Single or Double Insulated Older power tools were single
insulated. The wiring, motor, and switch were insulated from the
case, but the case was made of metal. It was able to conduct an
electric current. If an energized component within the tool
shorted to the case, the case could become energized. A third
wire was added to the plug. This third ground wire was connected
to the case of the tool. If an energized component shorted to
the case, a large current would flow through the low resistance
ground wire and clear the fault (trip the circuit breaker).
Newer power tools are double insulated. The wiring, motor, and
switch are insulated. The case is also made of a non-conductive
material such as plastic. Many of these newer tools do not need
a three conductor plug. They are much safer to use, especially
in damp conditions. Do not use an older metal case tool if the
plug only has two prongs. A three conductor plug can be added as
a replacement. The third conductor (green) is attached to the
case of the tool with