Title, really. I recently had to reverse the polarity of a badly installed receptacle. I read that the current will work either way but I could shock myself on the light bulb socket, for instance, if the receptacle has its hot and neutral reversed. But why any of this matters when the current is constantly changing direction anyway? Why is the neutral “safe” ?
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AC is more properly called an alternating voltage source. So voltage is almost always referenced – ie it alternates with respect to something else (it measures a difference in voltage not an absolute voltage)
So the neutral provides several functions.
1) The neutral line serves as the reference level for voltage. The voltage is alternating with respect to the neutral.
2) The neutral line provides a return path for the current. “Return” here being relative again since it alternates sourcing and sinking current BUT always at the reference “0” volts. Without a neutral, a circuit will not be complete and therefore will not function.
3) For most 110V or 220V (or similar) in a household context, the neutral is also typically tied to ground. But we differentiate neutral and ground for SAFETY reasons. A badly wired or misfunctioning device may cause parts of the “neutral” line to be at “live” voltage. Almost all higher powered devices will have the case or something similar tied to ground. This way a malfunction will likely (not always!) cause a short circuit to ground and trip the circuit breaker rather than allow the device itself to present a dangerous voltage to the unsuspecting user.
4) For higher voltage consumer products (ovens, boilers etc), neutral may not be equivalent to ground. As the neutral could be tied to a different phase on a 3-phase system.
In any case, wiring simple home devices is rather straightforward and generally safe with a little precaution. So a PSA is to never try to meddle with high voltage and high power devices unless you understand at least the basics of power systems.
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