From: krw@nowhere.com   
      
   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".   
      
   >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. BTW,   
   water pipes are no longer allowed as the building ground (just the   
   opposite, though - they are grounded).   
      
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