Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Ground and common clearing it up!

  1. Oct 11, 2011 #1
    Forgive me if this seems a rather basic question, but I would like to settle it once and for all.

    The concept of "ground", "earth" and "common" confuse me a little.

    I think I have earth right - this is a path that allows dangerous current to be carried away, eg. if a live wire touched the metal case of an appliance, one would get electrocuted upon touching the case, so, the case is connected to an earth wire which allows this current to flow and hence trip a fuse or breaker.

    Ground and common confuse me far more. I see in many circuit diagrams that the return paths to a battery are often not indicated, just a ground symbol is used. So, is ground just the return path for current to the source, and this path is at 0 V?

    Wikipedia says, "Many electronic designs feature a single return that acts as a reference for all signals. Power and signal grounds often get connected together, usually through the metal case of the equipment."

    Does this mean that the metal case of a device is actually the return path for current to the source??

    Perhaps an example of where this confuses me might help:
    In the bipolar voltage divider, there is a reference ground such that 2 terminals are positive wrt ground and 2 terminals are negative wrt ground...in an actual circuit, what would this ground be physically?
     
    Last edited: Oct 11, 2011
  2. jcsd
  3. Oct 11, 2011 #2

    cmb

    User Avatar

    Well, not really. If you were sitting on a plate charged to 500kV you would be 'quite safe', unless someone came near you with a metal rod at earth potential. Earth potential would then be very dangerous!!

    The principle is that the physical Earth is a body of net-neutral charge so large that if you dump an enormous lump of charge into it, it will still remain neutral. A 'common' or 'local earth' is also a 'sink' for charge with somewhere to go, but it would not necessarily remain 'charge neutral' (assuming it was, in the first place) if it received a big dump of charge (or a small one locally in a very short time, e.g. RF currents).

    So the 'potential' between two things is the difference of net charge on the first thing with respect to the next charge on the other thing. The only thing assumed to always be net neutral is the physical Earth, whereas a local ground/common may not always remain charge neutral but can (or is allowed to, or annoyingly happens to) float up or down with respect to the 'true neutral' of the Earth.
     
  4. Oct 11, 2011 #3

    cmb

    User Avatar

    Usually, yes it can, but if the internal parts are intentially designed to be isolated from the box, then no. It is common practice to do this with mains-powered kit, such that if there is then a current between the electronics and the box (held to earth) then you know there is a fault.

    Whatever you call ground. You could intentionally set that point 1MV above Earth potential, if you like, which would [simply] mean that your local 'ground' was not charge-neutral. The whole circuit would then treat that 1MV level as the 'return' level to which circuit currents flow.
     
  5. Oct 11, 2011 #4

    psparky

    User Avatar
    Gold Member

    Let me confuse you a bit further. The neutral and the ground are tied together in the electric panel!!! Literally!!!!! This is 100% factually correct and applies to commercial buildings panels as well.

    What you said about ground is absolutely true. However it serves additional purposes if you didn’t already mention it.

    If you just have a neutral and hot connected to an appliance it is 120 volts. If you don’t have a ground connected….the appliance will work just fine. Say a washing machine for example. What if over 20 years the hot wire vibrates loose and is now sitting on the chassis of the washer. Mom now comes over and puts her hands on the washer and gets electrocuted. Bad.

    Now lets say we use a ground wire in addition to the hot and neutral. The ground is tied to the chassis of the washer. Now when the hot wire vibrates loose, it hits the chassis and instead of staying live…..it is a dead short and it pops the breaker!!!! The washer voltage is now safely dead. Mom goes to reset the breaker and the breaker again pops….she tries again and it still pops….etc. She then calls a repair man…..he takes the washer apart….puts the hot wire back in its place, flips breaker back on and dear ole mom is safe. Now apply this prinicipal to any electric device and u can see how the ground is useful!!!

    If it were a 240 volt line to line dryer, same principal. Works fine without a ground…..but if wire comes loose and sits on chassis…….
     
    Last edited: Oct 11, 2011
  6. Oct 11, 2011 #5

    jim hardy

    User Avatar
    Science Advisor
    Gold Member
    2016 Award

    there was a lengthy discussion recently

    try this thread and see if it helps you.

    https://www.physicsforums.com/showthread.php?t=526008

    meantime contemplate this question:
    if we wired the whole solar system for electric power,
    (i know that's physically preposterous, but mathematically it's not..)
    which planet should we use for ground?

    old jim
     
  7. Oct 11, 2011 #6

    cmb

    User Avatar

    Jim, I have another one for you. If you raise an object to a sufficiently high positive voltage (that is, an absence of electrons in this object) can you get it to some potential where the molecules just fall apart because there aren't enough electrons in it?

