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Why do some plugs have three prongs and some have two?

Q:

Why do some electric tools’ plugs have three prongs, while others have only two? Is one version safer than the other?
—Tom Bowman, Conway, Ark. 

A:

Two-prong plugs are just as safe as three-prong plugs, Tom. All tools, whether grounded or double insulated, carry a first layer of protection: the rubbery insulation coating on the wires and other components inside the tool. This keeps conductive bare metal from being exposed. If this insulation should ever fail, such as from a chafed wire, then current could flow through conductive parts of the tool and shock you. To prevent this, manufacturers add a ground wire (the third prong) on some tools to safely carry errant current back to the ground circuit. 

Double-insulated tools, identified on the label with a double square, shown above, come with two-prong plugs and have nonmetallic tool housings. These tools may still have metallic parts, but they—and the user—are isolated from electrically live components by nonconductive parts of the tool body. That eliminates the need for the third grounding prong.

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