Why is it that in my old
house the
Neutral and
Ground wires end up at the exact same place in the
fuse panel?
In
house wiring, there is a
bit of a funny legacy issue that kind of set the
stage for this odd combination. Houses built early last centaury did not have grounded outlets. They only had two conductors, Hot and
Neutral. The problem with this is that wires are not perfect conductors. They are actually like big resistors, that
drop voltage if the load is heavy enough. What this means is that if you have a really long run of
wire and a heavy load, the
neutral conductor at the
plug end will actually have some
voltage on it! Because of this, old "two
wire" appliances did not connect the frame to either
wire. In fact, on older two blade appliances you will see that the blades are the same size so they can get plugged in either way! As the years went by, appliances started coming out with one blade slightly larger than the other. This "larger blade" side of the
outlet was suppose to be connected to the white
neutral wire, although I often found this not to be the case when refurbishing older homes. When wiring an older lamp, you were suppose to
wire the outer shell of the lamp
socket to the wider blade in hopes that the bigger part would be less likely to
electrocute people! (Still true on two blade table lamps) The code writers still prohibited any real connection between the "larger blade" side and the case. Finally, the idea came forth to have a separate third prong that would actually be a
ground. As this third prong would not be part of the
circuit path, it would always be at "true
ground" potential, and could be connected to the case of the appliance. As long as the
ground wires are run separate from the
neutral wires, no
voltage drop will occur. If it was combined with the
neutral, some
drop will occur and it will no longer be at
ground potential. The only time the
ground wire sees any
current is when there is a short in the appliance. In these cases, this
current is looped back through the
ground and hopefully the overload quickly trips the
breaker. Because the
ground conductor may have to handle the full
circuit current when this happens, it is required to be the full approved
gauge for the
circuit in question.
When you looked in your
circuit breaker box, you saw a buss where the grounds and neutrals came together. This buss also had your service entrance
neutral connected to it, and hopefully also had a thick copper
wire that was connected to a large metal stake in the
ground. It is usually also connected to a water pipe and a gas pipe so that all are bonded together.
In a funny twist of fait, technology is eliminating the "third prong" defense with the
introduction of
ground fault detectors. These devices work like a bouncer in a club, who only lets the same amount of people enter as are leaving! If any imbalance occurs, they trip.