neutral vs gnd in ac wiring

Discussion in 'General Electronics Chat' started by electronis whiz, Sep 8, 2010.

  1. electronis whiz

    Thread Starter Well-Known Member

    Jul 29, 2010
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    i am confuzed about what the diference between these is. my computer repair book says that a groung just conects to the ground. but i saw in another book these are joined. i have seen braker boxes wired with them joined. i also put one together and they were joined and the inspection passed. i also saw in the computer repair book that they said that the nutral at every pole is grounded. the one box that had them joined is in a house and there is no ground wire or pole at the house. it apears to be a 100 amp box. i have also wondered on polarized plugs the wide blade is nutaral and that is conected to ground so what is the point then in having 3 prong outlets if it is grounded with just 2 prongs.

    could someone please tell me what is right.

    i have also been confuzed with ac because if it is alternating then wouldn't you get shocked from both the hot and nutaral wires. that is what it sounds like but it works like dc. i don't understand this.
     
  2. marshallf3

    Well-Known Member

    Jul 26, 2010
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    You're dealing in a science that's in a totally different world.

    Ground refers to one single point of the system that's usually connected to the actual earth via a ground rod or cold water pipe.

    Neutral is essentially at the same potential but kept separate from ground except for at one single point - and that's usually at the point where the true earth ground is obtained.

    Unfortunately this will mean that due to the currents involved, neutral wires at some distance from the true ground will not be exactly at ground potential.

    There's a very logical reason for doing electrical distribution this way. The ground circuit is not supposed to carry any current at all - it's a safety. The neutral circuit, which will be carrying current, can rise above true ground potential thus we can't trust it as being a safety.

    A "computer book" is not to be trusted with information about things like this, the NFPA's National Electrical Code (NFPA 70) clearly defines the rules and regulations regarding legally wiring buildings according to code. It also takes an average of about 4 years as a registered apprentice electrician working for a licensed firm before you can get good enough at it to attempt to take the tests involved in getting an actual Journeyman's license. It's a very intensive test and if you're lucky enough to pass it that proves you've learned how to read, interpret and apply the codes as written as well as understanding WHY they were written that way.
     
  3. Wendy

    Moderator

    Mar 24, 2008
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    All of what marshallf3 said is true. I don't know when the transition happened, but back in the 50's they only used 2 wires, hot and neutral. As it happened it was a bad idea.

    If you have an ohm or so in the wires and 10 amps you can have 10 volts difference between neutral and ground. This is OK, it is supposed to work like that. Of course, having an ohm in your wiring is another issue, but it illustrates the point.
     
  4. awright

    Well-Known Member

    Jul 5, 2006
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    The ground and the neutral perform completely different functions despite the fact that they are at the same nominal potential.

    Under normal conditions, the neutral carries exactly the same current as the hot line and the ground carries no current. In the event of a fault in the powered equipment that would cause the line voltage to be applied to the chassis or frame of the powered appliance, the ground conductor would hold the voltage on the chassis at close to ground potential, thus avoiding a hazardous condition that could lead to shock or electrocution of anyone touching the appliance. The current that results from the line conductor being in contact with the grounded appliance frame would cause the breaker to trip or the fuse to blow, thus protecting the user.

    A polarized two-conductor plug does not provide this level of safety, although ideally it can assure that the hot and the neutral conductors are connected inside the appliance in the safest manner. Examples include connecting the line to the center contact of a lamp receptacle rather than to the shell of the base which can more easily be accidentally touched or connecting the line first to the fuse or the power switch of an appliance so that when blown or turned off no line voltage is present inside the appliance except on one switch terminal.

    The polarized plug is unable to safely provide the grounded chassis and, thus, does not provide the level of safety provided by a U-ground plug in appliances with metal chassis. Polarized plugs are too easily defeated by trimming the wide pin or by force so that they can be plugged in incorrectly.

    Regarding your last question: Under normal circumstances the neutral line is always at ground potential and you will not get a shock touching it (although it is a good idea to avoid touching it lest you get confused). The 120 volt "line" or "hot" conductor carries the AC voltage that varies sinusoidally between nominally +170 peak volts and -170 peak volts (which results in an "AC voltage" of 120 volts rms).

