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Physically Grounded

  1. Mar 1, 2013 #1
    I'm familiar with nodal analysis and assigning a reference node which is theoretically on paper is zero potential. However if I ground a node physically, for example as in the circuit shown below, and then I touch one of the source terminals, do I get electrocuted? or if I measure the voltage between the grounded terminal of the source and the ground (another point) should I read 0?

    Thanks.
     

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  2. jcsd
  3. Mar 1, 2013 #2
    After reading through some posts about grounding/earthing I came up to a conclusion that there's no certain clear answer about how it works. One electrical engineer I asked told me that the ground has the physical ability to "absorb" high current, for some reason only God knows, and that's it. That's why it's used for safety and only safety purposes. Some use it as a common connection (for shortening) as a chassis, and this works because the ground connections in all mains are wired together. Claiming it's the zero potential or reference point makes no sense.
     
  4. Mar 1, 2013 #3
    No, there is certain answer it's just that there are different types of earthing- Protective, functional and lightning suppression are the three different types of earthing/grounding.

    The big problem is that "earthing/grounding" is to much of a general term and many people tend to get confused between the different types and also the term "earthing/grounding" leads to allot of general confusion. Also take into account that countries differ on there own earthing/grounding practices so there is no one word answer one could give....

    Anyway from your post it sounds like your talking about protective earthing which is all about personal protection from shock; Wikipedia actually has a really good section on protective earthing. http://en.wikipedia.org/wiki/Earthing_system

    To answer your first question, no you won't be electrocuted because the battery is only 10 volts;)
     
    Last edited: Mar 1, 2013
  5. Mar 1, 2013 #4
    Suppose you have a boiler that makes steam for heating as in the attachment. It works by applying power to a heating element which heats the boiler until the pressure switch indicates a high pressure and opens the circuit. To begin with let's assume that it is totally ungrounded. In other words the grounds at A, B, and C do not exist. It might be in the basement of an apartment building and may work fine for decades but eventually the insulation at point A may start to crumble and contact ground. Still the boiler works perfectly fine.

    More decades go by and the insulation at point B falls off and it contacts ground too. Now what happens? Pressure builds up in the boiler and the pressure switch opens but because it is bypassed, it doesn't turn off the heater and the boiler explodes.

    But what happens if we connect an intentional ground at point C. Now if an unintentional ground develops at either A or B, the fuse blows and the electrician discovers what the problem is.
     

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  6. Mar 1, 2013 #5
    I appreciate your help but I still don't get it. Types of grounding?

    What about the third wire in the main? What type of ground is this?
     
  7. Mar 1, 2013 #6
    Forget about functional earthing at the moment. Functional earthing/grounding is about anything other than providing an earth/ground for shock protection-eg signal filtering, static protection, interference etc...

    Any earth/grounding conductors in your house will be protective earth/grounding. The purpose of this system is to create a low impedance fault clearing path back to the supply source, so anything we want to earth/ground for shock protection we connect to this path. This low impedance path is also commonly referred to as the "fault loop".

    So basically if an active conductor comes in contact with a conductive surface that is not meant to be live during normal operation, the magnitude of fault current flowing along this path will be so large that it causes the circuit breaker to trip or the fuse to blow.

    We can create this path by two ways, either we connect all our earthing/grounding conductors to our main neutral in the main switchboard (TN-C-S) or the supply authority will provide us with an earthing conductor to which we connect too. (TN-S)

    Generally speaking North America uses a TN-C-S system along with Australia whilst many European countries use the TN-S system.

    Just remember that this system doesn't actually have anything to do with the ground itself as far as fault clearing purposes go anyway.

    There is also a TT system which uses the actually ground itself for a fault clearing path but these systems are generally quite rare, and only work when there are RCD's(GFCI's) or ELCB's present due to the fact the ground is relatively high in resistance compared to copper or aliminiun conductors which are used to create the fault clearing path in TN system.

    Hope this helps:)
     
  8. Mar 1, 2013 #7

    jim hardy

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    The concept of "ground" is misunderstood, even by a lot of degreed engineers.

    I think we are imprinted early, by lightning, to believe electricity has some affinity for earth.
    When studying circuits, we pick up the water-in-pipes analogy which further confuses us - you can pump water out of the ground and water that leaks out of a pipe will soak back into the ground.
    So we come to think ground is some infinite reservoir or sink for current or charge. Which of course it's not.

    It will avoid a lot of confusion if you use a better name for your circuit reference point, call it "circuit common".
    And do as the British do, call "ground" "EARTH".
    The two are quite different.

    If you tie circuit common to earth, then your circuit is earthed. Tha's how US houses are wired and that's why sparks fly when a 'hot 'wire touches an earthed piece of metal.

