Ground Fault Circuit Interrupters
Lee Layton, P.E.
In the first chapter we will look at the theory of operation of GFCI’s. In chapter two, we look at the specifications and characteristics of the different types of GFCI’s. Chapter three covers code requirements, installation, and operational issues associated with GFCI’s.
After taking this course you should:
This course is intended for anyone who would like to better understand how ground fault circuit interrupters work.
Benefit to Attendees
This course will help you understand how a ground fault circuit interrupter works and the conditions where it provides protection. The course also gives the reader a good understanding of the conditions where a GFCI does not provide protection.
The physical nature of electricity is to seek the shortest path to ground. If a person makes contact with an energized conductor he may become a ready path for the current to flow to ground risking an electric shock or death from electrocution. In a residential environment the most likely cause of being shocked or electrocuted comes from an internal short in an appliance such as an electric drill, or from a damaged wire on an appliance or extension cord. The degree of current flowing through the person is dependent on the conditions at the point of contact. If the person is standing on a wet or damp floor in bare feet, the risk is much greater than if the person is wearing rubber soled shoes. Leaning against a metal appliance (e.g. washing machine) or contacting a metal pipe also increases the risk of electrocution. Of course, wearing rubber soled shoes and not contacting a metal appliance does not guarantee a shock or electrocution will not occur.
It only takes an amazing small amount of electric current to kill a person. Exposure to currents as low of 100 milliamps (ma) for only two seconds can cause death. Larger currents can cause death in much shorter times and currents as low as 10ma may temporarily paralyze muscles preventing the person from being able to release the appliance or source of the electric shock. The low currents and short times cannot be emphasized enough. It takes very little current for a very short time to kill someone. Consider someone using an electric hedge trimmer and he accidentally cuts the extension cord with the trim. The electricity has to go somewhere. If the trimmer has a metal case and he is standing on the ground, there's a very high risk that his body will form a short circuit—the path of least resistance for the current to flow through-creating a very strong likelihood that he will feel an electric shook or risk electrocution.
A ground fault circuit interrupter (GFCI) is an electronic device that can operate quickly to prevent electric shocks from electrical shorts. During normal operation of an electrical appliance, the current flowing through the conductors into a GFCI generally equals the flow of current returning from the appliance to the GFCI. However, if a “short” occurs in the appliance, or the cord, an imbalance is created between the current going to the device from the GFCI and the current returning to the GFCI. A GFCI is a device that can detect this imbalance in current flow, and, once the imbalance is detected, the GFCI opens the circuit to that no electricity can flow through the circuit, preventing electric shock or electrocution.
This course content is in the following PDF document:
In 2000 it was estimated that there was 400 million GFCI devices installed in residential applications in the United States. This is amazing considering that in 1968 there were virtually no units installed. Ground Fault Circuit Interrupters have increased the safety of electrical circuits in homes and businesses. These devices quickly isolate circuits with current imbalances to prevent electrical shocks. Of course, there are conditions where a GFCI does not offer protection, but they are still a tremendous safety device.