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In a DC circuit, is "negative" synonymous with "neutral"?

  1. Mar 16, 2016 #1
    Hi,
    I'll start by offering some context to my question. I am a mechanic by trade, and have a bachelor of technology in motorsport engineering. I have a friend who is an electrician by trade, and he likes to work on his car in his spare time. In my line of work I only deal with 12V/24VDC, negatively earthed vehicles, and use the common vernacular of positive(+) and negative(-)/earth/ground, in reference to the electrical system, such as the markings on a battery. In my friends line of work, he deals with AC circuits, and uses the vernacular of active, neutral, and earth, in reference to household power outlets or wall sockets.

    My question stems from an occasion when we were working on his car, trying to troubleshoot an electrical fault. When testing a standard relay, he stated that it was switching to neutral, where as I would say that it was switching to earth or ground(So when active, the relay completed the circuit between the component and the negative terminal of the battery). I'd never heard of the negative or return wire, in a DC circuit, referred to as neutral, so I told him that he was in error referring to negative as neutral. He disagreed.

    He proceeded to draw a 2 phase AC waveform, where the second phase was shifted 180 degrees so that it showed that the first phase and second phase were in opposite poles, and were effectively neutralising each other. He also stated that, electrically speaking, a neutral wire or terminal has no potential voltage, or has balanced charges. That all made sense, and i could accept the reason it was called neutral.

    I would argue, that in a car, in which the power is supplied by an alternator with 3 phase rectification, that the alternators circuitry only allows the peaks of the positive poles of the AC generated to enter the electrical system. In my mind, without the opposing negative poles being admitted to the system, that no neutralisation occurs, hence making the term 'neutral' not applicable to this application.

    Is this just a matter of each of us just using the vernacular of our fields to describe the same thing, or do I have a point? Or is it more complicated? Have I got it all wrong?

    Apologies for the length and complexity of the explanation for what is essentially a simple question, and also for the Queens English, but I'm Australian. Hope that doesn't put anyone off.

    Thanks in advance for any help provided.

    Regards,
    J Mc
     
  2. jcsd
  3. Mar 16, 2016 #2

    davenn

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    Hi J Mc
    welcome to PF :smile:

    nor have I, it's definitely an incorrect expression
    Neutral is used in an AC system, a single or multi phase system ... the hot wire has different names depending on your country of origin
    3 common ones ... Live, Active, Phase

    once the AC is rectified and has left the alternator as DC, it's +12 (24) and negative or 0V


    the neutral wire in an AC system has nothing to do with something being neutralised


    you are correct, his teaching is bad

    The best English :wink:

    Greetings from Sydney, Oz


    Dave
     
  4. Mar 16, 2016 #3
    Hi Dave,
    Thanks for taking the time to write back to me. It's always nice to hear that I'm right about something :smile:

    I'll probably never manage to convince my friend that he has it wrong, you know how Aussie tradies can be :wink: but that's OK, If hes the only one making that mistake then i can live with it.

    Thanks again Dave,
    J Mc
     
  5. Mar 17, 2016 #4

    sophiecentaur

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    Not the best terminology to describe what's going on.
    The output currents from each of the three phase windings add together, via the rectifier diodes, to produce a near smooth DC current out of the + terminal. There will be an equal and opposite current flowing through the Earth (-) terminal. There is no 'neutral' involved; just a + and Earth (-).
    The Neutral is only a relevant description of a conductor in a three-phase system in which it is possible to balance the loads so that all the currents flow between the phases and there is no current through the neutral. That could be where you are getting the idea of "neutralising".
    In a DC context, you could imagine a dual power supply with +12V and -12V and Earth terminals. Connect a 100Ω resistor from each terminal to Earth and you will get 120mA and -120mA flowing in each leg, but no current through Earth terminal. If you disconnect the Earth there would still be zero volts where the two resistors join.
     
  6. Mar 17, 2016 #5

    russ_watters

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    I'm a mechanical engineer, and while I get that the term used is different, I don't see much difference between the actual wires. In both cases, what is critical is that it is the wire after the load and it therefore carries current but essentially zero voltage.
     