    When somebody uses the word "Ground" you should immediately ask them : By 'ground', are you referring to EARTH or to CIRCUIT COMMON? "

    In your diagram above - by that three tiered 'ground' symbol attached to source + terminal, are you referring to EARTH or to CIRCUIT COMMON ?
    If your symbol represents, say a circuit common like an automobile chassis, recall automobile chassis is separated from EARTH by the tires so you could get a small shock by touching that node. Recall the shock from static when you exit your car on a cold dry winter day... but there'll be no sustained current flow. Touching other side would allow only whatever current the source can push through the tires to flow through you.

    If your symbol means EARTH, then touching that node won't complete a circuit it only parallels that earthing wire with you.
    But touching the other side of supply connects negative 10^4 volts straight across you.

    In circuits, earth(which is often called ground, unfortunately) is just another wire. It goes most everywhere, though. One can connect to it or not, choice is up to the circuit designer.

    Term "Ground" is also used often for signal common. Mis-used, i say.

    That dual use is an unfortunate state of affairs.

    old jim
     
    Last edited: Mar 1, 2013
  9. Mar 1, 2013 #8
    Thanks a lot all. Yeah I think I mixed the two terms "common" and "ground."

    I could try this circuit at home but I don't want to take the risk and do any connections to the main :D

    What would be the readings of the voltmeters across R1 and R2? How does the ground (earth, main-neutral, green wire) affect the results?
     

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    Last edited: Mar 1, 2013
  10. Mar 2, 2013 #9
    The readings would be the same as when you didn't have the physical earth connection. It is a going to be a voltage divider. So 6.6 Volts across the 2 ohm resistor and 3.3 volts across the 1 ohm resistor.

    The physical grounding won't affect the measurement result. The only result you will have is, that if you touch the node that you have grounded, then you wont get a static shock, because the node and earth now have the same potential, and you most probably also have the same potential as earth.
     
  11. Mar 2, 2013 #10
    Don't know about God, but for me the reason is that the big ball we're sitting on has a very big capacitance. Hence you can transfer to it a lot of charge without having its potential change in a perceivable manner.

    That's what makes 'earth' so special.
     
  12. Mar 2, 2013 #11
    Hello Rudenh :
    1- In your two diagrams, both cases are valid, the key to this discussion as well as any "circuit" discussion. Is when you ( assuming you are also grounded) touch any other part of the circuit you create another circuit loop. If there is more than about 50V present between ground and the point you touch - this is considered to be unsafe ( Per worker safety rules in the USA).
    2 - "One electrical engineer I asked told me that the ground has the physical ability to "absorb" high current, for some reason only God knows" - really? I would advise not to get info from this supposed EE.
    3 - Ground and Earthing ( depending on field of engineering and geography) have different meaning - so every discussion needs some context. In your case 1000V souse - they typical purpose for grounding is safety - and reliability. However grounding is also used extensively in EMI/shileding and low level signal protection.

    When it comes to safety - the point is that all of the surfaces , conductors that people may come in contact with are at the same potential. "the top of your stove is at the same potential as the water faucet"

    As for reliability - in HV systems ( similar to your diagram), with very high quality insulation, it is possible, depending on the circuit parameters, for an ungrounded circuit to build up a high potential between the isolated circuit and ground - eventually exceeding the voltage rating of the insulation between the circuit and ground and causing a breakdown in the insulation. Since this insulation failure occurs at only one point - perhaps not a big problem, but could be catastrophic. So many systems have a ground at some point to ensure the system is stable.
     
    Last edited: Mar 2, 2013
  13. Mar 2, 2013 #12
    Well...

    So any node I wire to the earth has 0 potential? This means in the first case I will not get electrocuted when I touch the grounded side of the source, in the first sketch?

    Does this apply to the Moon? Mars? other planets? What about spacecrafts?
     
  14. Mar 2, 2013 #13

    jim hardy

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    remember - voltage is potential DIFFERENCE.
    So any node you wire to earth has same potential as earth in that vicinity..

    Think about it - your feet are at potential of local earh.
    You are about to touch ( i assume with your index finger) a wire that's at same potential because it is earthed.
    What will be potential difference between your feet and index finger when you make contact?


    I'm not quite sure what you are asking. But clearly electric circuits work just fine on moon and spacecraft and Mars as we've been receiving radio signals from them for years.