    Could there be a planet in the universe which just doesn't have enough electrons, so a large fraction of the atoms are missing an electron? What would the chemistry of that planet be like?
     
  8. Oct 11, 2011 #7

    psparky

    User Avatar
    Gold Member

    I'll take a stab and say jupiter because it is the largest planet.
     
  9. Oct 11, 2011 #8

    jim hardy

    User Avatar
    Science Advisor
    Gold Member
    2016 Award

    wow i'd refer that one to a plasma physicist

    well Velikovsky certainly said as much (Worlds in Collision)

    and you might google on terms 'iron sun' and 'neutron repulsion'

    earth itself could maybe be an example - it has an electrical potential gradient (which got disturbed in vicinity of Fukushima), as if it had excess charge on its surface
    and it has a magnetic field as if surface borne charge were moving along with its rotation;


    but to your last inquiry i plead unqualified in chemistry to venture a reply

    it's a tightrope walk between keeping one's mind open and becoming a fanatic....
    that's why i like Sophie's intellectual discipline...
    one must poke at these crazy ideas until either they fail or you reach your mental limit and admit "i don't know"- and await more facts

    that's why i await with such interest investigation of these particles that so recently are reported to have sped across Europe at > c....
    .... remember Einstein thought up relativity because everybody who measured c was getting the same answer.

    old jim
     
    Last edited: Oct 11, 2011
  10. Oct 11, 2011 #9

    jim hardy

    User Avatar
    Science Advisor
    Gold Member
    2016 Award

    just re-read my last post and fear it might cause offense

    please do not take my phrase "crazy ideas" as any kind of put-down or slam;
    opposite was my intent.

    i constantly am working them in my head cross-testing them against each other
    most get ruled out on grounds of leading to impossible conclusions

    but to me, "indeterminate as yet" is an acceptable answer...
    for there's a zillion of them floating around in my head
     
  11. Oct 12, 2011 #10
    Thanks guys. I guess my idea of earth ground was right - the example above of the washing machine makes complete sense to me.

    The idea of a common also seems to make sense to me. In circuit analysis, one needs a reference point to measure voltages with respect to. One could take the negative end of a battery in a circuit for instance as 0 V (or in practice, a metal strip which is the return path for all currents to the source since by the time electrons get here, all the potential has been dropped across resistors etc. and it is at 0 V) ....this much makes sense!


    However, the ground in circuit diagrams is still confusing me. For instance, in a Wheatstone bridge (Electronics Fundamentals, Floyd, p. 237) with a transducer in place of one of the resistors, there is a node just after the bridge section. From this node, there is a connection back to the negative terminal of the battery, and another to ground. Why is this? Why is the ground needed here?

    Why wouldn't the device work without it?
     
  12. Oct 12, 2011 #11
    Why are you making this so complicated? Current can only flow in a loop. In this case it flows out of the positive terminal of the battery and back into the negative terminal. The ground symbol is there make it clear to you that the negative terminal of the battery is the circuit ground.
     
  13. Oct 12, 2011 #12

    turbo

    User Avatar
    Gold Member

    OP, "ground" and "earth" (usually used in Britain) are equivalent. If you live in the US, your power from the street-side transformer is supplied to your entrance and breaker box in 3 conductors. There is 120V coming from one side of the transformer, and 120V (out of phase with the other leg) coming from the other side of the transformer, PLUS a neutral conductor connected to the center-tap of that step-down transformer. The neutral tap is referenced (tied to) ground at your electrical entrance, giving a fault-path to ground.

    Any catastrophic shorts in your home's appliances or circuitry should shunt the current to ground, tripping breakers (or in the old days, blowing fuses) so that you can see where the problem is without anybody getting hurt. In cases where your neutral conductors lose ground reference (perhaps due to loose or corroded clamps/screws in the entrance panel) all kinds of very odd and potentially unsafe conditions may occur. This happened in our old home many years ago when entrance panels designed for copper feeds were fitted with aluminum wires and lost good contact. The power company reps were clueless, but my trouble-shooter electrician brother-in-law figured it out in a few minutes of phone conversation. Tighten one clamp and fixed!
     
  14. Oct 12, 2011 #13

    psparky

    User Avatar
    Gold Member

    Hey Turbo....perhaps you meant to say this....and I do believe it is your intention....

