From: null@void.com   
      
   On 7/14/2015 11:04 AM, krw wrote:   
   > On Tue, 14 Jul 2015 06:15:55 -0400, "J.B. Wood"   
   >  wrote:   
   >   
   >> Hello, all.  As an EE what should be a simple electrical   
   >> circuit/networks question has me puzzled as to the rationale, and I   
   >> can't find it online or in any textbooks.   
   >>   
   >> Assume we are discussing a standard 120/240 V residential service in the   
   >> U.S.  It is required that all ground (green) and neutral (white) wires   
   >> supplying the inside facility be tied together at the service entrance.   
   >>   That much I understand; if there's a hot (black) wire fault to the   
   >> metallic part of the device connected to the ground wire, then opening   
   >> of the breaker/fuse is facilitated (but not guaranteed depending on the   
   >> amount of the fault current).  IOW, the grounded (metal shell, etc) of   
   >> the device protects the user and ideally blows the fuse.   
   >   
   > Were that the only reason, there wouldn't be a need for separate   
   > ground and neutrals at all.  The reason to have separate grounds and   
   > neutrals (so they *have* to be tied together somewhere) is that if the   
   > neutral is opened, the device still has the safety ground.  The case   
   > cannot become "hot".   
      
   Actually ground and neutral don't have to be tied together. As I   
   understand it, some systems in the UK have the utility transformer   
   neutral earthed at the transformer, then an insulated neutral is   
   supplied to buildings. The building "ground" is earthed at the building,   
   but not bonded to the neutral. The ground/earth is not  adequate to   
   carry fault current to trip building breakers, so RCDs are used.   
      
   >   
   >> What I don't get is the need to also connect the neutral/ground wires   
   >> via a short path (connection to metal water pipe or ground rod(s)) to   
   >> contact with soil (earth). Other than perhaps lightning protection (at   
   >> least on the neutral/ground wire side) what other advantages accrue from   
   >> this earthing?  Conversely, what would you give up with no connection to   
   >> earth?  I presume once upon a time power distribution EEs came up with   
   >> this but I'd like to appreciate their thinking.  Thanks for your reply   
   >> and comment.  Sincerely,   
   >   
   > You're right.  It's mainly for lightning protection, though not so   
   > much for direct strikes.  Think of  a cork on the ocean.  If lightning   
   > strikes close to the house, the entire house will "ride" on the same   
   > voltage (wave).  You don't want to have two such grounds because you   
   > don't want the current from the strike passing through the house.   
      
   I like the cork analagy. If there is a lighting strike with strong   
   currents earthed at your building the "ground" at your building can rise   
   thousands of volts above nearby 'earth'. Much of the protection at the   
   building is that all wiring rises together.   
      
   > BTW,   
   > water pipes are no longer allowed as the building ground (just the   
   > opposite, though - they are grounded).   
   >   
      
   Depends on what you are saying. Under the US-NEC a water supply pipe   
   that is 10 ft or more metal in contact with the earth is REQUIRED to be   
   used as an earthing electrode, just as it has since time began.   
      
   Water pipes used to be allowed to be used as a 'ground' connection   
   inside the building. That is no longer allowed (except within 5 ft of   
   the water service entrance, which is the same location where earthing   
   electrode connections are made).   
      
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