Polarized electrical cord plugs are designed so that they can only fit into a socket/outlet one way. One of the two vertical prongs is slightly taller than the other, with a corresponding tall and short slot in the outlet, which makes it impossible to insert the plug backwards, ensuring that the “hot” and “neutral” connections cannot be reversed.

   Since household electricity at wall receptacles is a 60-hertz alternating current, meaning that the electron flow changes direction back-and-forth 60 times per second, it might seem like polarization is a pointless waste of time. But, although the current is flipping around rapidly, it is only coming from one direction—the hot wire—and the neutral simply provides a grounded connection to complete the circuit. You could visualize it as current flow being pushed and pulled from the hot side, but extending all the way around the circuit.

   Because an appliance switch is typically placed on the incoming hot wire side when it is wired correctly, everything past the switch is dead until turned on. If the wiring is backwards, the internal components of an appliance are electrified up to the point of the switch at the back end. No current is flowing, but it allows the possibility of electrical shock if someone inadvertently completes a circuit to ground with their body. Two examples would be poking inside a toaster with a knife to loosen a stuck piece of bread or touching the shell of a lamp socket—both of which could possibly shock you if the polarity of the wiring is reversed.

    Correct polarity can be defeated if the outlet itself is wired backwards, which is a safety defect we check for at a random sampling of wall outlets during a home inspection. Some appliance manufacturers overcome this potential problem by using a double-pole switch that disconnects the circuit at both the hot and neutral wires.

   Although polarized outlets and plugs were introduced in the 1880s, they were not popular at first and did not become standard until the mid-20th century. The earliest National Electric Code (NEC) that we can find that references polarized receptacles is the 1962 edition, which required outlets to be both grounding (3-prong) and polarized. We occasionally see an unpolarized outlet still in place in a vintage home with ungrounded 2-slot receptacles—like the battered, burnt, and ready-for-retirement one shown below.

    Also, see our blog post When should I replace electric receptacle outlets?

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  To learn more about electrical wiring, devices, and receptacles, see these other blog posts:

• What is the height requirement for an electric receptacle outlet? 

• Where are GFCI receptacle outlets required?

• When were GFCI receptacle outlets first required?

• Does a home inspector remove receptacle outlet cover plates?

• What is the minimum height for an exterior receptacle outlet? 

• When was the current receptacle/outlet spacing of 12-feet first required? 

• When was the three-slot (grounding) outlet/receptacle first required? 

• Why does painting an electric receptacle (outlet) make it unsafe?

• How many electrical receptacles (outlets) are required in a hallway?  

• What problems does having too many electric receptacle outlets on a single circuit cause? 

• Is a house required to have outdoor electric receptacle outlets?

• How I can tell if a receptacle outlet is tamper resistant?

• Why is there a GFCI breaker in the electric panel for the bathroom shower light and exhaust fan?

• What is a false ground, bootleg ground, or cheated ground receptacle? 

• How can adding wood paneling or a wainscot create an electrical safety hazard?

• How far apart should kitchen counter receptacles be spaced?  

• Is an ungrounded electric receptacle outlet dangerous?

• My bathroom electric receptacle/outlet is dead and there are no tripped breakers in the electric panel. What's wrong?  

   Visit our ELECTRICAL page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.