AC polarity isn’t right, just because ‘it works’
Polarity in AC circuits doesn’t matter; until it does.
A common mistake of amateurs (and some experienced but untrained installers) is to assume that because house AC wiring is ‘alternating current’ moving alternately in two directions, the polarity of the circuit conductors has no significance. Most loads will in fact operate when wired backward (or connected to reversed-polarity receptacles), and the ‘it works’ test often is as far as some installers go in their understanding of proper installation.
However, wiring safety codes for AC in buildings ensure that the most dangerous conducting parts remain most guarded from contact by people. When a circuit is closed (on), current indeed flows both ways through both conductors of the circuit, and both wires will be dangerous to touch. However, when the circuit is open (off), the white (‘neutral’, grounded) wire is connected to earth and has zero voltage to most anything in the building except black-insulated wires; the black (‘hot’, ungrounded) wire is connected via a breaker to a big transformer on a utility pole outside, and its voltage cycles between zero and peak voltage, many times per second (the RMS voltage of this sign wave equates to the nominal 120V of U.S. household circuits). Touching the ‘hot’ may shock you at all times (no matter if the circuit is ‘on’ or ‘off’. Touching a ‘neutral’ usually will not harm you (only when the circuit is under load, ‘on’, and even then, often only with a fraction of the voltage in the circuit, since the wire connection to earth conducts better than people — even people who may be grasping clean metal pipe or standing bare-foot on wet, on-grade floor slabs).
An illustrative example of the safety function of polarity in AC wiring and appliances is the standard Edison-base (E26) lamp socket, which applies voltage and current across the lamp filament (or high-efficiency lamp power supply electronics) via the female ‘screw-shell’ (the ‘neutral’, grounded, pole of a 120V AC circuit), and a spring-tab, which is protected from fingers deep at the base of the socket (the ‘hot’, ungrounded leg of the polarized AC circuit).
When connected to correct AC polarity, the lamp socket of a floor lamp will be more-or-less safe to fingers. When the circuit is open (off), the screw-shell is connected solidly to ground at the breaker panel, and its voltage will be zero to ground (and zero to a basement floor, a pipe, or the metal case of a stereo plugged into a grounded circuit). No current will flow through a person who is touching the neutral screw-shell and something grounded; they would have to jamb a finger all the way into the base of the socket to touch ‘hot’ voltage. By contrast, a lamp socket connected to a reversed-polarity receptacle will have a ‘hot’ (ungrounded) screw-shell that is cycling at significant voltage to ground, while the spring-tab at the base will be ‘neutral’, and at zero volts to ground. Someone touching both the reverse-connected screw-shell and something grounded would become a conductor for significant current moving from 120V RMS ‘hot’ to zero volt ground. A lamp (aka ‘bulb’) will function when wired with either correct or reversed AC polarity. When the lamp is installed, the incandescent lamp filament (or high-efficiency lamp electronics) will complete the circuit between ground, through filament (or electronics) to circuit breaker to utility pole transformer winding, making the formerly ‘neutral’ screw-shell dangerous to touch as voltage cycles through it. However, because the lamp is installed in the screw-shell, the lamp itself becomes an insulator keeping fingers out of the socket while the socket has the potential to become energized. The heat-damaged screw-shell Edison/A19 type socket shown at left would be an exception, since the insulation around the screw-shell has cracked loose enough that fingers could come in contact even with a lamp installed.
Reverse-polarized lampholders provide an easy initial example for why polarity matters in AC wiring, but polarity matters for other AC appliances, as well. All electric devices sold for use in the United States must be tested and listed for specific uses by an underwriting agency — some deep pockets to be responsible to ensure that these products are safe. Underwriter testing verifies the safety of devices when connected to a properly polarized circuit and used as directed, but does not confirm safety when the product is energized with reversed polarity. Among other possible problems, a reverse-polarized circuit connected to an appliance will energize the entire appliance circuit (and any faults within it), so that the appliance only needs the switch to provide a path to ground (correctly polarized, the switch would merely interrupt the appliance’s path via the breaker to the neighborhood utility step-down transformer). If a reverse-polarized appliance has a fault, then it may find a person to provide a path to ground, shocking or electrocuting them regardless of whether the switch is turned ‘on’ or ‘off’. A big part of the National Electric Code (NEC) revolves around visible disconnects (switches) for turning things off when they are hurting someone or causing other damage, such as starting a fire. The switch on a reverse-polarized appliance will not de-energize the appliance, even if someone is getting shocked by it (except, counterintuitively at the very moment of an emergency, if you turned it ON, since connecting the energized appliance to an intentionally-grounded copper wire will draw most current through the wire, taking it away from the person being shocked/electrocuted). After several confusing attempts to disconnect power using the appliance switch, someone might have the presence of mind to stop torturing their victimized friend with such a reverse-polarized switch, and yank appliance cord and plug from the receptacle, but what if that connection is behind a piece of furniture? What if the person being shocked is the only one around, and they can’t move precisely because their muscles and/or brain or heart are seized from electric shock? This is not a nice way to die.
In AC circuits, polarity doesn’t matter until it does.
- Wiki.Answers: Why do you care about polarity in AC circuits?
- AllAboutCircuits.com: More on AC polarity
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