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Reference point ground vs Earth-ground

  1. Feb 27, 2016 #1
    1. The problem statement, all variables and given/known data
    I have seen earth and ground used interchangeably. However, there appears to be some distinction with regards to reference point ground and earth ground.

    My textbook used an example of a loaded voltage divider and examined how the voltage across certain resistors can be measured to be positive or negative depending on how we define our ground (at-least thats what I gathered from it). This is because, regardless of where we place the ground, that location (node) by definition is at zero volts.

    So my question is, lets say we have a circuit with a single power supply. Is earth ground always connected to the negative terminal of the power supply while reference point ground is generally thought of as a ground that does not have to be connected to the negative terminal of the power supply?

    If I am wrong, can someone please provide a brief explanation as to the differences between the two?
     
  2. jcsd
  3. Feb 27, 2016 #2

    cnh1995

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    Reference ground can be any point in the circuit. In many of the single source problems, it is at the -ve terminal of the battery. However, you can assume it anywhere in the circuit as per your convenience.
    Earth ground is actually "earthed" i.e it is buried. It is done from the protection point of view. All the costly electrical equipment is earthed via an earthing wire. Three-pin plugs have an earthing wire, apart from live and neutral wires.
     
  4. Feb 27, 2016 #3
    Thank you for your response! Two questions...

    1. Can I think of earth as a big reservoir of charge?
    2. Can you elaborate a little on what you mean by earthing is done from the protection point of view?
     
  5. Feb 27, 2016 #4

    cnh1995

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    Yes.
    Consider a device with metal body. If live wire comes in contact with the metal body(after insulation failure) and a person touches the device, he'll get an electric shock if the device is not earthed. If the device is earthed, it's metal body will be connected to the ground(actual "earth" ground). Hence, it will bypass the person's body and fault current will directly flow into ground through the ground wire, without harming the person or other components of the device.
    images?q=tbn:ANd9GcQx5ILuQk46-1KdQ7sReiCRK4TP0M3Lkg0WDLw8OtwdJ2vnY3Ka.jpg
     
  6. Feb 29, 2016 #5

    CWatters

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    +1

    The point to realise is that all voltages are measured "with respect to" another node. You can never measure a voltage in isolation.

    Lets say you were working on a portable device powered by a 12V battery. You could choose to measure voltages on any node with respect to either the +ve or -ve terminals of the battery. What you are doing here is defining either the -ve or +ve terminals of the battery to be 0V for the purposes of measuring the voltage on the node of interest. For example a node that is at +3V with respect to the -ve terminal would be at -9v with respect to the +ve terminal of the battery.

    In many cases the -ve terminal is defined as 0V and people get lazy and just say "this node is at 3.564V" without saying "with respect to the -ve terminal of the battery" every time.

    Usually but not always. Some devices have no earth connection. In the example above it's a portable device. So "earth ground" isn't connected to either terminal. Either terminal could be at quite a high voltage with respect to a nearby water pipe buried in the ground if there is some static electricity around.

    Most modern cars are "negative earth" in that the chassis is connected to the -ve terminal of the 12V battery. However some old cars were/are "positive earth" and had the chassis connected to the +ve terminal of the battery. Most cars have rubber tyres and neither have the chassis electrically connected to the planet earth.

    Yes. Reference ground is usually the node that other voltages are measured "with respect to".
     
  7. Feb 29, 2016 #6

    CWatters

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    PS: Be careful when using an oscilloscope. These display the voltage difference between an earth lead (usually fitted with a crocodile clip) and probe tip. Unlike a portable multi meter you can't just connect either lead to any old point in the circuit being tested. That's because the earth lead is connected to mains earth inside the oscilloscope.

    For example if the device under test is a computer, these usually have 0V connected to mains earth inside the power supply. So if you connected the oscilloscope earth lead to the computers 12V rail you could short circuit the power supply via the mains earth.

    More here..
     
  8. Aug 31, 2016 #7

    David Lewis

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    Is the voltage of a water pipe buried in the ground considered to be absolute zero (i.e. no surplus or deficit of electrons)?
     
  9. Aug 31, 2016 #8

    gneill

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    It's just considered to be a connection to a very large, effectively neutral well of charges and is about the best we can do for a common reference potential that's available globally.

    We can't claim that it represents some "absolute zero" potential reference due to the existence of factors that can affect the local ground potential over time. For example, magnetic storms, aurora activity, and even local lightning storms can introduce currents that circulate in the Earth and cause regional variations in potential.

    The science behind establishing effective Earth grounding for installations is actually not trivial. But for the amateur radio fan, the quick and dirty cold water pipe ground is cheap and very effective. Of course, these days much of new indoor plumbing is now plastic of some form, so no more nice copper conduits to ground!
     
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