GFCIs — What they are and are not

March 8, 2012
Useful info on ground fault circuit interupters

Editor: Leland Teschler

Fuses and circuit breakers are included in an electrical circuit to prevent an electrical overload from damaging property, such as your house. But the electrical current needed to kill you is much lower than the amount that will cause the fuse or circuit breaker to burn out or “pop” and interrupt the electrical current flow.

When you look at the normal 120-V outlet in your home, there are two vertical slots and a round hole centered below them. The left slot is slightly larger than the right slot. The left slot is called “neutral,” and the right slot is called “hot.” The round hole is a ground connection. If an appliance is working properly, all electricity the appliance uses will flow from hot to neutral. The ground is not normally in the circuit that causes the appliance to function. The ground is normally connected to the shell of the appliance so if the hot wire touches the shell (a broken hot wire perhaps) the current from the hot wire will flow to ground and cause the fuse or circuit breaker to overheat, pop, and stop the current flow. The normal level of current necessary to actuate a breaker or fuse is generally 20 A. In contrast, the amount of current necessary to put the heart into fibrillation is less than 100 mA.

It is important to know what GFCIs do and don’t do. They are designed to prevent electrocution by detecting a difference in current flow between the hot-wire current flowing into the appliance and the current flow out of the appliance to the neutral. When this measured difference is as low as 6 mA, the GFCI will shut down the circuit within one-thirtieth of a second. This means if the measured 6 mA or more at 120 V is going through you, you will get the full impact of the shock, but the shock will not last long enough to put your heart into fibrillation, which could kill you.

As an example, consider a person using an electrical appliance outside in the rain. Suppose that person is barefoot (another of God’s ways of thinning out the gene pool). The appliance is wet, and there is a path from the hot wire through the operator to ground. Electricity flowing from hot to ground through the operator could be fatal. The GFCI will sense that current may be flowing because there is an imbalance between current flowing out of the hot wire and into the neutral wire. As soon as the GFCI senses this imbalance, it will trip and cut off the electricity.

The National Electrical Code requires GFCIs for receptacles in bathrooms, some kitchen receptacles, some garage receptacles, and outside receptacles. Because of the many electrocutions that resulted from dropping a hair dryer into the bathtub while bathing and the lawsuits that resulted from such incidents, most manufacturers of hair dryers now build in GFCIs as an integral part of hair dryers.

Remember, the GFCI will protect your heart from going into fibrillation but will not protect you from getting severe shock. And you can still get hurt. I tell my students when I am teaching Loss Control classes that, “If you are using a GFCI-protected electrical tool while standing on a 12-ft ladder and the tool shorts to the case, the electrical shock will not kill you. But the fall from the ladder could.”

Lanny Berke is a registered professional engineer and Certified Safety Professional involved in forensic engineering since 1972. Got a question about safety? You can reach Lanny at [email protected].

Edited by Leland Teschler

© 2012 Penton Media, Inc.

Sponsored Recommendations

Precision Motion Control for Robotics Systems

April 8, 2024
Learn more about Precision Motion Control for your Robotic Systems through Harmonic Drive's latest Webinar!

KNF Pumps Revolutionize Desert Dust Tracking

April 3, 2024
The Sahara Desert is one of the biggest sources of dust worldwide. KNF air sampling pumps help tracing this particulate matter air pollution.

Introducing the Mass Flow Rate Calculator

April 3, 2024
The KNF Mass Flow Rate Calculator makes it easy to convert volume flow rate to mass flow rate. Simply enter the volume flow and the gas or gas mixture.

Explore KNF's Cutting-Edge DC-BI Pump Drive Technology

April 3, 2024
With six new DC-BI pump series, KNF launches an entirely new pump drive technology relying on advanced BLDC motors and achieving unprecedented advantages.

Voice your opinion!

To join the conversation, and become an exclusive member of Machine Design, create an account today!