    In the U.S. system you get 240 volt power between two "hot" lines, both at 120 volts AC relative to ground but exactly 180 degrees out of phase. In this case, in the absence of a fault in the equipment, the neutral carries no current.

    awright
     
    Last edited: Sep 9, 2010
  5. Wendy

    Moderator

    Mar 24, 2008
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    The average DC level may be zero, but there is still plenty of power there, else it wouldn't be usable.

    There is some debate as to what happens if you get shocked by DC vs AC, but in either case you die. The particulars of how you die may vary a bit, but the end is the same.
     
  6. marshallf3

    Well-Known Member

    Jul 26, 2010
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    Odd this subject should come up, we just replaced a 480V 3 phase 600A feeder run yesterday that was butchered years ago due to copper thieves while the building spent 6 - 7 years as unoccupied. We're going to be using one area in a different way now and I had to get some decent current back there and later may have to send in another 400A.

    At some point in time that area must have been leased out. When we took it over the lighting worked and we rarely used it for anything but storage so I paid no attention until I tried to start up a 30 HP air handler the other day. Turned out that previously temporary tenant had patched in some 480 by sneaking some #4 AWG through a conduit run back to the original 600A breaker.

    I called my favorite commercial electrician friend out, I'm not exactly set up to pull 4 pieces of 600A conductors through a long raceway.

    Today I get to figure out a lot of other rather suspicious things I noticed while we were at it yesterday.
     
  7. electronis whiz

    Thread Starter Well-Known Member

    Jul 29, 2010
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    i am wondering too my i know some one that has an old sowing machine it has a non polarized plug but thay said that if it was pluged in one way thay got a tingle off the plate by the needle. but if you flip the plug it won't is this somthing like that.
     
  8. GetDeviceInfo

    Senior Member

    Jun 7, 2009
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    A couple of things need to be cleared up;
    Firstly, a 'neutral' cannot exsist in a circuit with less than 3 wires (excluding ground), and only carries the unbalanced current. The white wire in a 2 wire circuit is rightfully called the 'identified' conductor.

    Secondly, 'ground' is a potential, not a wire. The green wire that connects all metallic components is your 'bond'. This provides a safe path to ground for leakage currents.
     
  9. nsaspook

    AAC Fanatic!

    Aug 27, 2009
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    Third: Some circuits don't have a neutral or wired ground. Most of our critical cooling and pumping motor systems on the work site use this. Delta 3 phase 480/240 ungrounded system with no single phase loads. http://ecmweb.com/nec/electric_understanding_basics_delta/
     
    Last edited: Sep 10, 2010
  10. marshallf3

    Well-Known Member

    Jul 26, 2010
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    While true, it's just become commonplace to call the return path (white wire) as the neutral by many.

    And yes, I've got 480Y277 that only sends X1,2 & 3 to a motor. No neutral and the only ground being the conduit. 40 year old building though.
     
  11. Wendy

    Moderator

    Mar 24, 2008
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    I was going through an old book, copywrited 1917, that was a blast from the past. The concept of split phase, hot, and neutral hadn't been invented, along with basic sockets. Soldering of the wires was actively encouraged (now it is illegal if I'm not mistaken), and if you wanted to wire something it had to be hardwired into the house, or you used a light socket. Interesting reading for how much the technology has changed.

    Wiring Houses for Electric Light
     
    Last edited: Sep 11, 2010
  12. GetDeviceInfo

    Senior Member

    Jun 7, 2009
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    The code has a section dealing specifically with the 'grounding' wire. This wire is only found at the service entrance and runs directly to an acceptable earthing electrode. Nowhere else. The code seperately deals with the bonding of all equipment and metallic housings, that yes, connect back to the grounding electrode.

    The code calls for a bond, and rigid metallic conduit with joints made tight, is acceptable as the bonding path. For service connections, the bonding path has more stringent requirements. Double locknuts, lugged rings, etc.

    And even with those dramatic advancements, the code has largely evolved from the results of accident investigations.
     
  13. nsaspook

    AAC Fanatic!

    Aug 27, 2009
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    Cool stuff, I have a few old tech books. FREDERICK J.DRAKE & CO had books about everything.
    http://www.usgennet.org/usa/topic/preservation/smithy/cover.htm

    Our old farmhouse in Texas had knob wiring when I was a kid. We totally rewired the place about 1968 when we got indoor plumbing.
     
  14. marshallf3

    Well-Known Member

    Jul 26, 2010
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    If I'm wiring in something new I'll always run a separate ground wire, conduit alone isn't always that trustworthy for a good bond. I'm also a nut about using compression fittings, I wouldn't trust those single screw ones they sell for much of anything.

