A question recently posed on Quora went something like: “If the polarity in AC changes (i.e. no positive/negative, hot/ground), why is there a hot and ground wire in the real world (worse yet neutral)…”
I am an Electrical Engineer but I am not a licensed Electrician. This is a discussion of concepts, not specific advice on electrical wiring.
Current can be thought of as flowing through a single point, but voltage (also called potential difference) is always between two points. There is no ‘polarity’ at one point. Only with respect to some other point.
With AC (alternating current), the direction of current flow in the wires and the polarity of the voltage between the wires changes in a sinusoidal pattern. The voltage reaches a positive maximum at a particular instant and half a cycle later it reaches a negative maximum.
In AC wiring, “hot,” and “neutral” really have nothing to do with polarity. “Hot” and “neutral” define the relationship of the wires with respect to ground.
“Ground” itself is a relative term. It could refer to earth ground or some other common point. For example, most vehicles have a negative “ground” system so you could say the positive battery terminal is “hot” with respect to “ground,” but the system could be just as easily designed to have the positive terminal connected to chassis of the vehicle and then the negative terminal would be considered “hot.” Either way, the chassis is not connected to actual earth ground.
In the AC power distribution system, the secondary winding of a power transformer feeding a home or business with single phase power is center tapped. The center tap is connected by a wire to earth ground at the power pole. This establishes a definite relationship to “ground” at that location.
You typically see three wires twisted together going in to a building. Two “hot” wires that are insulated, with approximately 220V between them, and one bare “neutral” wire connected to the center tap. Half of the standard 110V circuits are connected to one of the “hot” wires and the neutral. The other half of the circuits are connected to the other hot wire and the neutral. If you have a 220 appliance like an electric stove or clothes dryer, it is connected across both “hot” wires.
There is also a local ground wire. All of the outlets and appliances have a connection to this local ground.
Creating a “neutral” wire by connecting one side of the power circuit to ground enhances safety because there is one less “hot” wire. If everything is working properly then you should be able to touch the “neutral” wire with no problem because back at the breaker box “neutral” and “ground” are both connected to the same point.
However the neutral wire is part of the power supply circuit which means current flows through it. The wire and the connections have some resistance. This means that at the point where power is being used there could be a small voltage difference between the neutral and ground. It should be extremely small unless there is a problem with the wiring.
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Original newspaper column published October 2019