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Definition of electrical ground

  1. Feb 26, 2010 #1
    In the short time I've been here I've already noticed this forum seems to lack a good definition of electrical ground or earth, posters seem to mean different things when they use it.

    Sorry if this is the wrong place to air this but it seems to have popped up in several different places in the forum.
  2. jcsd
  3. Feb 27, 2010 #2


    Staff: Mentor

    Could you provide some examples of places where you see some disagreement?
  4. Feb 28, 2010 #3
    A quick shufty reveals this in more than one thread.

    I am not saying this is factually incorrect, but is it complete & adequate as an explanation or definition? No disrespect is meant to the author.
  5. Feb 28, 2010 #4


    Staff: Mentor

    And who is that poster disagreeing with? You said there was disagreement, so that implies at least two different opinions.
  6. Feb 28, 2010 #5
    Dale, I don't understand.

    I said nothing about disagreement.

    I am of the opinion that there is more than one use of the term, but that they all stem from a unique property that is posessed by an earth.

    I have also seen the explanation 'a body with no charge'.

    I did not initially post my definition as I did not want to colour other's ideas, or provoke a futile argument. Obviously I will eventually have to offer something.
  7. Feb 28, 2010 #6


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    That's because the term "ground" (or "earth") can indeed mean slightly different things depending on the circuit and context. The central thing that is common to all definitions is that it's a common point or reference node. Sometimes this (a reference node) is all that it means while other times it also implies a physical connection to a devices metal chassis and/or physical earth.

    In my attachment I've drawn several symbols that are frequently used as "grounds". The top two (filled/unfilled really just personal preference) are usually used to indicate a signal reference node. The third one (bottom left) usually indicates that this node is connected to the metal chassis in which a circuit is mounted and the last one usually indicates physical "earth".

    Attached Files:

  8. Feb 28, 2010 #7
    Thanks for your contribution, uart.

    Connections to chassis, or even multiple connections, do not necessarily make it an earth or ground. It is possible for apparatus to use a chassis as a common point internally but still have a separate earth connection, not connected to this chassis.

    There are other uses of earths besides a voltage reference point - and they need not be a single point - how about PME?
  9. Feb 28, 2010 #8


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    Yes I know, that's what I meant by "ground" meaning different thing in different instances. The common factor here is that they all represent a reference node.
  10. Feb 28, 2010 #9
    Personally I distinguish three different uses for earths.

    1) Protective earths

    2) Reference

    3) Screening and shielding

    There may be more I haven't thought of so I welcome suggestions not covered by the above.

    They all use the characterisic of an earth that

    An earth is a body whose potential does not alter, regardless of the current flows into or out of it, within the design limits of the system.

    As far as I am aware the term earth is synonymous with ground - One is basically UK usage, the latter US.
  11. Feb 28, 2010 #10


    Staff: Mentor

    Sorry, I read "mean different things" in your OP and misinterpreted it as "have a disagreement".

    Obviously in physics it is common to use different definitions for the same word provided the definitions are equivalent. So, you can define force as "mass times acceleration" or equivalently as "the rate of change of momentum". Why someone might choose one over the other could be quite arbitrary. Some posters might use one form exclusively, others might switch between them depending on the context.

    I don't see that it matters much. If there is no disagreement then the various meanings are equivalent, and I believe it is not particularly valuable to try to force everyone to pick one equivalent definition over another.
  12. Feb 28, 2010 #11
    Unfortunately either sloppyness with words or a genuine lack of understanding can lead to incorrect statements.

    For instance I have seen the incorrect statements

    An earth has low, negligable or zero resistance.

    An earth is a single point.

    An earth requires a good connection.

    There can only be one earth in a system.

    An earth is at zero volts.

    If you adopt the above as true how, for isntance, would you go on to talk about earth resistance - an important subject to most of us.
  13. Feb 28, 2010 #12


    Staff: Mentor

    And do you believe that those are not in disagreement with each other or with a correct definition of ground?
  14. Feb 28, 2010 #13
    If you are referring to the five faux that I listed
    No this is not a belief system, go to church for that.

    You will find the answer to the first two in your national building codes, electrical section or in a decent geophysical textbook.

    I could easily demonstrate examples of the others, if I thought you were seriously addressing the issue.
  15. Feb 28, 2010 #14


    Staff: Mentor

    I am afraid that I have a really difficult time communicating with you. I don't understand why you initially went out of your way to point out that you weren't talking about a disagreement and then when I asked for clarification about your position you respond with some strange comment about church.

    If you can clearly state what your concern is then I will try to address it, but frankly I am not sufficiently interested to continue trying to guess.
  16. Feb 28, 2010 #15


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    Earth and ground are two different things as uart already shown. Earth is a ground, but not every ground is an earth ground. This has specific meanings in terms of circuits.
  17. Mar 1, 2010 #16
    So please be good enough to elaborate on your views.
  18. Mar 1, 2010 #17


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    As I said, uart and your own quotes have pretty much explained it.
  19. Mar 1, 2010 #18
    In the US, ground is a contextual designation in electronics. For electricians, it's always dirt ground.

    The ambiguities should be resolvable by context within the scope of the field by those knowledgeable within a field in question. If they aren't, the presentation could be better.

    It's quite common within electronics to designate multiple grounds. If you wish, 'ground X' is simply a designator for a particular node, usually having low impedance to other nodes (bias voltages) --or even a subsection of a node.
    Last edited: Mar 1, 2010
  20. Mar 1, 2010 #19
    Is it?

    What about the ground in an automobile electrical system?

    Thank you for your contribution, Phrak, the rest of this post is not specifically addressed to yours.

    Some people have no trouble with the concept and application of an earth or ground, but many do.
    It would be better if those to whom it comes naturally shared their knowledge rather than some casual dismissal.
    Is that not the purpose of this forum?

    How do you explain to someone with less knowledge what is a 'good earth' , considering the invocation by many manufacturers to 'connect to a good earth'
    Or the term in the wiring regulations 'an adequate earth'
    or the working of earth electrodes in a Wenner array in geophysics?

    People are asking questions like this every day. Often the answers they receive are inappropriate and the subsequent actions wrong. Many an otherwise promising electronic design or apparatus connection suffers from poor consideration of eathing issues.

    I consider this a serious issue and invite those with greater knowledge to contribute for the benefit of all at all levels.
  21. Mar 2, 2010 #20
    For automobile mechanics, the chasis is ground. These are not electricians.
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