And maybe you can clarify this for me, but what is the reason for keeping the neutral and ground separate in the breaker box?
Since many objects that don't normally carry electricity are connected to the Equipment Grounding Conductor (EGC); including the metal parts of handheld tools and appliances; it is important that the EGC not carry normal operating currents the way the Grounded Current Carrying Conductor (Neutral) does. The hazard here is twofold.
First there would be some voltage drop on the EGC. By that I mean that there would be a difference in the voltage between the far end of a circuit and the panel. That can cause current to flow on the metal objects and surfaces that were not meant to carry current. Even under the best of conditions that difference in voltage can be a hazard to the very old and the very young.
In the event of an open in the feeder neutral all of the current of all of the 120 volt loads would be returning to the Equipment Bonding Jumper; which s that green screw that you will put in the Service Entry Enclosure's neutral busbar to bond the enclosure to the Neutral of the Service Entry Conductors; at the panel you are calling the main switch; and back to the utility's transformer via the EGCs, main bonding jumper, and the neutral of the Service Entry Conductors. That would cause a markedly elevated voltage on exposed conductive surfaces that could be dangerous to everyone.
Those 2 scenarios are why the National Electric Code (NEC); of the United States; now requires that the Neutral and the EGC be kept separated from each other so that only the current from a fault will travel on the EGC.
Not so very long ago the NEC allowed the chassis of kitchen ranges, clothes dryers and similar appliances to be bonded to the neutral so that any contact between the Hot and the metal frame would create a short circuit which would open the Over Current Protective Device (OCPD) [Fuse or circuit breaker in homes]. But in some cases the Neutral conductor of such circuits would fail open. In the absence of a separate EGC there is no pathway back to the utility's transformer from which that current originated. That would cause the conductive surfaces of the appliance to rise in voltage to a full 120 volts. So you jump out of the shower,relize you need a piece of clothing that is in the clothes drier, run down to the basement, across the conductive concrete floor, grab the metal handle of the drier, and become the guest of honor at a funeral. That is not far fetched. There are hundreds of documented cases of such electrocutions. That is another reason that the NEC now forbids using the Neutral to ground equipment and requires a separate EGC to all loads that are in dwellings. The exceptions allowed for industrial premises are few and quite unique.
For this purpose any contact between the current carrying conductors, including both the Energized Conductors [Hot] and the Neutral Conductors that connects with the EGC is a Ground Fault. If the accidental contact were between the Energized Conductor and the Neutral that would be a Short Circuit.
I do realize that is a lot to take in. If it raises questions for you just ask. The only stupid question is the one you didn't ask!
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Tom Horne