Neutral and ground wires

Discussion in 'General Electronics Chat' started by shespuzzling, Sep 20, 2011.

  1. shespuzzling

    Thread Starter Active Member

    Aug 13, 2009
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    Can anybody explain why in a 3 phase 4 wire system the unbalanced currents return path is in the neutral, while when you have a phase-ground fault it is through the ground? I'm envisioning two wye systems which are connected by an equipment ground wire, and the neutral on each side is bonded to the ground bus. I don't understand how the return current (assuming the phases are not perfectly balanced) knows the difference between the ground wire and the neutral wire.

    Any help is appreciated. Thanks!
     
  2. #12

    Expert

    Nov 30, 2010
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    I don't really know all the aspects but I can tell you that the neutral wire is a "current carrying" conductor and the "bond" is not supposed to carry current unless there is a fault. This is about words that electricians use.

    When neutral and ground are connected together, as they often are, the current can't tell the difference and choose which wire to flow through. Electricians...go figure.
     
  3. colinb

    Active Member

    Jun 15, 2011
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    EDIT: sorry, this doesn't really speak to your question... read my next post.
    One case of a phase-to-ground fault is:

    (1) the metal enclosure of a device should be connected to a path ground so that if a hot wire (one of the phases) shorts to the metal enclosure of the device, this will instantly trip the circuit breaker or fuse. If the metal enclosure were not grounded, the circuit breaker would not be tripped, and the enclosure could stay connected to a high voltage until a human touched it and was shocked.

    (2) the metal enclosure cannot be connected to the neutral conductor since it is a current-carrying conductor and therefore by Ohm's law, since the conductor has a nonzero resistance between the device and earth ground at the building power entry, there is a potentially significant voltage between the neutral conductor at the device and earth ground -- thus you could sustain a shock by touching it.
     
  4. colinb

    Active Member

    Jun 15, 2011
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    OK, I read your post again and I think I didn't really answer the question.

    What you are suggesting is that there are two parallel conductors bonded together at each end. A certain current passes through the conductors together. This total current is divided between the two conductors based on the resistance of the conductors. Think of each conductor as a resistor in a parallel resistor network. WP: Resistors in parallel.
     
  5. shespuzzling

    Thread Starter Active Member

    Aug 13, 2009
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    But isn't the neutral bonded to the ground bus bar so that they are both at ground potential, and this ultimately goes back to a grounding electrode? So if you drew this scheme out it would be two wye connections with a neutral between them and also a ground wire connecting them (the equipment ground). That's where I'm not understanding how you can differentiate between the neutral and the ground.
     
  6. shespuzzling

    Thread Starter Active Member

    Aug 13, 2009
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    Thanks colinb, just got your revised reply....that is exactly my problem. I thought the ground wire is designed to only carry current during a ground fault. Since it's bonded to the neutral, I would assume that current flows just as you described in these two parallel paths, but I know that's not the case in reality and I can't figure out why.

    #12, How can you bond something so they are at the same electric potential but not allow current to flow between them?
     
  7. colinb

    Active Member

    Jun 15, 2011
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    What are these “two wye systems” you speak of? Two power sources with common neutral but different power phases, or two different power consumption devices (e.g., two motors)? Can you sketch a schematic or block diagram of what you are thinking?
     
    Last edited: Sep 20, 2011
  8. shespuzzling

    Thread Starter Active Member

    Aug 13, 2009
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    I'm envisioning two 3 phase 4 wire switchboards that are connected together via the 3 phases and the neutral and also a ground wire. Working on finding some kind of diagram now...
     
  9. colinb

    Active Member

    Jun 15, 2011
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    You need to ensure the ground carries no current under normal conditions. Otherwise it will have a potential difference (nonzero voltage) due to Ohm's law. Ground should be tied to neutral at only one point.
     
