Electrical Safety Online Textbook question

Discussion in 'Homework Help' started by salbando, Dec 12, 2012.

  1. salbando

    Thread Starter New Member

    Oct 13, 2012
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    In the online textbook on the Safe Circuit Design page in the seventh paragraph, it is stated, "To help ensure that the former failure is less likely than the latter, engineers try to design appliances in such a way as to minimize hot conductor contact with the case. Ideally, of course, you don't want either wire accidently coming in contact with the conductive case of the appliance..."

    However, in the sixth figure the ground wire is drawn in and this puts the neutral wire at the same potential as the frame of the appliance. Now, this makes sense to me, but what doesn't make sense is why it would be a problem to just connect the neutral wire to the frame and use a polarized plug to ensure that the frame is at the neutral's (i.e. earth) potential.

    Is the only problem really that is circumvents any GFCI protection or are there other safety concerns?
     
  2. #12

    Expert

    Nov 30, 2010
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    As you saw in the drawings, neutral is busy making the appliance operate. It carries current under normal operation and it might develop a short to the case. The ground wire is a redundant safety. It has no business carrying operating current, only fault current.

    One aspect of this is that the neutral wire has some tiny bit of resistance and it normally has current flowing through it. If that was the only "ground" and it was connected to the case, there would normally be a bit of voltage on the case. Only enough to give you a tickle, but not perfect. Adding the ground wire provides a path to ground that normally has no current at all or tiny leakage currents like a milliamp. That keeps the tickle voltage from developing and instantly blows the fuse if something goes terribly wrong with the current carrying conductors.
     
  3. salbando

    Thread Starter New Member

    Oct 13, 2012
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    Thanks, that makes sense, but since the neutral and ground wire are bonded at the breaker box, isn't that essentially connecting the frame of the appliance to the neutral wire via the ground wire (ignoring the small resistance from the wire run from appliance to breaker via neutral and from breaker back to appliance frame via ground)?
     
  4. n1ist

    Active Member

    Mar 8, 2009
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    There are four reasons that you do NOT want the neutral bonded to the frame.

    - As mentioned, since neutral is a current-carrying conductor, it will have a voltage drop. This can put it many volts above ground (think a 15A on 100 feet of 14AWG wire).

    - If you are contacting the frame of the appliance and a ground (ie, plumbing or a damp cement floor), you become a parallel path for the normal operating current flow

    - If there is a fault in the house wiring that results in an open neutral, the frame of the appliance will be at full line potential, causing a shock or fire hazard

    - Polarized plugs are not always wired correctly. Plugging an appliance with the frame bonded to neutral into a receptacle that has hot and neutral reversed would have the frame of the appliance at line potential.

    There used to be an exemption to allow this on stoves and dryers due to copper shortages in World War 2. It is no longer allowed for new or remodel installations.

    /mike
     
  5. salbando

    Thread Starter New Member

    Oct 13, 2012
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    Thanks. I think I get it. The main thing I was confused about was that I didn't see the difference between connecting the neutral at the appliance frame directly, or connecting the neutral to the appliance frame by a round-trip to the breaker box. I now see my theoretical circuit as follows:

    Hot wire resistance in series with appliance in series with neutral wire resistance going to the breaker box in series with ground wire resistance from that connection at the box back to the appliance frame. Then if I touch the frame and an earth ground, I am a parallel leg with the ground and any transient voltage will drop over me and the ground with most of the current choosing the lower resistance ground wire.

    I hope that sounds correct. I don't mean to obsess over these things, but I find that there seems to be wide spread confusion about the neutral versus the hot wire. Some of it is just poor wording, but I have to feel that the poor wording is coming from many many people not really understanding the difference between to two wires.

    They say things like, "The neutral is the return. It has zero voltage, but carries the return current."

    Of course I understand it has close to zero volts relative to ground, but my understanding is that it has different color insulation only so we humans know which incoming wire has been earth referenced. If a bird was to stand on the black wire his little floating circuitry could think the hot wire is ground and if he reaches his wing over to the white wire, he would certainly find a voltage across his little body.

    Lastly, I see from a practical point of view that a person could just think of the white wire as zero and the black wire oscillating above zero and below, but if a person was just to follow the conventional current flow, then they would have to think of it as coming in on the black wire and out on the white and then coming in on the white and out on the black 60 times a second..... Right?
     
  6. #12

    Expert

    Nov 30, 2010
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    I think you mean neutral vs ground, and I agree. I was working electronics for 20 years before I learned about why the electricians use a third wire called, "bond". I was thinking, "inside the box" (where all the transistors are) and never had to think about why a refrigerator needs 2 "grounds". Now I know, and it doesn't seem like just a stupid law any more. It really works.
     
  7. salbando

    Thread Starter New Member

    Oct 13, 2012
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    No, I meant neutral versus hot wire because my understanding is that the neutral and hot wire are one and the same wire inside the generator or turbine or whatever is creating the emf and what we see coming into our homes are just the two ends of that wire. One is color coded white because it is earth-referenced, but otherwise there is no difference between the two.
     
  8. WBahn

    Moderator

    Mar 31, 2012
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    Even if that were the only difference, it is a HUGE difference! Grab one and you look stupid, grab the other and you die stupid.

    We are also ignoring all of the transformers (and how they are wired) between the generator and your home. In most places (codes and customs vary) there is a transformer close to your home (generally serving a small number of homes - in rural areas it only servers a single home) and, in the U.S., it is centertapped and the center tap is earth grounded and serves as the neutral. The other sides are both "hot" and are out of phase to each other. Generally, half of the 110V circuits in the home use one of the hots and the neutral while the other half use the other half and the neutral. The handful of 220V circuits use the two hots directly. So only the 220V circuits resemble your notion of the two wires being basically the same.
     
  9. novice85

    New Member

    Sep 26, 2014
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    In the safety section of the textbook it talks about a down power line. If a person is standing between the ground and the hot wire touching the ground the person will be shocked. I would've thought the person would be safe because both feet are touching the same reference same scenario as a bird sitting on the power lines. Could someone explain this better for me?
     
  10. WBahn

    Moderator

    Mar 31, 2012
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    Is the person touching the hot wire, or just standing on the ground near it?
     
  11. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    This is an old thread; however, the practical need still exists.

    If a power line is on the ground, there is a voltage gradient from that line to some distance from it. If you are standing in that area of gradient, you can be shocked if your feet are separated by some distance, say 2 feet (60 cm). Hence the recommendation is to shuffle your feet, keeping them very close together when moving away from such a line. Alternatively, one should wait for help and for the line to be turned off. Some people suggest hopping. I believe that is a dangerous recommendation given the conditions that often are associated with downed lines (e.g., rain and strong winds) and the simple fact that most people will fall when trying to hop on any surface, much something like less wet grass.

    John
     
  12. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    I have called the neutral wire"return". You had AC Hot and AC return. Then we added the third wire called ground. Both ground and AC return was tied to the same point in the breaker box.

    I had systems where the alarming signals had hot, return, and shield. In a high RF environment, using just the shield as a return wire caused multiple false alarms. And in that case, I connected a return wire, the shield was tied to the ground point in both the equipment and the terminal box.
     
    Last edited: Sep 26, 2014
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