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How can I tell if the receptacle outlets are grounded?

Wednesday, October 3, 2018

Just because a receptacle has a ground slot (the round hole under narrow hot and neutral slots) does not mean it is grounded. Prior to the NEC code requirement in 1960 that all residential receptacles be grounded, receptacle outlets were 2-slot and ungrounded. An example is shown below left, with a conversion adapter—also known as a “cheater” plug” in the bottom half. The additional round slot that is required in modern receptacles (below right) provides a ground connection, which is essentially an alternate route for electricity that has gone astray in an appliance and has energized a metal part of it that could become a shock hazard.

   The older receptacles are often called “2-wire,” because they are connected to a only a hot and neutral wire. The neutral provides a return route for the electricity when the appliance is turned on. Newer receptacles are “3-wire,” and are served by a hot, neutral, and ground wire. The ground wire is typically bare copper.    Because many new appliances have a 3-prong cord that cannot be plugged into an older receptacle, we sometimes find pre-1962 homes that have had their 2-slot receptacles changed out to 3-slot—but the ground slot is not connected to anything.

   Also, receptacles in newer homes may have lost their ground connection somewhere between the receptacle and main electric panel due to a loose wire connection. The ground wire connection is typically daisy-chained between a string of receptacles, so a loose ground wire connection at one receptacle will cause a loss of ground connection at all the downstream receptacles.

   We recommend testing receptacles, especially in pre-1960s homes, for this defect using a 3-light tester available at most hardware stores for about five dollars. If only the center orange light comes on when the tester is plugged in, the receptacle is not grounded. If both orange lights come on, then the receptacle is grounded and correctly wired.
   Any other combination of red and orange lights indicates that the receptacle is wired improperly, with the most common defect being reversed polarity, where the hot wire is connected to neutral lug and neutral connected to hot. In other words, the receptacle is wired backwards. A diagram showing what the different light combinations mean is printed on the receptacle tester. 


   There are three ways to repair an ungrounded 3-slot receptacle, listed below from most-expensive to least-expensive:

  1. Install a ground connection to the receptacle, typically done by running a ground wire to the receptacle, which can be difficult to accomplish.
  2. Change out the receptacle for a GFCI-receptacle. GFCI stands for Ground Fault Circuit Interrupter. Essentially, if there is a ground fault (you are being shocked), the circuit is interrupted (shut off to prevent your death). The receptacle should be marked with a sticker, which comes in the box with the receptacle, stating “NO GROUND CONNECTION.” The reason is that some appliances, such as televisions and computers, require a ground connection to work properly and should not be connected to an ungrounded GFCI-receptacle. Also, surge protection devices do not function properly without a ground connection.
  3. And last, the cheapest remedy is to change the offending receptacles back to 2-slot, which are still available at most hardware stores.

    FINAL NOTE: An uncommon defect, in which both the ground slot and neutral slot are connected to the neutral wire, called a “false ground,” is not detected by a 3-light receptacle tester. A more sophisticated electronic test device is necessary. To read more about it, see our blog post What is a false ground, bootleg ground, or cheated ground receptacle? 

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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?

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?

Why are electrical outlets and plugs polarized? 

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? 

   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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