Bonding

Discussion in 'General Electronics Chat' started by qitara, Jan 5, 2014.

  1. qitara

    Thread Starter Member

    Jan 18, 2013
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    Hi Folks

    If there is a neutral-earth bond at the service panels at our homes why do we need a ground rod if the fault current will flow back to the utlity transfomer trough the neutral and not trough the ground rod ?


    Happy New Year : )
     
    Last edited: Jan 5, 2014
  2. BillB3857

    Senior Member

    Feb 28, 2009
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    What would happen without your ground rod if the neutral back to the transformer were to open? Does that help answer your question?
     
  3. qitara

    Thread Starter Member

    Jan 18, 2013
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    0
    So the rod serves as a backup ?
     
  4. WBahn

    Moderator

    Mar 31, 2012
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    What ground rod are you talking about? The ground rod IS the neutral-earth bond at the service panel. Or are you talking about the ground rod at the utility transformer end?

    What fault current are you talking about? There are lots of different kinds of faults. Which one or ones are you talking about specifically?
     
  5. MaxHeadRoom

    Expert

    Jul 18, 2013
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    See PDF's.
    Max.
     
  6. qitara

    Thread Starter Member

    Jan 18, 2013
    87
    0
    I meant the ground at the house/building not at the utility transformer, and for the fault current i mean an earth fault current
     
  7. #12

    Expert

    Nov 30, 2010
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    6,768
    I regret to inform you that the ground rod at my house doesn't work. My neutral is connected to buried water pipes hundreds of feet long, but when the neutral wire to the power transformer got a bad connection, surge protectors all over my house melted.

    In theory, the ground rod stops this from happening. In practice, don't count on it.
     
  8. inwo

    Well-Known Member

    Nov 7, 2013
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    Ground rod is the last resort if no other ground points are available. It's also required as a backup in most cases because it's easier for the inspector to see.:)

    The ground is needed to limit potential difference between the neutral/ground system and earth.

    It would be likely that the earth, and things connected to earth, like plumbing, concrete, and bare feet, would be at a different potential from neutral, if not electrically connected.

    I once found a situation where a customer was being shocked when touching wash machine.

    Ground to machine checked out fine.
    By turning off breakers one at a time, it was determined that an apartment on the other end of the building had an uninstalled outdoor light fixture feed imbedded in wet stucco.

    The ground system was fine. The whole building and floor was hot.:eek:

    If it wasn't for the ground rod keeping the potential down, it could have been very serious.

    ps. reference above post.
    Ground rod isn't intended to shunt fault current, even in theory.
    It can help keep things at equi-potential with normal small leakage currents.
     
  9. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
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    Don't forget about a lightning strike. If lightning strikes the utility pole behind your house, you want the surge current conducted to earth under the utility entrance to minimize what comes into your house...
     
  10. WBahn

    Moderator

    Mar 31, 2012
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    Then what you are asking can be restated as, "Why do we need a ground rod if there is a ground rod?"

    I think the real question you are trying to ask is, "Why do we need a ground rod?" Period.

    It's a reasonable question to ask. From a superficial perspective, it might seem like it would be safer if we didn't have any ground connection anywhere. If we didn't, then we could handle the hot lines all day long (provided we only handled one at a time). Isn't that much safer than possibly being electrocuted by touching a hot line while standing barefoot on damp concrete?

    But what happens if the transformer that is providing us our 120V/240V power (here in the U.S.) develops a winding-winding fault and we end up with 13,200V coming down our lines? Bad things. So the neutral on both sides of the transformer are grounded and, by doing so, many of the faults that could happen will result in a large current flowing through one or more terminals of the transformer and trip a breaker at that point. But, in doing this, we create the situation in which a person standing on the ground and contacting just a single hot conductor out of the transformer could potentially be electrocuted.

    So let's minimize the chance of this happening as the result of an equipment fault. We'll do this by making it so that if a hot-to-chassis fault happens, it will result in significant current flow such that the circuit break will blow. The best way to do this is through the grounding conductor in the three-wire plug by connecting it to the equipment chassis. But what if the equipment doesn't have provision for this and we have to deal with the possibility of a line-to-ground fault? Well, we need a good enough ground conduction path between the ground at the fault and the transformer neutral to get enough current (~15A) to trip the breaker. If we don't have this, then it is very possible that we can support more than enough current to be lethal (<1A) while not tripping the breaker. This is why we want a local grounding point at the service entrance and not just a grounding point at the distribution transformer some arbitrary distance away in which the ground fault current has to flow through who-knows-what type of soil/rock to get back to the transformer. Instead, it only has to travel through a few dozens of feet to the service entrance.
     
  11. MaxHeadRoom

    Expert

    Jul 18, 2013
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    In some parts of the world the Neutral is not bonded to the earth ground at the panel, just back at the supply transformer, and the service Co. does not always supply a ground conductor anyway.
    In these types of installation the ground resistance is measured from the local ground conductor and/or ground rod back to the supply transformr, and if insufficiently low, a main ELT or GFI is fitted at the main panel.
    Max.
     
    Last edited: Jan 6, 2014
  12. alfacliff

    Well-Known Member

    Dec 13, 2013
    2,449
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    atr the last house I lived in, the "ground" wire from the electric meter was just burried inches under the surface of the ground. that would not have made a very good safety ground at all. also, the NEC was written to protect wireing from problems like people putting 3 wire recepticals in when only two wires exist.
     
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