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The Concept of Grounding

  1. Jun 4, 2013 #1
    Hello everyone. I'm very...very new to electronics, and am looking to get into the 'maker' world, of building small electronics, and projects. One concept though that has bothered me is the concept of grounding. Now I've come to understand that ground wires in homes have that bottom plug reserved for ground pins. And if there is a spike in voltage it is sent through that ground wire, out from the house, and literally into the ground on the utility pole outside. But how does ground work in small electronics? The literal sense of sticking a wire in the ground is lost, so how can electronics have 'ground' wires as well? such as this? Are they used for the same purpose?


    for example pin one is a ground pin, but what is is grounded to?
     

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  3. Jun 4, 2013 #2

    phinds

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    There are 2 kinds of ground. Circuit (local) ground and Earth Ground.

    A portable, plug-in, drill for example, will have a metal case that is connected to Earth Ground so that if there is a short between the hot wire and the case, it will short to Earth Ground. This is considered to be better than the alternative which is electrocuting the guy using the drill.

    A portable radio (battery powered) however, obviously isn't connected to Earth Ground and does not need to be. It has a local ground which is just the reference point for all the voltages in its circuitry.
     
  4. Jun 4, 2013 #3
    Thanks for the speedy reply! So something like a plug-in drill has the ground wire obviously, connected to ground from the outlet. But again, what protects the person if a battery is used instead of the wire for the drill? Can you elaborate on how 'local' grounds protect the user?
     
  5. Jun 4, 2013 #4

    FOIWATER

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    The ground here is used differently than grounding in a house.
    It's a misconception that electrons have some affinity for ground naturally.
    They have a tendency to return to the source of voltage.
    If, as in the case of electronic circuits, the negative terminal of the voltage source is connected to "ground", then grounding another part of the circuit THEN gives electrons an affinity for ground (to ultimately get back to the voltage supply) does this make sense?
    The thing is, ground is typically a very low impedance path back to source.

    In power systems, it has different connotations. The ground IS still used as a low impedance path back to the source, but it does not normally carry current.
    In the event of a line-to-ground fault (say without a continent path back to the voltage supply) a high voltage line could cause potential rises in the earth surrounding the fault with no tripping of any fuses or circuit beakers towards the sub. If though, the transformer has a grounded wye setup, when the line-to-ground fault occurs, there is a low impedance path back to the source (through the ground) and a large current flows which quickly activates fault protection equipment.

    In your house say, where grounds are ran all over the house with your circuits, it has much the same purpose as the power system case. As you said the transformer outside your house is grounded. Normally, the ground wire does not carry any current. If, for example, a hot wire came off a terminal and contacted a metal junction (assume you do not have any ground protection) you have a potential rise situation (as w/ the power system). And you, coming to contact with the box, experience that same rise in potential. If however, the metal box is continent with the "ground" of the transformer, then a large current will flow, and trip the breaker feeding that circuit. As well, the transformer is connected directly to earth ground, and in the house your ground bus in the panel is connected to the ground (because of its low impedance characteristics)

    So, in your provided picture, the source has a positive and a negative right? You connect the positive terminal say to +V, then the negative terminal to GND. On the timer, connect GND to the same GND you connected the voltage supply to. you can connect in fact any other device properly to +v and GND to power it....

    Hopefully this makes some semblance of sense. If it doesn't someone will set us both straight.
     
  6. Jun 4, 2013 #5

    FOIWATER

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    I would say that a 'local' grounds as in electronics do not protect the user in the same way they would a ground circuit in your house.
     
  7. Jun 4, 2013 #6

    phinds

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    But for portable (battery driven) devices, they do not need to.
     
  8. Jun 4, 2013 #7
    If we have a power source as in the picture, then I like to call the negative terminal the "return path" and the positive terminal simply the "positive" or "power plus".

    Here the "ground" symbol could mean two things:

    https://www.physicsforums.com/attachment.php?attachmentid=59270&stc=1&d=1370364983

    Type 1: The circuit common
    If we mean it is the circuit common, then we use it as our "0 potential" from which all other voltages are referenced. If this is the case, then the "ground" wire on the picture is not an actual wire but simply a symbol to remind us that this is our "0 volts".

    Type 2: "Earth ground", "Chassis", "M-ground", "Structure", "Case"
    In the latter case, which is the one that has to do with safety and EMC, the "ground" wire on the picture and the symbol has a completely different meaning. Sometimes, we also use different symbols for these grounds, see below.

    [Broken]

    Lets talk about safety first.

