The reason, as I recall, for neutral and ground being separated has to do with Ohms law.
Neutral is a working conductor in the sense that it SHOULD normally carry 100% of the current the hot does. So, 15-20 amps in a typical residential application.
Due to many factors, the impedance of neutral can become significantly non-zero.
15 A * 0 Ohms = 0V
15 A * 5 Ohm = 75 V
etc. etc.
So it is quite possible, if not probable that neutral can be non zero. Now imagine if that the ground wire no longer exists (as in old homes/appliances), but the chassis of your equipment is bonded to neutral (old washer/dryers). See the problem? You now have gear which can develop a significant shock factor by touching it. Happened to me with an
old dryer once. :)
The entire point of a safety ground in appliances is to have any voltage which accidentally touches the chassis to drain to ground and hopefully blow a fuse or breaker.
Of course, faults can happen in the ground conductor as well, but operationally, if the ground develops a high impedance, it’s not going to normally develop a voltage because current = 0.
This part is kind of important:
Because each panel can introduce a new impedance problem, the neutral and ground must remain unbonded except at the service panel. The service panel is the first one after the meter. So, if you add subpanels, you run 4 conductors. +120, -120, Neutral and Ground. One trick here is that the ground may form a loop. You don’t necessarily have to pull the ground from the panel....but that leaves open the possibility of a future contractor not knowing this, so don’t.
This concludes my Ted talk.
Neutral is a working conductor in the sense that it SHOULD normally carry 100% of the current the hot does. So, 15-20 amps in a typical residential application.
Due to many factors, the impedance of neutral can become significantly non-zero.
15 A * 0 Ohms = 0V
15 A * 5 Ohm = 75 V
etc. etc.
So it is quite possible, if not probable that neutral can be non zero. Now imagine if that the ground wire no longer exists (as in old homes/appliances), but the chassis of your equipment is bonded to neutral (old washer/dryers). See the problem? You now have gear which can develop a significant shock factor by touching it. Happened to me with an
old dryer once. :)
The entire point of a safety ground in appliances is to have any voltage which accidentally touches the chassis to drain to ground and hopefully blow a fuse or breaker.
Of course, faults can happen in the ground conductor as well, but operationally, if the ground develops a high impedance, it’s not going to normally develop a voltage because current = 0.
This part is kind of important:
Because each panel can introduce a new impedance problem, the neutral and ground must remain unbonded except at the service panel. The service panel is the first one after the meter. So, if you add subpanels, you run 4 conductors. +120, -120, Neutral and Ground. One trick here is that the ground may form a loop. You don’t necessarily have to pull the ground from the panel....but that leaves open the possibility of a future contractor not knowing this, so don’t.
This concludes my Ted talk.