Thanks to Jim for his explanation of why the ground wire and the neutral
wire in an AC power distribution system are never to be connected at the
load.
Here is another explanation:
Consider a 120VAC circuit fed by a single circuit breaker in the breaker
panel. Lets say we have 120VAC measured between L1 coming out of the
breaker and the Neutral bus in the panel. We have a 15 Ampere circuit
breaker, and 14 AWG copper wire, three of them, black, white and green,
feeding several 15 amp rated outlets. Now we plug a device that is
drawing 10 Amperes from an outlet that is 39 feet (wire length) from the
circuit breaker box. Now if the power drop to the house is real good, we
still have 120VAC between the neutral buss and L1 in the load center
(breaker box) panel, but at the outlet where the 10 Amp load is plugged
in, we have only 119.8VAC. There is a 0.1 volt drop on the black (L1)
wire, and a 0.1 volt drop on the white (neutral) wire. There is no
current flowing in the green (ground) wire, which is connected to the
chassis of this 10 Amp load device, so the chassis is at zero volt
potential, relative to ground. So far, so good.
Now lets suppose that someone thought it was okay to connect the neutral
and ground together in this device. Now the return current, which should
be completely on the neutral wire, is being divided between the neutral
and the ground wires. In this example, since both the neutral and the
ground wire are both 14 AWG, the current divides equally. Now we only
have 0.05 volts drop from the 10 Amps ( 5 Amps on each wire) on the
combined parallel resistance of the ground and neutral wires. The
chassis of the device, connected to the ground wire, is now at 0.05 VAC
above ground. Not only that, another device plugged into the other half
of the duplex outlet, which is not even turned on, has it's chassis at
0.05 VAC.
Do you think 50 mV is insignificant? Think of it this way: How would you
feel about having your loved one standing barefoot on wet concrete, and
touching the chassis of this device?
Ground is ground and neutral is neutral, and never the twain shall meet.
Except in the load center panel.
DE N6KB
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