    Now - if you ran a wire down from the moon's surface to just above your driveway so you could measure THAT potential difference, what would be voltage between earth and moon? I don't know.

    http://science.nasa.gov/science-news/science-at-nasa/2008/17apr_magnetotail/

    http://www.sciencedaily.com/videos/2008/1007-preparing_for_a_walk_on_the_moon.htm
     
  15. Mar 3, 2013 #14
    It's a bit tricky. This is one of those situations best described as "the spherical cow approximation".
    Potential is defined uniquely up to an additive constant. You are free to pick the value that best suits your calculations for that constant, as long as you stick to it for every other potential in your circuit or system. For example, you could choose the potential of one wire of a high voltage line as your 0 potential and express the voltage difference of nearby lines with respect to that. But you would not want to touch that line while sitting on the mast just because you arbitrarily called its potential 'zero'.

    When dealing with potentials there are some arbitrary choices that - notwithstanding their arbitrarity - make more sense than others. For example, with constant fields and linear potentials, any location at a finite distance will best suit your needs. The gravitational potential near the surface of the earth (g h) is usually referred to a zero height of your choice, be it the lab floor or sea level.
    When dealing with fields and potentials that decrease with distance to a finite asymptotic value, it is easier to consider a zero reference at infinity (the reason is that you can set the additive constant to zero and forget about it). For example, the electric potential of a point charge[*] decrease as 1/r, and 1/r goes to zero as r->infinity. By setting the conventional zero potential at infinity you will end up with a zero value for the additive constant, so that you won't have to drag it along in all your calculations.

    Now, back to our spherical cow, the one we all live on.
    The Earth, in a very wild approximation, can be seen as a very big spherical conductor. A very big spherical conductor has a very big capacitance (i.e. can gobble a lot of charge almost without changing its potential) and - by definition - has a very big radius. If this radius were infinite, we could actually give the earth the potential of zero volts.

    Now, where is the catch?
    Ok, the radius is not infinite but from a practical point of view, we can give zero potential to points so distant from us that any change in their distance won't appreciably affect the potential we can measure. In a way it's just like calling the antipodes 'infinity' and giving zero potential to the potential we are all accustomed to: the potential of the earth we walk on.

    But it's the conduction part of our spherical cow that can be really tricky. Conductivity is not uniform and depends on many conditions, like terrain, soil composition, humidity, weather conditions...
    You cannot trust that if you put an electrode into the ground near your house, then it would be at the same potential as an electrode in the ground at the local electric facility. Heck, you cannot even be sure that electrodes in the ground around your house and those around your garden shack would be at the same potential!
    But locally, if you plant enough electrodes deep enough, close enough and in an average enough type of soil in average enough conditions, you could at least have a nice 'earth' connection with all electrodes at about the same potential and capable of gobbling up all the charge you throw at it without batting an eye.

    As for your question about the potential on other planets, if the other planets surface can be considered conductive (and that remains to be seen, in absence of an atmosphere and water in the ground), the same spherical cow trick can be pulled on their surface too. You will call 'zero' the potential on the surface of that particular planet.

    Is the 'moon zero' the same as the 'earth zero'? Good question, I wish I knew the answer. Because that's where the spherical cow approximation breaks up: the distances involved between the Earth and the Moon are greater than those of our supposedly infinite spherical cow. Moreover, the environmental condition are different - there is plasma in between I believe, solar winds, magnetic fields and who knows what (recently Nasa has found a new set of radiation belts). I (and I mean I) don't know what would be the potential difference between the Earth and the Moon.

    As for the spacecraft, I guess that you will find measurable limits in the amount of charge you can place on it without having its potential changed. But not in the range of the charge any electrical circuit on board could displace.

    Corrections and/or integrations are most welcomed.

    Note: [*] Or, more importantly, any distribution of nonzero total charge at a reasonable distance - call this the 'monopole approximation'.
     
    Last edited: Mar 3, 2013
  16. Mar 3, 2013 #15
    0 - 0 = 0 I will be safe yay! I will try this at some power plant.

    Only earth that magically makes a relative quantity zero?

    I understand node voltage or potential is relative to some node called reference node, which has a zero potential because it's relative to itself. This is used in calculations to find other nodes relative potentials which in turns used to find the actual voltage difference between them except for the reference node.

    But for some unclear physical reasons the earthing of some node makes its potential "absolute zero", meaning that it's has the node voltage of 0 w.r.t any chosen reference node in the circuit.
     
  17. Mar 3, 2013 #16

    sophiecentaur

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    The capacitance of the Earth is not particularly 'high' - just 700nF. But its 'self capacity' is hardly relevant (only relevant for visiting spacecraft and charged particles).
    The Earth can be treated as a 'free and for gratis' extra wire which can be connected between two points on its surface. Once you have established a good connection to it (spikes, copper radii or a copper mat), the resistance can be made low enough to carry as much current as you like with a very small potential drop.
     
