From: tw@dionic.net   
      
   PeterD    
     wibbled on Wednesday 10 March 2010 19:47   
      
      
   > Well, we had a nice day today... I went out and did some scouting to   
   > see how it is *really* wired (vs what the books say) and at each pole   
   > where there is a transformer, there is also a ground rod (or rods)   
   > that ground the neutral conductor at that pole. The neutrals are also   
   > bonded between poles, too, with an overhead ground, so that they are   
   > all connected together (this prevents problems if one pole's neutral   
   > to ground rod or wire is damaged). This is likely quite different from   
   > your setup in the UK, however.   
      
   That sounds quite a lot like our TN-C-S system (earth/neutral combined). The   
   multiple rodding of neutral down to the real ground is referred to as PME   
   (protective multiple earth) or MEN (multiple earth neutral). The Electicity   
   Supply regulations I believe state how often this must be done by the   
   supplier.   
      
   >>Thus, we are required to switch out the neutral in the OP's scenario,   
   >>which is pertinent, as with certain types of supply fault on our 3 phase   
   >>system, the neutral can potentially move up to 240V away from earth (and   
   >>lead to a local 1 phase outlet experiencing a large voltage between the   
   >>live and neutral, eg >300V - a scenario I have witnessed).   
   >   
   > In the US, if your neutral ever rises above ground, there is a fault   
   > and that fault must be corrected. Again, a difference in systems.   
   >   
   > Code (again USA) specifically says: "do not switch neutral."   
      
   Very interesting. We always isolate neutral along with the one live/phase at   
   the breaker panel via a DP switch which is mandatory (also supplies to   
   things in bathrooms like showers must be DP switched too). However we must   
   not switch the earth which is separate by the time the consumer sees it.   
   Neither must we ever cross bond earth and neutral within the installation   
   even if it is known to be bonded at the main fuse/meter.   
      
   It's usually the earthing systems that diverge AFAICS - even the IEE seems   
   to change its mind on the exact method of dealing with earthing from edition   
   to edition.   
      
   --   
   Tim Watts   
      
   Managers, politicians and environmentalists: Nature's carbon buffer.   
      
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