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 a screw. The ground screw is usually colored green. The ground conductor can be tested without disassembling the tool. A multimeter, set to measure resistance, can be connected between the ground prong of the plug and the case of the tool. The resistance should be very low. It is a good idea to use a ground fault protected receptacle with a single insulated tool. A GFCI should be used when any tool is used in a damp location.
Read about GFCI receptacles in my article here…
I had an old (and I mean really old) metal case jig saw and cut through a wire in a wall component a long time ago, and found out the hard way about single insulation….lol
You often hear people say that electricity will take the path of least resistance.
That is not true.
It will take EVERY path it can; Kirchhoff’s current law.
The lower resistance path will get the most current flowing through it.
Just because a tool is double insulated doesn’t mean you can’t be shocked. DAMHIKT
There is an interesting biography on the fellow who patented the first double insulated tool the Miller Falls No. 1144 Safe-T-Drill over on oldtoolheaven.com , just look for Leonard C. Pratt there.