From: jerry@example.invalid   
      
   Dave Platt  wrote:   
   > In article ,   
   > Jerry Peters   wrote:   
   >> In theory, the Neutral and the Ground should be at equal potential -   
   >>thereby avoiding false trips - as that is what the GF device is looking   
   >>for - current going to Ground (or somewhere), not Neutral. If the Ground   
   >>and Neutral are not at equal potential - there may be something for the   
   >>GF device to detect.   
   >   
   > That's almost exactly backwards, in practice.   
   >   
   > When the system is in use (that is, when the Hot line is drawing   
   > current), the Neutral and Ground wires are at the same potential   
   > *ONLY* at points where they are actually bonded together - that is, at   
   > the panel or sub-panel or transformer.   
   >   
   > At other points (for example, at the outlet) they won't be at the same   
   > potential.  They can't be, because the neutral wire is carrying   
   > current and has non-zero resistance, and thus has a significant   
   > voltage drop between the outlet and the panel.  The protective-ground   
   > wire won't be carrying current, and thus has no voltage drop between   
   > the outlet and the panel.   
   >   
   > So, if a GFI considered "ground and neutral wires are not at equal   
   > potential" to be a fault condition, it would trip every time somebody   
   > turned on a light or appliance.   
   >   
   > If you want to see this demonstrated, it's not difficult to do, if   
   > you have an AC voltmeter with properly-shrouded insulated test leads.   
   > Using one half of a standard outlet, measure the voltage between the   
   > neutral and ground contacts.  If nothing on that circuit is drawing   
   > current, it should read 0 volts, or within noise-factor of that.   
   >   
   > Then, plug a 15-amp space heater into the other half of that outlet,   
   > and turn it on.  You'll almost certainly see a significant voltage   
   > develop between ground and neutral, caused by the current flowing   
   > from the outlet back to the panel through the neutral wire.  I'd   
   > expect something on the order of a volt or so to show up on   
   > the meter.   
   >   
   > If you don't see a voltage drop between ground and neutral under   
   > these conditions, it may very well mean that your outlet is   
   > mis-wired, and has ground and neutral connected together at   
   > the outlet... which is a definite no-no.   
      
   You've misquoted me, that's from Peter W. This is what I wrote:   
      
   This make no sense, the GFCI doesn't care about the ground wire, it   
   measures the difference in current between the 2 supply wires.   
      
   Look up a datasheet for the LM1851 IC, it will show sample circuits   
   for a GFCI, there's no connection to the ground wire at all. In fact   
   you can use a GFCI on an ungrounded circuit and it's still functional.   
      
   In other words I agree if the ground & neutral being at differing   
   potentials matters then there's some sort of leakage and the GFCI   
   should trip.   
      
   	Jerry   
      
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