    If you have a circuit which is just a power source and it looks like the following.

    attachment.php?attachmentid=59273&stc=1&d=1370367314.png

    If the wires are just directly connected to the resistor and the circuit is floating above the ground suspended by some kind of magic.

    Before we go any further, lets have a quiz, and EVERYONE IS INVITED TO ANSWER..

    Question 1: What is the voltage difference between me and the return path without me touching the wire? (The return path is the wire connected to the negative terminal).

    Question 2: What is the voltage difference between me and the "hot wire" without me touching the wire? (The "hot wire" is the wire connected to the positive terminal).

    Question 3: What is the voltage difference between me and the return path if I am touching that wire?

    Question 4: What is the voltage difference between me and the "hot wire" if I am touching that wire?

    And YES I know that there are no values in the diagram, but this doesn't matter ;)
     

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    Last edited by a moderator: May 6, 2017
  9. Jun 4, 2013 #8

    Integral

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    Question 1: What is the voltage difference between me and the return path without me touching the wire? (The return path is the wire connected to the negative terminal).
    Undefined
    Question 2: What is the voltage difference between me and the "hot wire" without me touching the wire? (The "hot wire" is the wire connected to the positive terminal).
    Undefined
    Question 3: What is the voltage difference between me and the return path if I am touching that wire?
    0V
    Question 4: What is the voltage difference between me and the "hot wire" if I am touching that wire?
    0V
     
  10. Jun 4, 2013 #9
    The winner is Integral :D

    In the beginning, the voltages will be undefined (could be 0 volts, could be 5000 volts, could be 10.000 volts), but when we touch these wires, I get the same potential as the wire.

    However, no current is running through me. So I am not being electrocuted. None of the wires are dangerous to touch. Even if the source is a staggering 100.000 volts, I can safely touch the wires.

    The reason for this is of course that no current is running through me.

    Current needs a loop. The circuit in the picture shown is a loop and current going out is also the current going in. If I touch either of the wires, I do not provide any path for the current, and thus, no current will flow.

    The problem happens at the instance that something unforseen comes into the picture. If this magic circuit is floating near a tree, and it so happens that one of the branches on the tree gets in contact with one of the wires, lets say the hot wire.

    In this case, when I touch the return path I will suddenly have made a loop, and current will be able to flow through me.

    attachment.php?attachmentid=59275&stc=1&d=1370374038.png

    In this case, I may be electrocuted, since a current will run through me.

    How much current will run through me? Depends on my resistance. (BUT ONLY A FEW MILLIAMPS CAN KILL).

    Until this point, I think most of us are following, and nothing wierd has happened yet.

    Let us now take a wire and connect to the "return path" and stick it into the ground.

    attachment.php?attachmentid=59276&stc=1&d=1370374470.png

    The "ground" wire is an ACTUAL CONNECTION to ground now. In the beginning, no current is flowing into the ground, because there is no current loop.

    But lets imagine the tree being a douche-bag again and touching the "hot wire". What happens now?

    Well for one: The tree and the earth now provide a current loop. Lets see a picture.

    attachment.php?attachmentid=59276&stc=1&d=1370374933.png

    So if I now touch the "hot wire" also I may no longer be the easiest way for current to travel, and in this way, I am protected. However, current can still flow through me, but there is a protective device that can make you safe.

    The GFI (Ground Fault Interrupter).

    This piece of hardware measures the current that goes out of the positive terminal and the current that returns through the return path.

    If the tree is touching the "hot wire", then there will be current through the tree and through the earth and into the negative terminal from the ground wire. If we measure the current in the "return path" before the ground connection, then we will be able to see that not all the current going of the the positive terminal, is returning through the return path. Some of it must be going through the ground. The GFI will then disconnect the power source and we're all safe again.

    When I return, I will try to explain how grounds are used on the ISS. When I started working, I suddenly understood everything about grounding when I had to design things for the space station. The reasons is that there is no "earth" that makes you wonder. But you still have a ground.

    Stay tuned!

    In the meantime. Answer these questions:

    Question 1: What happens if the tree touches the return path?

    Question 2: Could instead of connecting the grounding wire to the "return path" instead connect it to the "hot wire"?
     

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  11. Jun 4, 2013 #10
    Grounding Part 3: The International Space Station


    On the space station, power is generated from solar panels. In this case, we will simply consider them to be ideal voltage sources. The simplified solar array will have a "hot wire" and a "return path".

