Posted on: November 22, 2016
By: Alan O'Neill
Posted in: Electrician
You may know in what situations the NEC requires you to install a ground-fault circuit interrupter (GFCI), but do you know how as it works? A GFCI is specifically designed to protect people against electric shock from an electrical system, and it monitors the imbalance of current between the ungrounded (hot) and grounded (neutral) conductor of a given circuit.
This video explained how GFCI works.
However, a GFCI doesn’t give you a license to be careless. Severe electric shock or death can occur if you touch the hot and neutral conductors in a GFCI-protected circuit at the same time because the current transformer within the protection device won’t sense an imbalance between the departing and returning current and the switching contacts will remain closed.
In addition, GFCI protection devices fail at times, leaving the switching contacts closed and allowing the device to continue to provide power without protection. According to a 1999 study by the American Society of Home Inspectors, 21% of GFCI circuit breakers and 19% of GFCI receptacles inspected didn’t provide protection, leaving the energized circuit unprotected. In most cases, damage to the internal transient voltage surge protectors (metal-oxide varistors) that protect the GFCI sensing circuit were responsible for the failures of the protection devices.
A GFCI is much more subtle. When you look at a normal 120-volt outlet in the United States, there are two vertical slots and then a round hole centered below them. The left slot is slightly larger than the right. The left slot is called “neutral,” the right slot is called “hot” and the hole below them is called “ground.” If an appliance is working properly, all electricity that the appliance uses will flow from hot to neutral. A GFCI monitors the amount of current flowing from hot to neutral. If there is any imbalance, it trips the circuit. It is able to sense a mismatch as small as 4 or 5 milliamps, and it can react as quickly as one-thirtieth of a second.
So let’s say you are outside with your power drill and it is raining. You are standing on the ground, and since the drill is wet there is a path from the hot wire inside the drill through you to ground (see How Power Distribution Grids Work for details on grounding). If electricity flows from hot to ground through you, it could be fatal. The GFCI can sense the current flowing through you because not all of the current is flowing from hot to neutral as it expects — some of it is flowing through you to ground. As soon as the GFCI senses that, it trips the circuit and cuts off the electricity.
GFCIs will also fail if you wire them improperly. The most important thing to remember when wiring them is to connect the wire originating at the breaker to the line side of the GFCI and the wire connecting downstream to the load side of the device. The GFCI terminals are clearly marked “Line” and “Load.” As an added safety improvement, one manufacturer markets a 15A, 125V receptacle with a built-in line-load reversal feature that prevents the GFCI from resetting if the installer mistakenly reverses the load and line connections.
The primary purpose of the GFCI is to protect people from fatal or severe electrical shock. However, this device has also been shown to prevent electrical fires and reduce the severity of other fires by interrupting the flow of electric current. The average homeowner might not even know what a ground fault is, but when a house’s ground fault is compromised, it can put the entire property in danger. Call (713) 812-7070 us for your home service and repair needs.
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