I see many power tools with large areas of exposed metal on the
outside, yet they're still classified as "double insulated." How can
this be? For example, corded electric drills are often aluminum in
the front 2" of the body, circular saws almost always have a metal
blade guard, and in both situations this metal is in direct contact
with metal parts of the motor.
Double insulation refers to the fact that the armature has two levels
of insulation - the windings are insulated, and the bobbin that the
windings are on is insulated from the motor shaft with an epoxy insert
that provides an additional layer of insulation. Generally, the
brushes and wiring are also contained in a plastic housing. The metal
portion principally houses the reduction gears and bushings/bearings.
"Double insulated" doesn't mean there are no exposed metal parts (although
an all plastic outer shell might be one way to achieve it). It means there
are two, separate, layers of insulation between the live parts and anything
exposed to human contact. The theory being that if one fails the other
still provides isolation and, further, that the one likely to fail would
be, for example, the motor windings, rendering the tool unusable (I.E. a
second failure unlikely since why would you be trying to use a non working
tool?), with the armature insulating bobbin (second layer) still intact and
isolating the metal parts.
That doesn't mean the thing is utterly safe from abuse or something stupid
like dropping it in a filled bathtub or some nut shoving bent paper clips
into the vent holes. It simply means that it has double protective layers
to reduce the chance of shock when it's *properly handled*.
It is, of course, still better to have the third wire ground as well as the
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