    ....but the line to line voltage coming out of the streetside transformer is 240 volts. Because the transformer is centertapped (our neutral wire), we can achieve 120 volts by going to line to neutral....which is the case in most cases except for electric dryers, ovens, hot tubs, AC compressors and the like.....which obviously are line to line 240. (unless they are gas)

    And yes, there are clearly two 120 volt vectors out of phase by 180 degrees....so 120-(-120) equals 240.

    I know I'm splitting hairs here...but the voltage delivered to a USA home is clearly 240 volts. With 120 volts at are fingertips as well.

    I'm simply pointing this out so someone not familiar with the home panel doesn't get confused.
     
  15. Oct 12, 2011 #14

    turbo

    User Avatar
    Gold Member

    This is a clear re-statement, and I'm OK with it. A little refinement - the electrician that wires your home should try to balance the loads on the legs of your breaker-box. This often was not the case when older homes were wired years back, and people started adding freezers, refrigerators, dryers to the mix. It was much easier for electricians back when AC was meant to provide lighting and smaller loads like hand-mixers, pop-up toasters, etc.
     
  16. Oct 12, 2011 #15

    psparky

    User Avatar
    Gold Member

    Yes, just a clear restatement, indeed.

    Also, the centertap is grounded like you said......and the nuetral and ground tied together in the panel.

    That being said....it is my belief there is some "trickle" current running thru the ground between the transformer and house panel. The current can easily go thru the hot conductors or neutral....but there is also a path thru the ground. Way, way higher in resistance, but the high resistance will still take on trickle current......V=IR if I'm not mistaken.

    I could be wrong....if so, someone please correct my logic here. Then again, I may be right. It happens.
     
    Last edited: Oct 12, 2011
  17. Oct 12, 2011 #16
    https://www.physicsforums.com/showthread.php?t=382007&page=2

    In addition to the definition I offered in that thread (similar to that by cmb here).

    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.

    I now add the following.

    An earth need not be part of the circuit or indeed any circuit.
     
    Last edited: Oct 12, 2011
  18. Oct 12, 2011 #17

    phinds

    User Avatar
    Gold Member
    2016 Award

    Taken by itself, I'd take that as evidence AGAINST. Velikovsky was a nut case.
     
  19. Oct 12, 2011 #18

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    Before anyone makes a statement on a thread like this they should have to declare what Country they are in and what electrical distribution is used. I spent a long time at cross purposes with US residents who have the whackiest electrical system you could ever imagine - unless you happen to live in the US, in which case you would say that the UK system is the whackiest possible system.

    In the US, the power is generated with three phase generators and the high voltage distribution is three phase but one pair of wires from a transformer 'on the pole' feeds each house (220V), with a third wire from the centre tap of the transformer. This is referred to (afaik) as the neutral. This neutral is, essentially, a local neutral. High power is taken using the 220V but low power uses the 110 V between one side and the neutral. Any imbalance in load will only affect the individual consumer. Historically, I believe the US system was 110V but the currents required for high power 110V appliances forced the modification of providing a 220 supply as well.
    In the UK, three phases are distributed right down to the lowest, domestic voltage. The 'UK neutral' is connected to all the consumers who are fed by the local transformer. Normally the houses are fed with phase 1, phase 2 and phase 3, in rotation, all along a street (230V) - hoping that the three loads will near enough balance out. Any imbalance will turn up as a small voltage (just a few volts max) between the (everybody's) neutral and the actual Earth. This neutral is just allowed to 'float' and shouldn't be connected to Earth (water pipes or stakes in the ground).
    Many statements made about one system don't apply to the other.
    Except the one that says - If your washing machine is connected directly, by a network of wires to the Earth and to every other piece of exposed metal in the house then there can never be a lethal voltage between the case and anything else that can touch in the house.
     
  20. Oct 12, 2011 #19

    cmb

    User Avatar

    I am sure I will hear those words echoing in my head, if my house ever spontaneously discharges 300kA of electrons upwards, inducing residual currents throughout!
     
  21. Oct 12, 2011 #20

    psparky

    User Avatar
    Gold Member

    Good stuff.

    I would say the three phase system is way better. But I guess USA believes single phase can deliver enough for cost comparison.

    Interesting about the floating neutral. I guess you wouldn't have any "trickle" current through ground in that case.

    Also, it is my belief that our supply voltage is 240 and center tapped to 120. Our load voltages are 230V and 110V to account for voltage drop.

    And yes, I live in USA.
     
    Last edited: Oct 13, 2011
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Ground and common clearing it up!
Loading...