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Grounding wire in household circuitry?

  1. Jun 27, 2015 #1
    Hi guys,

    I know this sounds crazy but I do not understand why household circuits are grounded. It is this grounding that causes more harm than good in my opinion. I understand the need for it, e.g. if your washing machine's hot wire shakes loose and contacts the housing and it weren't grounded then you'd be in for quite a surprise with the next load of laundry... but if the transformer weren't grounded in the first place it wouldn't matter if you touched the housing because neither the hot or neutral wires individually would shock you.

    Without grounding you would have to complete the circuit and touch both wires to get shocked. Now I am proposing that the neither the transformer or the circuit breaker panel itself be grounded for this to be the case. Just three wires (the two ends of one side of the transformer and the center tap for 120VAC) from the pole transformer coming into the house and distributed via the circuit breaker.

    I am assuming that perhaps this is more of a solution to the needs of the power company in that they need periodic grounding of their transformers such that a lightning strike to a line would rapidly dissipate instead of surging more widely across the grid. In this case I would understand the need to ground the house's circuity as well as the transformers but otherwise this makes no sense to me.

    Any thoughts????


    http://hyperphysics.phy-astr.gsu.edu/hbase/electric/hsehld.html#c1http://postimg.org/image/f9niinv2h/6f933d44/
     
    Last edited: Jun 27, 2015
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  3. Jun 27, 2015 #2

    sophiecentaur

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    This is very true - as far as it goes. But if you have a house full of devices, some of them could have been installed years ago. It is very possible that there could be a resistive connection from 'one side' of your supply to Earth. No one would know about that; everything would be working fine. However, the 'other side' of your circuit would now have full mains volts on it so you could no longer go ahead and touch it, secure in the knowledge that you would be safe. The value of leakage resistance wouldn't need to be that low, for the 'hot' side to be able to pass a significant enough current to kill you without blowing a fuse.
    What you are suggesting is, in fact, done when connecting individual electrical devices in outdoor applications. An isolating transformer with floating outputs is often used and the protection this gives is well worth having. However, the appliances are built to a higher standard of insulation so there is very little chance of the fatal combination of a leakage to ground on one side and an exposed conductor on the other side.
    Of course, nowadays most supplies use a 'residual current' circuit breaker which cuts the supply when there is any unbalanced current in a circuit - so the Earth conductor is not so necessary. But there's no such thing as a free lunch and a circuit breaker that will protect you from serious shock will also trip at the slightest excuse - unusual loads and old equipment - which means that sometimes you can't get the supply to stay on when it would be actually considered as being quite safe. In that case, the Earth system gives you excellent protection
     
  4. Jun 27, 2015 #3
    Here's a good reason: lightning strikes. If you have a ground, everything rides up and down with the ground potential. If you don't have a ground, the lightning will couple capacitatively in to your wiring and destroy loads of equipment.

    In general, if you could maintain isolation in everything, it would be fantastic. But that isolation isn't perfect. Things DO short to ground all the time. It is important to have a reference to that ground so that you can trip a breaker.
     
  5. Jun 27, 2015 #4
    Another issue with grounding the neutral wire is the difficulty in detecting ground faults. This is becoming a significant issue with the introduction of large photovoltaic systems.

    Some high profile fires have been caused by ground faults that went undetected because the neutral was at ground already. Then, at a later time, the intended path was cut and large current flowed through the ground fault.

    This is a tough problem since, as the other posters mentioned, there are often good reasons to ground the neutral.

    http://www.solarabcs.org/about/publications/reports/blindspot/
     
  6. Jun 28, 2015 #5

    sophiecentaur

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    My point was that you do not need a 'short' to ground (implying a very low resistance path). A perfectly harmless 100mA leak will give a lethal situation.
    Also, you do not need a ground reference to detect current to growing. A modern RCD looks at unbalanced current. This is not the same as an old fashioned Earth Leakage trip, which measured the actual current in the Earth conductor, going to the Earth Spike.
    I don't know why there should be a problem detecting unbalanced current in a PV system. Is there a reference to it? [Edit: I read what those links (other post) said and they just make the point that systems should be engineered 'properly'. That really goes without saying, imo. Otherwise, I son't see any difference between PV systems and any other.]
     