  7. Mar 17, 2016 #6

    sophiecentaur

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    Yes - zero voltage referred to Earth. The same net Current has to flow in and out of your system but the local neutral conductor goes back to the three phase supply transformer and, at that junction, currents in the various neutrals from other users will tend to cancel and produce a much lower net value than their numerical sum.
    One of the points of using three phase systems is that the total amount of copper can be less. The Neutral conductor in the distribution network can be smaller and, of course, it is also cheaper to instal because it's at nearly zero voltage. People find the neutral very confusing and it can be if you don't start at the beginning and see why it's used in the first place. PF is chockablock with threads about it. Not helped because there are at least two major types of domestic power distribution system, used throughout the World and there is a lot of shouty arguefying by people who think that theirs is the only system. :smile:
    Good luck with this one. (Whoops - I only just saw who I was replying too. haha - this is how to suck eggs :biggrin:)
     
  8. Mar 17, 2016 #7

    jim hardy

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    Yes it is disparate vernaculars.

    Yours is better.

    One of my pet subjects....

    We EE's are guilty of terrible misuse of language.

    I prefer the concept of "Power Supply Return" , the place where currents from all loads join up to get back into the power source.

    That is vehicle chassis in metal bodied cars, vehicle skin in airplanes and missiles, circuit common in electronic gizmos.
    In aircraft it's obviously not connected to earth, nor in rubber tired vehicles. But many call it earth or ground anyway.
    In some US appliances it is tied to the third prong of the power cord which makes its way(in a properly wired building) to earth, in many appliances it is not so tied.

    We abuse the concept by mis-naming it "Ground" instead of "Power Supply Return" or "Circuit Common".
    That leads to much misunderstanding by the public, most of whom think electricity has some magical affinity for earth.
    That mistake is re-inforced by use of "Water Analogy" to teach basic electricity, for everybody's childhood experience with a garden hose is that water falls to the ground and soaks in. But Gravity and Electricity don't mix... Gravity does not pull lightning down to the ground, as it does water..

    Your concept seems to me solid.

    Have you yet in your profession encountered any pre-1956 Chryslers, or pre-1970 Volkswagens, or pre 1952 Ford tractors?
    You will find in all of them the electrical system's positive not negative is tied to chassis.
    A friend in a car stereo shop let the smoke out of three expensive audio systems before he figured out his new dune buggy was built on a 'positive ground' VW chassis.

    In house wiring (US at least) Neutral wire is at nearly earth potential, elevated only by the voltage drop along it. So neutral is a good name for it. Here in US it's white to distinguish it from green Earth wire.

    I hope you can help your friend with his terminology.

    As Lavoisier said, approximately, 'In improving our language we reason better.'

    old jim
     
    Last edited: Mar 17, 2016
  9. Mar 17, 2016 #8

    sophiecentaur

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    Sounds like a medical problem that old geysers like us could get. Well - I guess that's what it is.
     
  10. Mar 18, 2016 #9

    sophiecentaur

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    I was gobsmacked when I saw my friend's Ford Popular battery connection. (in 1963) I was convinced that was why it wouldn't start. haha. How I thought he could have ever driven it there in the first place . . . . . .
     
  11. Mar 18, 2016 #10

    meBigGuy

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    I'm positive he should not call the negative terminal neutral. I'm pretty well grounded in this subject (and not neutral), and must point out that the negative terminal could be the "active" terminal and the positive terminal grounded to chassis, and is therefor closer to being what he is referring to when he incorrectly says neutral.. Nature is neutral about whether the positive or negative terminal is grounded to chassis.
     
  12. Mar 19, 2016 #11

    sophiecentaur

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    If that is the current way of thinking then there is a potential difference of opinion. If members cannot conduct themselves better then wire they not being induced to choose their words more carefully?
     
  13. Mar 19, 2016 #12

    anorlunda

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    Hats off to those people so skilled at bending the English language.
     
  14. Mar 19, 2016 #13

    jim hardy

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    Hijack Alert
    When he killed the Mudjokivis,
    Of the skin he made him mittens,
    Made them with the fur side inside,
    Made them with the skin side outside,
    He, to get the warm side inside,
    Put the inside skin side outside;
    He, to get the cold side outside,
    Put the warm side fur side inside.
    That's why he put fur side inside,
    Why he put the skin side outside,
    Why he turned them inside outside.

    i wasn't clever enough to modify it for subject of this thread.

    credit to www.the-athenaeum.org/poetry/detail.php?id=66
     
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