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Ground Fault Circuit Interrupter (GFCI) explained

blog GFCI 2One of the most common electrical problems I find during home inspections is a lack of Ground Fault Circuit Interrupter (GFCI) receptacles.  You should see these devices in several places around the house, and you’ll recognize them as having a test and a reset button.  In this blog post I’m going to describe how GFCI’s work, why they’re important, and where they should be in your house.

There are two basic risks associated with electricity: fire and electrocution.  GFCI’s help to protect people against electrocution.  They do this by constantly monitoring the electrical current flowing through them.  If everything is working properly then the current flowing through the hot and the neutral conductors will be balanced and equal.  After all electricity makes a circuit, so what goes in must come out.

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But if the GFCI detects an imbalance in the currents flowing in and out then the assumption is that some of that stray electrical current is flowing out through you and you’re being shocked, so the GFCI shuts the circuit off and stops the flow of electricity.

GFCI’s do this by passing both the hot and neutral conductors through a current transformer (CT), as shown in the figure above.  If the CT detects balanced current then power is delivered to the receptacle.  But if a person touches a live conductor, or touches an electrical enclosure that is being faulted to a live wire, then some of the current goes out through that person.  The resulting current imbalance is picked up by the CT, which sends a signal through the sensor relay and the circuit is opened and the flow of current stops.

GFCI receptacles also have a test button.  When the button is pressed a little bit of current (limited by the resistor shown) is shunted around the CT, creating an imbalance that trips the GFCI off.

The UL standard that covers GFCI’s (UL 943) requires that they trip off within 1 second on a 6 mA (milliamp) fault.  Their actual performance is typically much better, tripping off in about 0.1 second and in less than 0.03 seconds on a 20 mA fault.  By way of comparison, a healthy adult can generally tolerate a shock of up to about 50-100 milliamps before death becomes a real likelihood.

Note that GFCI’s don’t have to be built into a receptacle where you’ll plug in your toaster or hair dryer.  GCFI protection can be built into the circuit breakers in your electrical panel, or they can simply be installed in-line with the wires of the circuit in the form of a GFCI switch.

In 1975 there were approximately 650 deaths by electrocution in a residential setting in the U.S.  In 2008 there were only 50, according to the National Center for Health Statistics.  That’s a dramatic drop, and there are several reasons, including newer housing electrical systems in general, the use of double-insulated power tools, and of course the introduction and expanded use of GFCI protection.  So even though you probably don’t know personally anybody whose life has been saved by a GFCI, and you may not have heard any stories about GFCI’s saving a life, there is no doubt that the use of GFCI receptacles has saved thousands of lives over the last decades and I urge you to consider them a very important safety component of your house.

The National Electrical Code (NEC) first required GFCI’s in 1968, for underwater pool lights.  In 1971 that was expanded to include all outdoor receptacles in residential use.  The locations requiring GFCI’s have been expanding ever since, and today GFCI protection is required for all receptacles in these locations, according to the NEC section 210.8(A) and the International Residential Code section 3902:

  • All receptacles in bathrooms.
  • All receptacles that serve a kitchen countertop.
  • Receptacles that are outdoors.
  • All receptacles that are in garages.
  • Receptacles that are in unfinished basements.
  • All receptacles in crawl spaces.
  • All receptacles that are within six feet of the outside edge of a sink other than the kitchen.

There is an exception in the basement for receptacles that serve a fire or burglar alarm as long as the receptacle isn’t accessible to the homeowner.  There’s also an exception outdoors for receptacles used for de-icing equipment, again as long as the receptacle isn’t readily accessible.

Garages and unfinished basements require GFCI protection not because of water, but because concrete is actually a pretty good conductor of electricity.  So if you’re standing on a large slab of concrete then electrical current will want to go through you and through the concrete to get to the ground.  So there’s an increased risk of shock and so GFCI protection is required there.

There is a myth that in the kitchen only receptacles within six feet of the sink require GFCI protection.  This used to be true, but in 1996 the NEC was changed to require GFCI protection for all receptacles that serve a kitchen countertop, regardless of how far they are from the sink.

Older GFCI electrical receptacles seemed to be more prone to nuisance tripping, which happened when the GFCI trips off even though there is no problem.  This was usually associated with issues of electric motor startup from a clothes washer, freezer, garage door opener, sump pump, or even a bathroom exhaust fan.  Newer GFCI’s have virtually eliminated nuisance tripping.  I occasionally get push-back from people who don’t want to install a GFCI receptacle because, “It might trip off when you don’t want it to.”  But that’s exactly the point: it might trip off.  It’s supposed to trip off, because that’s how it protects people.  And with newer GFCI equipment the chances of it tripping off when it shouldn’t – nuisance tripping – has been reduced to almost zero.  And isn’t that a good tradeoff?  A very small chance that the GFCI might trip off and inconvienience you versus the chance that the GFCI might save your life.  Or the life of someone you love.

If your home was built or remodeled before the codes required GFCI protection then strictly speaking you are not required to install them.  The codes aren’t retroactive.  But as your home inspector I’m going to recommend that you install them every place that the NEC currently requires them.  Let’s all be safe out there.