Why are electrical outlets and plugs polarized?

Wednesday, July 11, 2018

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?

• • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • •

  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?

What is the code requirement for GFCI protection for receptacles near a wet bar sink? 

What is the requirement for a service receptacle outlet for heating, air conditioning, and refrigeration (HACR) equipment? 

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? 

What is allowable voltage range at a wall receptacle outlet in a house?

When should I replace electric receptacle outlets? 

Does an electric receptacle outlet in a storage shed require GFCI protection? 

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

What is the difference between an electrical receptacle, an outlet, and a plug?

Does a washing machine receptacle outlet require GFCI protection?

What is the building code requirement for receptacle outlets at stairs and stair landings?

Can I remove a 240-volt range receptacle and hard-wire the range? 

What is a "backstab" receptacle outlet? 

Why are some electric receptacle outlets upside down (ground slot up) in a house?   

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?  

How far above a kitchen countertop do electrical outlets have to be?

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

How high above the floor do electric outlets/receptacles in a garage have to be?

How far apart should electric receptacles be spaced in a bathroom? 

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?

What are the most common problems/defects found with electric receptacle outlets during a home inspection?

   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. 

 

How To Look At A House

McGarry and Madsen's home inspection blog for buyers of  

site-built, mobile/manfuactured and modular homes

(placeholder)

Search

This

Site

Attics

Air Conditioner & Furnace Age/Size

AFCI, CAFCI,

DFCI, & GFCI

Bathrooms

Aging in Place

Appliances

Click Below  

for Links

to Collections

of Blog Posts

by Subject

Cracks

Doors and Windows

Electrical

Energy Efficiency

Fireplaces and Chimneys

Heating and Air Conditioning

Home Inspection

Hurricane Resistance

Electrical Receptacle Outlets

Electrical Panels

Garages and Carports

Common Problems

Exterior Walls & Structures

Insulation

Insurance

Life Expectancy

Mobile/Manufactured Homes

Older and

Historic Houses

Mold, Lead & Other Contaminants

Modular Homes

Metal Roofs

Plumbing

Radon

Pool and Spa

Roof and Attic

Remodeling

Safety

Site

"Should I Buy A..."

Stairs

Termites, Wood Rot

& Pests

Structure and Rooms

Wells

Water Heaters

Water Heater Age

Septic Tank Systems

Plumbing Pipes

Sinkholes

When It First

Became Code

Park Model Homes

Shingle Roofs

Stucco

Wind Mitigation Form

"Does A Home

Inspector...?"

"What Is The Difference Between..."

Brick

Concrete and

Concrete Block

Foundations

Rain Gutters

Condominiums

Crawl Spaces

Building Permits

Clay Soil

Floors

Toilets

Generators

HUD-Code for

Mobile Homes

Flat Roofs

Sprinkler Systems

4-Point Inspections

Frequently Asked Questions (FAQ)

Building Codes

Inspector Licensing

& Standards

Washers and Dryers

Kitchens

(placeholder)

Electrical Wiring

Plumbing Drains

and Traps

Smoke & CO Alarms

Top 5 results given instantly.

Click on magnifying glass

for all search results.

Lighting

Sinks

Electrical Switches

Siding

Water Intrusion