the term "common"

Discussion in 'General Electronics Chat' started by shelly50, Jan 17, 2006.

  1. shelly50

    Thread Starter New Member

    Jan 17, 2006
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    We have been having a conflict at work. I have worked in the electronics field for 10 years. I recently landed a new job doing Industrial electronics and my new boss insists that the hot wire (Black) is considered the "common" wire. I have always known the neutral wire to be common in AC and the black, or ground wire to be common in DC.

    Is there ever a time when the term is switched????
     
  2. n9xv

    Senior Member

    Jan 18, 2005
    329
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    Never! At least not here in the USA. Common refers only to nuetral which is ultimately connected to ground.
     
  3. alim

    Senior Member

    Dec 27, 2005
    113
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    You may be having a problem with standards.IN THE British system BLACK is neutral.in AMERICAN system BLK. IS HOT.The black wire in DC is called NEGATIVE or COMMON. In the british system theBLK. is called neutral. I dont know how a HOT wire COULD BE CALLED COMMON.Common to what? YOUR BOSS probably comes from a commonwealth country.
     
  4. n9352527

    AAC Fanatic!

    Oct 14, 2005
    1,198
    4
    I thought in the UK the neutral is blue and the live wire is brown. At least that is how a UK plug should be wired. There is no black wire in the standard UK mains cable, only blue, brown and green/yellow. Are the cables different between fixed wiring and the plug extension?

    Edited: Just ignore this post. Fixed wiring in the UK does use red for live and black for neutral. Blue and brown are reserved for flex cables. I'll remember that when I attempt to re-wire my house in the future :p Thank you Google!
     
  5. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Hi,

    Just to get through the colors in US 120 VAC, green is ground - can also be bare wire. White is common. Black is hot, as is red. I'm not sure about industrial 480 VAC colors.
     
  6. n9xv

    Senior Member

    Jan 18, 2005
    329
    1
    shelly50,

    Here's how it is for residential wiring;

    for a 2-phase 240-VAC system (two 120-VAC lines 180-degrees out of phase with each other & ground).

    Black is "hot" because it carries voltage & current. White is nuetral or "common" because it does NOT carry voltage & current but provides a "common" tie point for the return path of current and is ultimately connected to ground. The bare wire (sometimes green) is a ground wire tie directly to earth ground. The bare "ground" wire and the "nuetral/common" wire are one in the same electrically speaking. Common, nuetral, & ground are the same thing in residential wiring.

    Are you working with something radically different?
     
  7. shelly50

    Thread Starter New Member

    Jan 17, 2006
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  8. shelly50

    Thread Starter New Member

    Jan 17, 2006
    3
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    We are working with PLC's. I know he's wrong but can't convince him of it. They have a terminal called DCM, or Common, he believes because it is used to give power to the output modules that it is essentially "hot". I need to ohm it out to prove him wrong. I can't get too pushy, he is my boss but is also self taught when it comes to electrical.
     
  9. Gadget

    Distinguished Member

    Jan 10, 2006
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    Common can also refer to a switch.... i.e. 3 terminals, Normally open, Normally closed, and common. In this instance the phase would go to the common terminal, as the neutral shouldn't be switched.
     
  10. n9xv

    Senior Member

    Jan 18, 2005
    329
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    I'am a bit unclear as to what he actually thinks.

    Scenario 1;

    Does he think the Black wire which he calls common is actually nuetral/ground?

    Scenario 2;

    Or, does he accept that the Black wire carries voltage/current but he just calls it common.

    If its scenario 1 then he is endangering not only his life but the life of anyone working with him. If Black is indeed "hot" then surley a simple demonstration with a DMM would prove otherwise. In any case, if he is putting you at risk with a potential of being electricuted then I would go above him if possible and clear up the color code misconception. The bottom line is - does he respect the "hot" wire as "hot" or does he think something is ground when in reallity its "hot"!

    I am wondering if he is "loosely" reffering to the Black wire as being a common supply to the output modules. Still, he should use proper terminology.

    I'am not sure of the type of personallity your dealling with there but you could try this;

    Find a typical 120-VAC outlet in the plant and take off the faceplate and ask him what he would read on a DMM. That would at least prove the color coding for 120-VAC sources. I know thats a sticky situation being your boss and all. I've worked with people before myself that seem have to an assinign way of doing things.
    But, just going by what your telling us I would say stand your ground if you know your right! There's no room for technical misconceptions in an industrial enviornment with machines that could potentially kill you.

    Keep us updated!

