The third wire: using three-prong plugs with two-prong adapters

Johnny Morgan, Bankston, Jr., Joseph D.  |  9/15/2009 7:45:42 PM

News Release Distributed 09/15/09

When dealing with electrical appliances in the home, it’s often aggravating to have a three-prong plug with only a two-prong outlet, such as with extension cords.

Dr. David Bankston, an LSU AgCenter engineer, explains this third prong is definitely not there to give you the blues.

“The term ‘the third rail’ in relation to politics and topics that a candidate dare not discuss derives from subways in which the train wheels ran on two rails while the third rail provided electrical power to the train,” Bankston said. “In contrast to being a source of danger, the third wire in a residential electric circuit is a source of safety.”

Modern 120-volt residential electrical circuits have three color-coded wires. The three wires are a hot wire, which should be black or sometimes red; a neutral wire, which should be white; and a ground wire, which should be green or bare.

“The words should be are used in describing the wire color because the wiring may not have been installed correctly or the wire colors could have been switched,” Bankston said. “The circuit will still work, but safety is compromised. In operation, the hot wire can be thought of as the source of the power.”

The LSU AgCenter engineer said when the wiring is working as designed, the current flows from the hot wire through the appliance and back to ground through the neutral wire, which is essentially at the ground.

“Ideally, the ground wire is not involved in normal operation,” Bankston said. “The third wire comes into play when something goes wrong and an electrical path is created between the current-carrying wires and the appliance case.”

Bankston explained that when this happens, some of the current can flow through the case to ground. How much depends on how good the path to ground is. If you are touching the case, you could be the path to ground.

Without a third wire, all of the current flows through you to the ground, he said. This current could be sufficient to severely injure or kill without causing a circuit breaker to open.

With the third wire, an easy path to ground through the third wire could carry most of the current and cause the circuit breaker to open without causing bodily injury, Bankston said. With the third wire you may feel a tingle or shock, but you stand a much better chance of avoiding electrocution than without it.

Bankston said not all residences have grounded receptacles. In fact, many older residences do not have a third wire and often feature outlets that accept only two-prong plugs.

Many appliances of that era came with instructions to ground the case before using the appliance, he said. Today, three-plug-to-two-plug adapters are commonly used to plug three-wire appliances into a two-prong receptacle.

This will work, but don’t forget to properly ground that little metal tab attached to the adapter plug, Bankston said. That metal tab, when properly grounded, completes the third wire circuit. Using the adapter without grounding it means that you could be the path to ground.

“Some appliances, such as double-insulated drills, have plugs with only two prongs,” Bankston said. “These appliances cannot take advantage of the third wire. They may have been designed with additional electrical insulation, such as a plastic case on the “double-insulated” drill, or may come with instructions to ground the case.”

Double-insulated appliances rely on extra insulation to prevent you from being part of a path to ground. Some rely on you grounding the appliance. Others may limit conditions under which the appliance should be used. If in doubt, read the instructions that came with the appliance.

For more information, contact Bankston at dbankston@acenter.lsu.edu, or visit the LSU AgCenter Web site at www.lsuagcenter.lsu.com.

Johnny Morgan 

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