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

Confusing terms? Live, Common, Neutral, Ground...

  1. Mar 29, 2016 #1
    OK, so I've gone through probably 90 tutorials on electricity and related topics, and there's a bundle of terms I ran into, used by people ages 12-80 with a dozen different accents and twice as many time zones. (So I'm not sure how many terms are universal and how many are provincial).

    Live, Common, Ground, Neutral, R/S-Terminal, and Switch line.

    I have my own extremely limited understanding of Live (power flowing, right? is that an AC term only or does it also apply to DC?), and Vaguely understand Ground to be attaching a third wire to an object with low resistance to act as a place for electrons to seek to be at their most rested Disney Land like state of well being (I like that analogy better than the one about tortured electrons fleeing for their lives).

    But Neutral and Common? Are those variations of Ground? And R Terminal? Switchline?

    Are there terms I missed? When I do a circuit, it seems like there's only 3 wires, so why have 6+ terms?
  2. jcsd
  3. Mar 30, 2016 #2


    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    hi there

    this subject has been thrashed to death on this forum..... there are a number of references at the bottom of the page to some of those threads

    here's some basics

    Live ( also known as Hot or Active) and Neutral are usually ALWAYS related to AC power systems

    Common and negative are usually the same in a single rail DC power supply
    BUT in a dual rail PSU, you can have, say a, +12V, 0V ( common) and a -12V

    Ground has various meanings depending on what electronics school institute you attended

    Ground generally refers to the "common", "negative" or 0V rail of a PSU or piece of circuit

    There are several common references to "ground" in circuits
    chassis ground ( connected to the metal case), this may also constitute an EARTH ground that is also
    connected to actual earth via the PSU and the mains earth wire

  4. Mar 30, 2016 #3


    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    not sure about these terms specifically ... please clarify where you read these and in what context

  5. Mar 30, 2016 #4

    jim hardy

    User Avatar
    Science Advisor
    Gold Member

  6. Mar 30, 2016 #5
    I'm going to guess "switch line" is some kind of hybrid jargon of "line" and PSU switch, or some other switch. R Terminal probably came out of automotive circuits.

    So to clear things up, Common can be negative in DC, and ground in AC?
    So Neutral is also Negative in DC, and also ground in AC?
    Which means Neutral = Common? Is this never untrue?

    Chassis Ground is the version of ground I'm most familiar with.
  7. Mar 30, 2016 #6

    jim hardy

    User Avatar
    Science Advisor
    Gold Member

    "Can be" as in "Could be".

    Every author ought to define the terms he's going to use in his presentation.
    If you try to lock your mind into fixed meanings of the terms Common, Ground, and Neutral you'd better keep them general because different authors working in different fields use them differently.

    "Common" usually means the point (node) in a device where the currents returning from various sections of the device all join up to get back into the power supply.
    They might be returning to the positive side of the supply if for example the device is built with PNP transistors.
    More often they will be returning to the negative side, which is a holdover from vacuum tube days.
    In a split supply system as often used with op-amps "Common" is usually the junction of the positive and negative supplies.
    Really it's up to the draftsman who draws the schematic to decide what point he is going to call "Common".

    "Neutral" is most often used in AC power distribution. In that field it means the conductor carrying current back from a load to the source.
    In US residential wiring the neutral is connected to earth near the service entrance .
    In industrial wiring the neutral may or may not be so earthed.
    Neutral is NOT the "Ground" wire in residential wiring, Neutral is the white wire and it carries load current.
    There's a green wire for Ground and it carries current ONLY if something went wrong and allowed current to get into the ground wire by accident. Its purpose is to prevent electrocution.
    Do not confuse the two terms.

    "Neutral" in DC is an unusual use of the term and you will have to figure out what was meant by whoever mentioned it .
    No. Neutral is NOT Ground in AC distribution even though in your house wiring it's connected to earth.

    It's sometimes true.

    The complicated world will not bend to our preference for simplicity.
    Read those old threads we suggested.
    It really is a simple concept once we "get our thinking straight" (old Southern saying).

    But "Ground" is the most misunderstood concept in EE. Get it straight now and you'll be way ahead.
    IEEE 142 , the Green Book , is a great introduction.

    old jim
    Last edited: Mar 30, 2016
  8. Mar 31, 2016 #7


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Switched line or switched live?
  9. Mar 31, 2016 #8


    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    There is your main problem, I think. The Power is no more flowing in the live than in the neutral, in a single phase system. The power flows because of the currents and voltages associated with all the conductors and not just one. That statement looks long winded but it takes account of what goes on with three phase systems.
    You will often find conflicting terminologies amongst the various articles and books you may read. However, if you read an article (a reputable one) then there will be some definitions given (perhaps in a diagram) and that is what will apply consistently to that particular article. Hopping between articles and just looking at the terms used is risky and won't help your understanding. This is a modern problem, of course. Articles that you can get on-line can be full of absolute rubbish - especially those that claim the 'simplify' the topic - and you cannot rely on them. That can even apply to some of the posts on PF!!!. A text book (on-line or paper) is a far more reliable source.
  10. Mar 31, 2016 #9

    jim hardy

    User Avatar
    Science Advisor
    Gold Member

    I was unaware it had a name R , but i learned about it on my 1953 Ford which was a 6 volt system.
    There exists a "Ballast Resistor" whose purpose is to set the current through ignition coil while points are closed. They want a couple amps.....
    When you're starting the car battery voltage drops because the starter pulls so much current, maybe clear down to 4 volts.
    That causes spark to be weak right when you need it most.
    So the starter solenoid on old 6 volt Fords , which is mounted on the fenderwell adjacent battery, has a contact that bypasses the ballast resistor during starting , to give a hotter spark while cranking.
    Ford kept that feature with its 12 volt cars .

    Your description of the Dodge starter describes the same feature but the solenoid is mounted on the starter instead of the fenderwell.
    I have no idea why they call it the R terminal. Maybe the first letter of some German word for that function ?

    I have no idea if it's called R on other makes.

    Understand the principle and the jargon becomes unimportant. A relay by any other name will switch as sweet.
    Last edited: Mar 31, 2016
  11. Mar 31, 2016 #10

    jim hardy

    User Avatar
    Science Advisor
    Gold Member

    It should be called "Chassis" not "chassis ground".
    Chassis is insulated from "Ground" by the tires ( in England, from earth by the tyres) .
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted