and reconcile various statements about system grounding. It is clear that most of us believe that ground rods do not accompish much, if anything. Is this true for system grounding in general, or just ground rods? There have been statements by some of the grounding 'heavy hitters' on the forum that a CEE is much better than ground rods. So does this mean that as compared to a ground rod(s), a CEE is more likely to "limit voltages due to lightning, line surges, and unintentional contact with higher voltage lines"? All three? just lightning induced surges? Just trying to understand when system grounding accomplishes somthing, and what that something is most likely to be. The primary motivation for this resulted from statements made in the book "photovoltaics", by solar energy international where they make the classic mistake and state,

    "A ground fauld occurs when a current carrying conductor comes into contact with the frame of chassis of an appliance...The frame or chassis of an appliance is deliberately wired to a grounding electrode...the grounding wire must be continuous, connecting every non-current carrying metal part of the installation to ground."

    Although this is technically correct, it strongly implies that during a fault, the electricity will 'go to ground'. Then they have a diagram credited to home power magazine which again is technically correct but strongly implies that the power will 'go to ground'. Anyway, I feel an article to home power magazine clarifying system grounding vs equipment grounding is in order and to help get the point across, I would like to make a statement such as, "system grounding plays a very minor role in the operation of an electrical system and typically does not provide any benefit" or "system grounding with rods alone typically wil not provide any benefit" or "system grounding can provide some protection from lightning induced surges, but is unlikely to provide much benefit beyond that" You get the idea, just wondering what others think would be the best way to put it. At the very least I would state that system grounding offers no protection from when a live wire comes in contact with a chassis or frame. I feel the article would/will be very beneficial to the renewable energy community. I plan to also discuss equipment grounding, since this is not as intuitive and works a bit differently than a utility supplied system because PV is current limited. Thanks for any advice.

  2. #2
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    That is a good point.I know that we're required to install ground rods, wiring,and the such believing that it provides a safer path for stray voltage or shorted ungrounded conductors to earth but I've also questioned it's effectiveness due to the fact that it naturally follows the path of least resistance to a grounded point of which is often closer to the incident's point of origin ie the person holding or touching the metal object in question.

  3. #3
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    It is my understanding that the ground rod, CEE will work to some degree to protect the premise from lightning. In most cases you wil not achieve 25 ohms to ground with a ground rod. Even if you had 10 ohms to ground you still would not clear a fault from a ground fault. The sytem ground will clear the ground fault.

    If you take a 20 amp circuit and hook it diectly to a ground rod, do not do this, that is just driven in the ground, then it will not trip the breaker.

    V=IR or I=V/R

    120/10= 12 amps.

  4. #4
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    WOW!!! Understandably, describing what an electrode function does in any unstabilized Hi-voltage event and its difference from Fault safety grounding is a big order to a PV audience that deals in DC predominately. Maybe I got that in reverse importance.
    Anyway, IMO the key is to describe grounding two ways as, 1) an Electrical load and equipment Ground-Fault system that is connected to 2) an Earth Embedded Ground-electrode System through the common single point ground of the Service panel disconnect main jumper bonded terminal bars. (Emphasizing that the embedded ground-electrode system does not provide an electrical circuit ground-fault function.)
    The Equipment ground-fault system path through the equipment grounding does provide a low impedance path back to the AC System source for operation of over-current protection breakers or fusing.

    As far as the electrode system quality between a UFER and Supplemental Electrodes, both can only function to dissipate unstablized high voltages of unpredictable transients through parallel paths intended to earth. The NEC [250.52] stipulates that the UFER and metal water pipe common electrodes are to be used as available. If the UFER is not available, then the supplemental rods are required. Any other 'tapped' electrodes present are also mandated to be bonded to either common electrodes. Now, which one is better? The UFER wins IMO due to less cost and time to install.

    In determining vertical soil driven electrodes versus a horizontal soil electrode effectiveness, according to data from Sandia Lab Lightning tests, the National Lightning Safety Institute has verified that approximately 85% of a stroke dissipates over the earth surface compared to a 15% depth penetration. Google up National Safety Institute or Sandia Labs testing data. Its been a while. The url didn't connect.

    Edit: url changed
    Last edited by gndrod; 01-12-08 at 07:41 AM.
    rbj, Seattle...Safety is a Professional Courtesy.