  7. Jun 28, 2015 #6

    dlgoff

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  8. Jun 28, 2015 #7
    Agreed. That was the reason behind the GFI breaker. It is commonly used anywhere household appliances are likely to contact water (example: an electric shaver or a hair dryer).

    These things do exist in larger machinery. For example, three phase motors in a delta configuration are isolated from ground. The devices you are talking about are feasible and they are used for exactly the reasons you state. The problem is that you need a lot more of those protection devices and you need them to be closer to the load to avoid complications from having more than one connection to ground.

    The ground based circuitry is a result of the fact that many places actually have to deal with ground potential up close: water pumps, HVAC air handlers and compressors, washing machines, and the like. In the isolated wiring situation, you need advanced detection capabilities EVERYWHERE, or you risk injury. Note that experienced electricians have all sorts of war stories about things that made contact to ground in weird ways where they weren't supposed to. However, with devices based at ground potential, you can detect these faults over a larger number of nodes in wiring.

    The faulty assumptions here are

    1. Such devices are cheap (they're not, and you'd need more of them to trip reliably in the presence of multiple faults).
    2. There will never be more than one connection to ground

    The issue here is safety. If we had perfect devices, we wouldn't need the safety. It is usually less expensive and easier to design systems that are inherently safe around grounded wiring than around isolated wiring.
     
  9. Jun 28, 2015 #8
    On the link that I posted, check out the full report pdf link: The Ground-Fault Protection Blind Spot: Safety Concern for Larger PV Systems in the U.S:
    Particularly the two paragraphs at the top of page 1, and figure 1 (also on page 1). Then skip down to section 3 (page 4).

    The issue is apparently that in PV systems, in practice, there is always some ground leakage currrent, and the larger the PV array the larger this leakage current.
    Thus, the specification for the trip threshold for ground fault interrupters increases with the size of the PV system. This leads to a large enough "blind spot" in large PV systems that dangerous ground faults (from a fire perspective) are not detected because they are below this threshold.

    Their recommendation for making sure the system is "engineered properly", seems to be to not to do anything in the installation that could cause a ground fault in the first place (insulation techniques, wire management), and to perform specific annual preventive maintenance/inspection.

    In other words it sounds like they are essentially saying: since we cannot reliably prevent fires caused by undetectable ground faults in larger PV systems, for heavens sake make sure you don't nick any insulation when you bury that darn cable.
     
  10. Jun 28, 2015 #9
    There are grades of direct burial cable designed for certain uses. The key is to ensure that this cable is used. A lot of PV system installers are not familiar with industrial wiring standards and so they bury cable with insulation that is not suitable for exposure to rodents, or they put in a conduit bank, not realizing that often these underground conduits get condensation or flood. It's not an issue of nicking cable. It is about water, critters, sunlight, and other enemies of wiring.

    Anyone can put a system in that will last for a year or two. But most install such systems with a notion that it will last for a decade or more. That takes real engineering.
     
  11. Jun 28, 2015 #10
    Agreed,
    But, at the same time, even when all of the correct cable is used and all of the best practices are followed, we still install ground fault protection presumably because it provides the safety net for when something goes wrong with this "perfect" install.

    The papers that I cited are essentially saying that, for fire safety purposes, we may not have that safety net in large PV systems.

    10 years is not a very long time. We lost power in a section of our neighborhood because a buried cable failed shorted to ground after 27 years (I got to watch them do the repair hot!).

    Losing power once in 27 years is no big deal, but having undetected ground faults that can lead to burning down buildings should never be able to happen.
     
    Last edited: Jun 28, 2015
  12. Jun 28, 2015 #11
    That safety net is still being written. You are correct, there are very few standards dealing with large PV arrays.

    Go look up Arc-Flash hazards. This is no joke. You should stand a good distance away from these guys. Hopefully they were wearing face masks, ear protection, and Nomex suits.

    That said, you are right, and such systems typically linger for much longer than the designers ever expect them to. I'll bet the people who installed knob and tube wiring in homes back in the 1920s never dreamed that stuff would still be around to cause problems almost a century later.
     
  13. Jun 28, 2015 #12

    jim hardy

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    The reason is real simple.

