Is safety ground different from battery ground?

Discussion in 'General Electronics Chat' started by geratheg, Aug 5, 2014.

  1. geratheg

    Thread Starter Member

    Jul 11, 2014
    107
    3
    Is Earth ground different from battery ground used in circuits?

    I'm curious about the ground application used for safety, such as the round plug in a 3 prong power plug.

    I learned that the ground is supposed to collect any excess electrons and could be connected all over the device, including its frame to protect the users.

    But since electrons flow from negative to positive, then would this ground be considered positive so that it can collect electrons. Or is ground neutral in this case meaning it will still collect excess electrons, but it's so big it would remain neutral.

    I always thought ground as a source of which electrons come from such as a battery's negative, not a source of which electrons go into.
     
  2. BeerBelly

    New Member

    Dec 16, 2013
    29
    5
    The safety ground is used to protect the user from electrical shock and to provide a common electrical potential as a point of reference. It has nothing to do with positive or negative, although on a DC circuit, it is more common to see negative connected here.
    The safety ground is connected to the case of an appliance so if the hot side of a circuit shorts to the case of the appliance, the fuse will blow before a person comes in contact with the case of the appliance.
     
  3. to3metalcan

    Member

    Jul 20, 2014
    228
    23
    Geratheg - ground is relative! Earth ground is ground for the wall outlet mains...it's "safety" ground because if the chassis is connected to it, the circuit can't shock you (it'll just short to its own ground instead of grounding through you.) Also, someone is confusing you about conventional current...current flows positive to negative. The carriers are "holes," not electrons. Ground (zero) is more negative than + by definition, so it can be viewed as either providing electrons or receiving holes...same thing.

    On the other hand...and I hope this isn't confusing...if your voltage supply is negative, ground IS more positive!
     
  4. geratheg

    Thread Starter Member

    Jul 11, 2014
    107
    3
    Thanks for the responses.

    I understand that current flows from positive to negative, but electrons are actually flowing from negative to positive. It's just thought as from positive to negative due to Ben Franklin's error lol. Though mathematically it doesn't affect circuits.

    What I'm trying to understand here is the flow of electrons rather than the current.

    I've never heard of the term "holes", are they considered positively charged?

    Can you elaborate on the negative voltage supply and ground part? Are you referring to the negative terminal on the power supply relative to ground? Is that what you are saying is more negative than ground?

    Edit:
    Another question: Does it matter whether ground is negative or positive? Let's put it in simple terms in the way a beginner like myself thinks: For example, right now we're using "negative" for safety ground. If we used "positive" for safety ground, would that still protect you?

    Another edit:
    I think I confused myself because I thought that every device that is grounded means you won't get static shocks. Yeah I know.... lol.
    Then thought about my question and realized you can get static shocks even if it's grounded.

    But what you will be protected from is that electrical shock of current flowing through you regardless of which direction the current is flowing, thus electrons can flow either way and you will be protected since they are most likely to go through the path of lowest resistance regardless of which direction if the device is connected to whatever ground is, whether it be negative or positive.
     
    Last edited: Aug 5, 2014
  5. to3metalcan

    Member

    Jul 20, 2014
    228
    23
    A dual power supply (say, for an op amp circuit) has three major nodes, or levels:

    (+)
    |
    |
    (0)
    |
    |
    (-)

    0 is ground. It's most accurate to call ground "zero" rather than "negative." negative numbers are less than zero, so ground looks "positive" to them. Positive numbers are bigger than zero, so ground looks "negative."

    With a single battery, you have (+) and (0). Or, if you flip the battery around (some early PNP transistor circuits are printed this way!) you have (-) and (0). This is what I mean about ground being relative...

    On the other hand, if you connect two 9V batteries in series, you have three terminals...if you treat the point where they're joined as "ground", the other two terminals are going to be +9 and -9 relative to that point.

    Yes, holes are positively charged...they're the carriers that are "missing" the electrons. Most circuits make more sense in terms of hole-flow than electron-flow.
     
  6. vk6zgo

    Active Member

    Jul 21, 2012
    677
    85
    The argument between "Electron Flow" & "Conventional Current flow" as you rightly surmised,is not important.
    "Holes" are a concept used in Semiconductors to describe "positive charge carriers",& are not commonly used in reference to normal conductors.

    "Current flow" is in fact,how Electromagnetic energy travels along a conductor.
    It doesn't work quite the way you seem to think,as Electrons don't make the full trip from one end of the conductor to the other at the speed of light(or close to ),but the Electromagnetic wave does,with the energy swapped between successive Electrons.

    In a closed circuit there are no "Excess electrons",& your understanding of Earthing is incorrect.

    In your car,one side of the battery is connected to the car body/chassis.
    Motor Mechanics often call this "Earth",although it is not connected to the actual physical Earth.
    Some older cars connected the positive terminal to the body,but modern practice is to connect the negative side to the body.

    The main reason for this practice is to save wiring,as the body becomes the return path for all the Electrical equipment in the car.

    Early transistor radios using PNP transistors & an old style radio chassis had the positive side of the battery connected to this chassis.
    More modern practice using NPN transistors has the negative side of the battery connected to a common rail.
    As in the case of the car,this connection services as a dc return path (& saves wire).

    Historically,wire Telegraph systems returned one side of the battery to an actual physical Earth ,& keyed the other onto a single "Earth return" line.
    Copper wire was expensive,so using this method saved a lot of money.
    At first sight,it looks ridiculous to use "dirt" as a conductor,but the Earth (eg,the World) has a very large cross-section,so the resistance of the connection is down to how good a connection can be made with an Earth stake.

    The very early DC power distribution systems tried using the same method,but as current draws became higher.losses in the Earth connection became a problem,so two conductor lines became the norm.
     
  7. vk6zgo

    Active Member

    Jul 21, 2012
    677
    85
    Following on,there were some advantages at the Generating end,of having one side connected to real Earth,as most buildings & infrastructure was inadvertently so connected.
    It made it easier to insulate large generators & later alternators if only one side needed to be separated from Earth.

    It therefore became the practice to connect one side of the supply to an Earth stake at the Power Station,although still using 2 wires for the power network.

    With the advent of AC,this practice was continued,with the connection referred to as a "Neutral" connection,although polyphase generation made things more complex.

    "Protective Earthing" is a practice used where the incoming Neutral line is again connected to an Earth stake where it enters a building.
    Two lines are brought away from this point---the Neutral line & a protective Earth line.
    These appear at the power socket,along with the "Live" line.

    Until quite recently,most Electrical appliances had metal cases,so there was the possibility of either the Live or Neutral lines becoming shorted to the case due to mechanical wear & tear.

    If two appliances had the opposite lines shorted to the case,they would operate perfectly OK,but if you touched
    both at once,you were "toast".

    Connecting the case to the protective Earth line meant that,at least in the case of the Live to case short,the fuse at the entry point would blow,protecting the user.

    Various countries have slightly different versions of the above systems,so a proper study of this would be a good idea.
     
    Last edited: Aug 5, 2014
  8. alfacliff

    Well-Known Member

    Dec 13, 2013
    2,449
    428
    a simple circuit may or may not need a ground. does a flashlight have a ground? but it has a + and- on the battery. ground is necessary if its part of the circuit, like the power lines behind your house, and necessary for safety, in the case of home powered apliences, and good for reducing noise in sensative equipment, but sometimes is not necessary to run the equipment.
     
  9. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,539
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  10. MrChips

    Moderator

    Oct 2, 2009
    12,440
    3,361
    Don't confuse earth ground with ground, chassis, case, shield, return, neutral, common, zero volt reference. They don't all mean the same thing.
     
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