What is reversed polarity at an outlet/receptacle? Why is it dangerous?

Sunday, August 19, 2018

Reversed polarity is when the hot and neutral connections at a receptacle are wired “backwards.” Home wiring is color-coded, and the black wire is “hot,” meaning that it is electrically charged or, as it is sometimes called, the “live” wire. It’s the one that will shock you if you come in contact with it in a way that will complete a circuit to the earth. The white is called the “neutral.” It completes a circuit when connected with the hot wire through a switch, providing electric power to an appliance, and will not shock you. 

    The screws at wire terminals on the sides of receptacles are also color-coded, with brass-colored screw being for the black hot wire and the silver screw for the white neutral connection.  Also, the two blades at the end of an appliance cord are size-coded: the smaller blade is hot and larger one is neutral. Receptacles have a small and large slot, so that the cord cannot be installed backwards.

    So, between the color-coding of the wiring and the terminals, plus the different sizes of the blades and receptacle slots so that the neutral cannot go into the hot slot of a receptacle, it’s obvious that getting the hot and neutral connection right is a big deal. The reason is that reversed polarity can create a shock hazard in certain situations. 

    Because the switch is positioned before the hot wire side enters the appliance and the neutral is connected to the other end of the appliance circuitry, when the polarity is reversed the appliance circuitry is electrically charged all the time, but only functional when a switch closes the neutral wire connection and the current begins flowing. 

   The diagram above illustrates the difference in reversed polarity with a toaster. When wired correctly, the circuit is energized only up to the open switch (left). But, with reversed polarity (right), the entire circuit within the toaster is “live” up to the backside of the switch.

  So, the heating element wires in a toaster (the ones that turn red) would shock you if you stuck a knife in the toaster with reversed polarity to prod a piece of toast loose. Also, the metal shell of the light bulb socket in a lamp would cause a shock if touched when the polarity is reversed. Both of them are harmless if the wiring is correct.

   A three-light circuit tester available at most hardware stores can verify if your receptacles are wired correctly. If the two orange lights turn on when its plugged in, like in the photo below, then the receptacle wiring is correct. A red and orange light combination indicates reversed polarity.

   Although reversed polarity is usually caused by incorrect connections at the receptacle, it can also be due to wiring reversal in the electric panel or at wire connections between the panel and the receptacle. 

    Also, see our blog posts Why are electrical outlets and plugs polarized? and What are the most common problems/defects found with electric receptacle outlets during a home inspection?

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

 • What is the difference between what trips a GFCI (ground fault) receptacle and a circuit breaker?

Why is an opening in the wall around the side of an electrical receptacle outlet a safety defect?  

When was GFCI-protection for kitchen dishwasher receptacle outlet first required? 

When should I replace electric receptacle outlets? 

What are "self-contained" electrical receptacle outlets and switches?

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?  

Is a house required to have outdoor electric receptacle outlets?

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

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?  

Is there an adapter that can be placed on a two-slot receptacle to make it safe? 

How do the new tamper-resistant electric outlets work?

Why is there no bathroom electric receptacle in this old house?

How can I tell if the electric receptacle outlets are grounded? 

How far apart should the electrical receptacles be placed?

    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.

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