  10. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    Consider any three phase wye load with a neutral connected. Suppose it's a 3-phase heating furnace system with slightly different resistances on each phase. A five core cable connects the power board to the three phase load with the neutral and earth conductors wired directly to their respective links and the phase conductors wired from a 3-phase protective circuit breaker or line fuses. The frame of the heating system will be earthed via the earth conductor in the cable and the neutral conductor in the cable will be wired to the common connection point on the load. Neutral and Earth are not linked at the load - only at the supply board.

    With normal operation current imbalance at the 3-phase load flows via the neutral conductor. No current can enter the earth connection at this location, since only the frame is connected to the earth conductor at the load - not at the neutral connection point.

    If a fault develops from a phase insulation breakdown to the heater frame then current will flow from that phase conductor back to the board in both the hard wired earth and neutral conductors. The split in current will depend on the conductor gauges used for N & E. It's also possible (likely) that the heating equipment is mechanically connected with metallic / conductive fixings to other equipment in the vicinity or perhaps to a metallic building frame. The other equipment and the building frame will likely have conductive paths to earth through other electrical cabling or simply by virtue of the incidental conductive earth connections formed. It's therefore possible that some part of the fault current flows in these unspecified paths as well. In multiple earthed neutral systems the multiplicity of return paths for the total fault current can become very fuzzy indeed. It's this diversity of multiple return paths that in part adds to the electrical system safety.
     
  11. GetDeviceInfo

    Senior Member

    Jun 7, 2009
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    This is the key to your query. The neutral is designed to carry a current during normal operation, while the bond is included to drain fault current only. Counting on a conductor to drain fault current, which may already have current from normal operation, would result in unwanted residual voltage levels.

    Just to specify a bit more, the Neutral and Earth are not linked at the load - only at the service panel.
     
  12. shespuzzling

    Thread Starter Active Member

    Aug 13, 2009
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    Okay, that makes sense then if the Neutral and Ground aren't connected at the load. So if you have any number of 3 phase 4 wire distribution boards connected in series, the neutral bus and ground bus will only be connected in the first board?

    And in reference to the ground wire there to drain fault current only....why is it sized so much smaller than the regular lines if in reality during a fault its current will be much larger?

    On a slightly related note...in this section, image #3: http://www.allaboutcircuits.com/vol_1/chpt_3/3.html

    Is the comparatively high resistance of the human and the ground the only reason why current won't flow from the source, through the person and ground and back to the source?
     
    Last edited: Sep 21, 2011
  13. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    In my country a distribution board (as distinct from a consumer unit) normally has an earth and neutral link - otherwise how would N & E be wired to the various loads (&/or consumer units & sub-boards) from that board?

    The wiring regulations differ from country to country.

    Presumably the expectation is that the fault current is rapidly cleared by the upstream line conductor protective device such as a fuse or breaker.

    It might be useful for you to take a look at this link which is quite comprehensive on a range of electrical installation matters - such as protection of the neutral conductor.

    http://www.electrical-installation.org/wiki/Main_Page

    http://www.electrical-installation.org/wiki/Protection_of_the_neutral_conductor
     
  14. GetDeviceInfo

    Senior Member

    Jun 7, 2009
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    Correct. Often, manufacturers of service panels include a link from Neutral to Ground. This must be removed if the panel is not the primary panel.

    Earth fault currents can be very high, but due to overcurrent protection, are typically of very short duration, as such, smaller gauge wires are adequate. The CEC (and I suspect the NEC) does have provisions for HV faults where some bonds must be replaced if an earth fault occurs.

    In North America, the neutral is not switched (nor fused). If one employs interruption, they must garauntee that all current carrying conductors open together.
     
    Last edited: Sep 21, 2011
  15. #12

    Expert

    Nov 30, 2010
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    Referring to post #6, you misunderstood me. The current can not tell which wire to flow through. It just distributes according to the actual resistance of each grounding wire.

    and just a note in passing, I see that you are in the USA, so the standard for the bond wire is 2 wire gauges smaller than the current wires. A dual 30 amp breaker would supply 10 gauge power wires with a 12 gauge bond wire.
     
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