    However, in order to create the correct voltages that are needed, the wires are connected to different power converters. So the voltage source shown is imagined to be the output of one of these power converters.

    attachment.php?attachmentid=59278&stc=1&d=1370376321.png

    The return path is connected directly to the hull of the space station. The astronauts will most likely be at the same potential at the space station structure, so touching the return path of such a wire, will not give them any static shock.

    Think also a moment that, if the astronaut is floating without touching anything of the space station hull, he can safely touch any of the wires.

    Next step is connecting this power to some kind of equipment. For example the super gigantic death ray that they have up there, or the stargate.

    attachment.php?attachmentid=59279&stc=1&d=1370376327.png

    The picture above shows that there are three wires in such a connector. "Hot wire", "Return" and "Structure". The structure is simply the "ground" on the space station.

    When you connect some equipment to this power source, the "structure" will be connected to the "case" of the equipment. So if we're talking about the death ray, the metal surrounding the inner workings of such a thing will have the same potential as the hull of the space station. Thus, touching the case will not give you any static shock, and no current will flow through you, when you put one hand on the "case" and one hand on the hull of the space station.

    IF an error happens inside the death ray, so that some "hot wire" is suddenly broken off an gets in contact with the "case" then a current will suddenly flow through the case and through the structure of the space station and back to the power source. This will make the GFI (Ground Fault Interrupter) trip and cut off power.

    Of course this is a rather simplified version of what really happens in the power network up there.

    ------------------------------------

    I realize I've been talking and talking and what do I really want to say? A lot of places "ground" is used in a wrong way. Most times it means the "common".

    The best way to talk about power sources are in terms of a positive terminal and a negative terminal. DON'T call the negative terminal "ground", as many people do. This is simply wrong.
     

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  12. Jun 4, 2013 #11
  13. Jun 4, 2013 #12
    Excellent explanation about grounding!

    For the OP: If you're building battery-powered small electronics, the ground is just a convention, a reference node that you arbitrarily call zero for your circuit calculations. For example, imagine you made a simple unity-gain opamp buffer powered by a single 9V battery. You would call, arbitrarily the negative terminal of the 9V battery ground (or 0V), then the positive voltage you would call 9V (or ground + 9V). Voltages only make sense physically when they are compared to something else. The voltage of a given node is meaningless unless you say what it is compared to.

    I've had systems where for technical reasons opamps were powered at 100V and 95V (compared to the chassis ground). As far as the opamps were concerned it was exactly the same as running at 5V and 0V (in fact it WAS 5V and 0V) because only voltages differences are meaningful.
     
  14. Jun 4, 2013 #13
    If your in the US, which I assume you are, they use what known as TN earthing. Which means all your "grounding" wires are connected directly to your main neutral in the switchboard. This insures that a fault with a bit of equipment turns into an active to neutral short circuit this in turn causes a large amount of current to flow which trips the circuit breaker or blows the fuse.

    The "ground rod" outside your house really doesn't have anything to do with the facilitation of protective devices, well not in a TN system anyway.

    If your interested in protective earthing than you might want to have a read through this.
     

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    Last edited: Jun 4, 2013
  15. Jun 5, 2013 #14

    sophiecentaur

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    It may be worth while summing up the above information. There are three reasons for having a 'ground / Earth' and they are all distinct.
    1. Safety. If all the exposed metal parts of electrical equipment are connected to a common point and that point is connected to all pipes, the steel frame of a building and a spike driven into the Earth (perhaps quite literally) then it is impossible for anyone to get a shock by touching any equipment, whether or not it's faulty. Everything you can touch is at the same potential.
    2. Convenience / cost. It saves wire if you can feed all your equipment with one wire and use the metal frame / chassis of a vehicle, for instance, as the 'return path' for the current. It's not an ideal way of doing things because car chassis can get rusty and this can introduce high resistances into the circuits but it's pretty common. This system is often used on a smaller scale inside some small, low voltage, appliances too - particularly for feeding the power around. I can't think of a serious practical power system that uses the actual Earth as a return path. The connection is too unreliable.
    3. Reduction of interference / noise / cross-talk. When you're dealing with low level signals and amplifiers etc., particularly for high quality Audio and Radio Frequencies it is common practice to use 'single sided' transmission of signals, with a signal wire and a common ground (signal return) wire (such as with co-ax cable / screened leads and signal connections between units within a case). Signals only are carried on these conductors (co-ax and screened leads). The layout of the signal paths is often very critical to reduce interference. In extreme cases, signals are sent along two, 'balanced' conductors and there is no actual ground conductor at all (twisted pair). Twisted pairs can also be encased in a further screen which may also be grounded to reduce interference further but not attached to either side of the pair.
     
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