    Stuff happens.
    A system that was built ungrounded will not remain ungrounded - something will get wet or a mouse will eat the plastic off a wire.
    Then you have a system that's not ungrounded like you think it is and that's dangerous.

    So we ground the system. Now an accidental ground will trip the breaker which announces that something needs to be fixed.

    That's the basic thought.
    For an in depth tutorial look up IEEE Green Book. It is well written and starts with the basics.
     
  14. Jun 28, 2015 #13

    sophiecentaur

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    But let's just plough ahead regardless!!!

    It certainly does

    This third quote is the most scary. It seems to reflect the 'fast buck' mentality (plus the political capital to be made) that's associated with the sustainability drive.
     
  15. Jun 28, 2015 #14
    I hate to be the bearer of bad news, but...

    In any new endeavor like this, there will be accidents until the industry can gain enough experience to set standards for likely safety problems. Some of them are quite predictable. Some of them may already have similar, but not prescriptive notions in the National Electric Code (The NEC is used in the US, I'm sure Europe has similar documents). The problem is that when someone is under contract to provide an installation at the lowest price, they do exactly what is legally required and not one bit more or less. So even though it might be advisable to use a better grade of cable, you can bet that nobody will use it unless they are told to via a consensus document such as the NEC.

    This is one of those areas where lawyers make loads of money until people get a clue. I have nothing nice to say about the process except that this is how standards happen. Like many such standards, it exists only because of someone's massive property loss or death.
     
  16. Jun 28, 2015 #15

    jim hardy

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    e.g. who publishes the NEC ? Not the academic IEEE but NFPA, National Fire Prevention Association.

    Imagine how the boiler and pressure vessel code came about..
    xxx.jpg

    http://machinetec.blogspot.com/2010/11/safty.html
     
  17. Jun 28, 2015 #16

    dlgoff

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    And the problems aren't just fire and shock. My parents old house doesn't have a ground wire anywhere and I can't get it sold because it's not cost effective for prospective buyers to bring it up to code.
     
  18. Jun 28, 2015 #17

    jim hardy

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    Oh no, Don -

    is it two wire Romex or conduit?

    Plaster or drywall ?
     
  19. Jun 29, 2015 #18
    The basic thinking is valid - all original wiring, not just housing had this thinking ( as well as saving cost) -- but also all industrial was underground Delta - it was learned, that is the real world grounding is safer, more relabel, easier to find the inevitable problem - etc. The final nail in the un-grounded vs grounded debate in commercial and industrial settings was probably electronics - computers, industrial controls, more sensitive devices - they suffered greatly at the voltage spikes & noise that exist in the ungrounded set up.
     
  20. Jun 29, 2015 #19

    jim hardy

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    A system that's completely floating is by definition at unknown potential respect to earth.
    So the voltage can be anything.
    Experience in industry showed that around big systems with rotating machinery, ungrounded system voltage to earth can get high enough to pierce insulation.

    So systems are "grounded" by something.
    In your house wiring that's a solid bolted connection, the "Bonding jumper".
    That s known as "Solidly Grounded System".
    In industry it common to use some impedance instead of a bonding jumper in order to limit fault current, with detection to announce presence of an unintended 'ground'. That lets equipment keep running if the flaw is non-fatal to it, so they can shut down the process gracefully.
    That is known as "Impedance Grounded System".. It is used in industry where it is expected that well trained and attentive people will keep the system in good shape.

    IEEE "Green Book", standard 142, is an excellent tutorial. I think EE curricula should include a one hour course on it.
     
  21. Jun 29, 2015 #20

    russ_watters

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    I just skimmed this, but I'm not seeing the most direct answer:
    That isn't true unless you are well insulated. If you are in a carpeted house, made of wood, you might have a chance, but what if you are leaning against the faucet while messing with the washer? Sitting on an air conditioning supply register while working on the outlet? Or have a flood? Or using an outdoor receptacle? Or have metal studs?

    Yes, there are some situations where it won't shock you to touch a hot wire, but instead of judging each individually it is much safer to not have to do that, but to provide a better path for the electricity to take than through you.

    And that also reduces fire danger.
     
    Last edited: Jun 29, 2015
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