    Kevin
     
  11. alim

    Senior Member

    Dec 27, 2005
    113
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    HI n9xv describes this as a 2-phase 240vac system.......This system is known as a single phase three wire system,it is derived from the 3-phase distribution thru a step-down transformer.In earlier times two-phase systems were used, but were 90 degrees out of phase.If you are to connect a 240v motor to the source it would be a single phase motor.WHEN 110volt loads are connected,the neutral or common certainly has current, in fact both 110volt loads current PASSES thru the neutral,and the wire size is larger.
     
  12. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    The neutral wire of a 120VAC circuit does indeed carry current and in theory there is no voltage difference between the neutral and ground wires. In reality there will be a small voltage difference due to load imbalances.

    In theory, the neutral of a 240VAC ciruit carries no current and has no voltage difference with the ground wire. Reality again involves small load imbalances. (The two hot leads of a 240VAC circuit are 120VAC to ground, and 180 degrees out of phase with each other.)

    The ground wire carries no current in normal operation, but carries fault current when one's boss incorrectly wires things.
     
  13. n9xv

    Senior Member

    Jan 18, 2005
    329
    1
    Your right its "single phase" or split phase as it come off the secondary of the transformer. I was meaning 2 lines that were out of phase with each other.
     
  14. n9xv

    Senior Member

    Jan 18, 2005
    329
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    The neutral wire in a 120-VAC circuit does NOT "carry" current. It is a "return" path for current. If you open up a residential 120-VAC outlet and touch the neutral (white) wire with one hand and touch an earthed ground rod with the other hand you wont get electricuted because there is no voltage present and thus, no current.
    The voltage origionates from the Black (hot) wire and pushes current through the load and "returns" to ground via the neutral (White) wire. If you put an ammeter in series with the neutral wire you will of course indicate current because your completing the circuit with the return path and ammeter.
     
  15. Spoggles

    Well-Known Member

    Dec 2, 2005
    67
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    In DC control circuits where you are switching loads, you would use a + Common bus, when using solid state (Open collector NPN transistors).

    Of cource then you would be in the electronics world, not the electricial one.

    Spoggles
     
  16. Spoggles

    Well-Known Member

    Dec 2, 2005
    67
    0
    In DC control circuits where you are switching loads, you would use a + Common bus, when using solid state relays (Open collector NPN transistors).

    Of cource then you would be in the electronics world, not the electricial one.

    And also as noted eariler Black would generally be 0 volts or 'ground' and would be used as a common point (Unles you work for the phone company where RED is common +., and -48V is "Battery"

    Of cource this does not include Signal Ground, Digital Ground, Analog Ground, Building Ground..etc.etc!



    Spoggles
     
  17. alim

    Senior Member

    Dec 27, 2005
    113
    1
    Hi I do not like to respond to responses unless I feel there is an absolute need to.I n our responses we should be at least 90% accurate if not 100%.I fully agree with thingmaker3 post's, but the reply from n9xv says "neutal wire is a return path for current and does NOT "carry" current. IS this not a contradiction. I would suggest a TEST,- LOAD-UP the 110 volts cct. and place an ammeter in series with the neutral or use a clamp- on meter around the neutral---check the reading.
     
  18. n9xv

    Senior Member

    Jan 18, 2005
    329
    1
    I just said that in my previous post. I did'nt say you would'nt indicate any current when connected in series so as to "complete" the circuit. From the point of view of the source, the neutral wire is just that - neutral, ground or 0-volt reference. The neutral wire never has voltage present on it but can indicate current flowing through it to ground as it is the "return" path for the flow of curent.

    I dont know what the confusion is with that?
     
  19. alim

    Senior Member

    Dec 27, 2005
    113
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    Are we confusing members of this forum? The neutral wire is an integral part of an AC CIRCUIT.It is not there toINDICATE. Whether you put an ammeter or NOT, current will flow. Use aCLAMP ON meter and check your reading .The cct does NOT need an AMMMETER to complete the circuit. These ccts. are paralell circuits.To sum up considering only 110volts circuits---the neutral wire would have the sum of the two HOT WIRES CURRENT FLOW.
     
  20. n9xv

    Senior Member

    Jan 18, 2005
    329
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    The branch wiring extending out of the breaker pannel is all parallel from the source. A load connected to the individual outlets however, constitutes a series circuit. For example, A light bulb connected to an outlet is indeed a series circuit.
    You can expand the idea to 100s of light bulbs "parallel" connected across the outlet. However, the 100s of light bulbs reduced to their equivalent resistance constitutes a series circuit with regard to that individual 120-volt outlet.

    A 120-volt circuit has only one hot wire. A 240-volt circuit has two hot wires.

    Above, you said;

    "---the neutral wire would have the sum of the two HOT WIRES CURRENT FLOW."

    Thats exactly right! because at that point you have a SERIES circuit and would indicate the sum total of ALL the individual return currents from ALL of the PARALLEL branches.
     
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