Hi all!
I started thinking about the different systems for grounding and stumbled across a potential problem.
In the US and much of Europe the standard way of grounding a building is to connect the ground wire to the neutral at the panel, meter or main fuse. (I won't use the terms grounded and grounding wire, since I only end up confusing myself. I'll also ignore the ground rods in this case.)
At the panel you have single pole breakers, which trip if there is too much current in the hot wire. There can never be a dangerous short between neutral and ground, since they are tied together.
Now: If I add a subpanel, using a cable with a separate ground wire I will no longer have neutral and ground tied together next to the breakers. When there is load on the circuits at the subpanel, there will be a voltage drop in the neutral in the cable from the main panel. Hence, there is a potential between the ground and neutral in all circuits on the subpanel. This includes those without load. If I short circuit neutral and ground at e.g. a socket, there will be current flowing due to this. This current never goes trough the breaker. Isn't this a danger? Have I missed something somewhere?
If we assume the voltage drop is 10V in the cable from the main panel, there vill be a 5V difference between ground and neutral. Suppose there is a 2.5 mm2 cable (13 AWG) cable with a neutral-ground short circuit 3 meters (10 feet) from the subpanel. The current flowing in this will then be:
I=V/R=5/(2*3*0.017/2.5)= 123 Amps (!)
A GFI will of course eliminate the problem, but most circuits don't have GFI:s.
[This message has been edited by C-H (edited 09-30-2002).]