  18. Mar 3, 2013 #17

    jim hardy

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    You might use a voltmeter or test light first... just in case what should be earthed isn't.

    Here's a practical example... in the US this is a common lamp socket.

    365px-Light_bulb_socket_E26_three_way.jpg
    photo courtesy of: By Hustvedt (Own work) [CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0) or GFDL (http://www.gnu.org/copyleft/fdl.html)] [Broken], via Wikimedia Commons

    Observe the threaded sleeve into which you screw the lightbulb. It's just inside the cardboard insulator.
    That sleeve is connected to the "neutral" wire of the lampcord.
    The neutral wire is the conductor with a rib in its insulation. The hot wire is smooth.
    The neutral wire is connected to the wider of the two prongs on the plug.
    That's so the plug will only go into the receptacle one way, neutral wire in wider slot..
    The wider slot in the receptacle is or should be connected to a white conductor inside the wall that is earthed at the house power panel.

    The reason you wire the lamp socket/cord/receptacle/panel that way is there's a good chance your fingers will touch the metal threads of the lightbulb while you are installing it.
    When they do , you will not get a shock if the house wiring is in good order. That's because the metal sleeve, and hence the metal threads on the bulb, are earthed through the wiring.
    If you DO get a shock something needs to be fixed.
    I once purchased a cheap lamp that was wired backward - sleeve was hot and the lamp would have been a shock hazard. Nowadays i always check them.


    Nothing magic about it. You might be confusing the concepts of voltage and potential.

    Remember - voltage is potential DIFFERENCE. That's why voltmeters have two wires...
    When you earth a node, its potential becomes same as earth where you tied the earthing wire.
    So there is zero difference of potential between it and earth.
    There is still potential difference between it and other nodes.
    Potential DIFFERENCE = voltage.

    let's see if next paragraph helps..
    See, you are using terms voltage and potential as if they were equivalent...

    Okay - what then is potential?
    Absolute potential is the work done in bringing a unit charge from infinity to wherever you are.

    Webster's: http://www.merriam-webster.com/medical/electrical potential

    Now that's quite impractical - we can't go out to infinity and carry back a charge just to measue local potential. So we assume earth is zero. It's a mighty convenient starting point...

    That concept took me a long time to absorb. It's why i asked earlier about voltage between earth and moon....
    I'd say "...earthing of some node makes its absolute potential same as that of earth, which is unknown. But we can use it , whatever it is, as our reference point from which all others are measured. We assign it value zero for convenience."

    Mechanical engineers do have an absolute zero for their pressure, and chemical engineers have an absolute zero for their temperature.
    We unfortunate electricals can only imagine an absolute zero for our potential..

    Earth actually has a potential gradient at its surface, here's an amateur built instrument to measure it.
    http://www.scientificamerican.com/article.cfm?id=detecting-the-earths-elec
    The principle has been proposed as altitude sensor for aircraft autopilots, but wisely not pursued.
    Point being - earth's absolute potential may not be zero.
    But we can use earth for our "Circuit Common" if we wish.

    I may be splitting hairs here. But for me this was an important concept.

    i hope it helps you.

    old jim
    There's other threads on this subject - perhaps somebody wiser than me can help...
     
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  19. Mar 3, 2013 #18

    dlgoff

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  20. Mar 4, 2013 #19

    jim hardy

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    Pressure too is a differential measurement.

    Mechanical engineers generally use local atmospheric pressure as their reference.
    Pressure measurements that are referred to local atmosphere are "gage pressure" , psig, but usually the g suffix is omitted as in that picture.
    Pressure measurements that are referred to absolute zero are called "absolute pressure", psia.
    Absolute pressure differs from gage pressure by local atmospheric presure, of course.
    An absolute pressure gage has an evacuated chamber for reference. In a barometer it's the empty space in the glass tube above the mercury.

    Observe that in Dlgoff's example from http://hyperphysics.phy-astr.gsu.edu/%E2%80%8Chbase/electric/watcir.html [Broken] ::
    Provided the reservoir in the picture is open to atmosphere it has an absolute pressure of ~14.7 psia. Well, that is if it's someplace like Miami which is at sea level.
    The gage showing 50 psi is relative to atmosphere. Were it an absolute gage it'd show 64.7.
    If instead it's in the mountains west of Denver whewre atmospheric pressure is more like 12 psia, the same absolute pressure gage would show 62 psia..

    We poor electicals don't know earth's absolute potential so we assign it value zero. That may be obvious to most folks but i struggled with the concept. Sorry if i bored you, and dont think i was in any way "talking down"...

    old jim
     
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  21. Mar 4, 2013 #20
    Thanks a lot Jim, your very helpful information is greatly appreciated. I will have to struggle myself with the concept too.
     
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