    I don't trust cold water pipes either, nor metal buildings on cement slabs. I'll always drive at least one long rod and more at the proper spacing if necessary.

    One thing I will make of note - the inspector will ALWAYS find something he wants you to do no matter how perfect a job is.
     
  15. GetDeviceInfo

    Senior Member

    Jun 7, 2009
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    I too always pull a bond. How many times do you find a conduit joint where someone has used the run as a ladder, only to pull the joint apart.
     
  16. marshallf3

    Well-Known Member

    Jul 26, 2010
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    That, or conduit was a convenient and sturdy support to hang things off of.

    You would not believe some of the things I've seen over the years and this building we moved into two years ago has had more than its share of problems. It was vacant for 6 - 7 years before we got into it and during that time was raided several times by copper thieves. The landlord got most of the obvious damage repaired but a lot remained hidden. In some places enough of a circuit was repaired to get some lighting circuits back on yet several cut runs they hadn't pulled the wire on yet remained in the boxes and plates were put back on - I suppose enough to make the circuits appear as untouched in order to satisfy the fire inspector.

    The building was originally used as a printing company and there were boxes everywhere; must have been a ton of equipment fed off of them. Until recently we had most of the building and I had our portion in decently code-worthy operation, still have some dead areas but I don't really need them at present. The part that pissed me off most about that end of the building was they grabbed the solid copper bus bars out of two 400A switch panels and due to the age GE couldn't deliver replacements in any decent time frame. I just got the specs and had them custom made.

    As of the first of this month we took over another 24,000 square feet a nearby company had been using for storage. They only needed minimal lighting and that's all that was working over there, now I needed the full HVAC systems, outlets, water heater, rest of the lighting etc back up. Last week I had to replace a 600A run that had been bypassed with some plain #1 AWG and get all the stepdown transformers hooked back up. I've got most of the HVAC back in order so they'll have heat but there won't be any air conditioning unless I replace an 800A run they also got a good portion of.

    Ah well, it's no worse than the building we used to be in. 58 years ago it seems that any color could be anything, (including white as a hot) ROmex was fine so long as you did it yourself and I cringed whenever I had to open a junction box as it was usually so full of wire that I haven't a clue how they ever got it closed, and you could usually expect some cloth covered in the mix.
     
  17. electronis whiz

    Thread Starter Well-Known Member

    Jul 29, 2010
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    i had a power strip i was given when the place upgraded to ups's and one of my parents house was so old there was know and tube wireing. and there was no ground on most of the outlets so i brought it here to use on my computer the house has normal wireing. the power strip somehow saw the no ground and would not work. we had a wireing problem a while ago we were working in the basement and bumped a wire that wus just soldered and taped together. it is like cloth wire. no way to tell black or whit so it still hasn't ben fixed. is this a big problem if there wired backwards if that happened. or should we just look at running new wire and puting in grounded outlets. i work on electronic stuff like repairing computer power supplies. would it be smart for me to make a gfci cord for when i test this stuff. i have a gfci extention cord i made i got the plug and cord at a yard sale for $1 it is about 25 ft but only 2 prong i didnt think gfcis could work on 2 i thought a ground was needed. or should i just use this when i test stuff. i have some old gfci outlets i could make one in a box with an old cord.
     
  18. marshallf3

    Well-Known Member

    Jul 26, 2010
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    Most GFCI's merely ensure that the amount of current going back into the neutral wire is the exact same as what's coming into the hot wire, they rarely pay any attention to the actual ground conductor.

    If you're going to be doing enough work over there to justify it I would consider having an electrician put in a small junction box for you with a proper ground. He won't like seeing the knob and tube wiring but if they've got a fair amount of experience they've all seen it at one point or the other. He will likely want to make a home run straight from the service entrance.

    Around here it can be left as is, "grandfathered in" so to speak and it's probably the same where you are but I wouldn't be surprised if some states or municipalities haven't considered requiring houses that are found to be wired that way to be rewired properly. If it's left ABSOLUTELY untouched it doesn't pose much of a hazard if it only sees low load conditions, plug a space heater into one of those old circuits and you could be asking for a house fire.
     
  19. electronis whiz

    Thread Starter Well-Known Member

    Jul 29, 2010
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    thanks that helps i think i can use that cord when testing repaired things then. so if something happens it will trip.
     
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