  5. #5
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    The issue I have with Ufers is now electrical inspectors, engineers and electricians think this is the holy grail of grounding. As in durn we drove 20 rods wished we had used a Ufer.

    Neither would have solved the issues at hand...

    Fact is for most power quality issues the connections to ground/earth are not the issue. The problem is with the ground/bonding with in the facility.

    http://www.psihq.com/iread/ufergrnd.htm

    My question has been, has anyone seen or know of damaged concrete from lighting strikes.
    Brian John
    Leesburg, VA

  6. #6
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    Quote Originally Posted by brian john


    My question has been, has anyone seen or know of damaged concrete from lighting strikes.

    The average electrician usually has very little to say about this topic other than "rumblings" of what they have heard.

    There are engineers who have "tested" this type of scenario you are describing who say that their findings are proof of the fact that CEEs are not safe for foundations and footings.
    I have read some of those papers. Their argument seems very convincing. Although I am not convinced based on my personal experiences. I think that in any laboratory experiments that results could be pushed to the limit.

    I have not seen many CEEs and results from lightning hits, so I cannot personally answer your question with any authority.
    What I will say (remember this is MY Opinion), in the area where I work and play, we do get some horrific lightning storms. But they are for a very short period of time in the summer. We also get "normal" thunder and lightning storms for other portions of the year. In all my years (I also ask this question of my students in almost all of my classes) I and they, have rarely seen a service hit by lightning. Buildings, yes; trees, yes; the grounds around buildings, yes.
    Have I seen light poles in parking lots that are attached (electrically) to buildings and have no ground rods driven hit by ligthning. Yes, many times. In those instances I do not recall seeing the concrete pillars they are supported by (and electrically connected to) damaged, cracked or blown up.

    So much for my opinion.
    Instructor, Industry Advocate

  7. #7
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    Quote Originally Posted by brian john
    The issue I have with Ufers is now electrical inspectors, engineers and electricians think this is the holy grail of grounding. As in durn we drove 20 rods wished we had used a Ufer.

    Neither would have solved the issues at hand...

    Fact is for most power quality issues the connections to ground/earth are not the issue. The problem is with the ground/bonding with in the facility.

    http://www.psihq.com/iread/ufergrnd.htm

    My question has been, has anyone seen or know of damaged concrete from lighting strikes.
    brian john ,
    Thanks for posting the link. That's a very interesting write up , I'd personally always thought concrete was merely used because of it's strength, I wasn't aware of, or even considered, the relationship for it's use in lightning strike protection through the grounding aspects.

    Carl :smile:

  8. #8
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    Electrofelon
    Sorry about my ranting prior, without really answering your question.
    My opinion is that a properly installed CEE will provide a better portion of the grounding system than a properly installed set of groundrods (usually 2-8 ft groundrods).

    There is generally less earth contact resistance, better earth contact (due to more of the surface area of the footing in contact with the earth than rods), less resistance in the electrode, etc...
    Instructor, Industry Advocate

  9. #9
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    My thoughts on the matter would be if you house is unfortunate enough to be struck in such a manner where your electrode system takes any or all of the energy of the stroke, more is better, Ufer, multiple rods, water piping, kid with a kite and key.

    I have been involved in several post lighting damage inspections, where the sites had sustained severe damage from lightning, this one site (with a arge tower as part of their operation upgraded EVERYTHING to a very substantial grounding, lightning protection, and it worked for several years and numerous thunderstorms. Till that stroke or strokes mother nature threw at them one night, FRIED everything in the site, site next door, 2 houses in the neighborhood, all of 7-11's equipment, Point of sale computers and at gas station.


    You might push her around for a while maybe trick her a bit, but when she is ready to throw that BIG BOLT Katie bar the door.
    Brian John
    Leesburg, VA

  10. #10
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    I found another one in that photovoltaics book this morning, "check to see that equipment grounding conductors and system grounding conductors have as short a distance as possible to ground"

    Yes the NEC does require the bonding of non-current carrying mettalic objects to earth (dont have my code book handy), but of course this is not for ground faults as the statement implies.

    I dont feel the need to go deep into system grounding theory - nor do I have the knowledge to, but I would just like to downplay its importance.

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