Re: Casing gives current



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.
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larrymoencurly said:

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.
FWIW,
Greg G.
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<Greg G.> wrote in message

And of course it's all hermetically sealed such that when you drop it in salt water no current escapes...I didn't think so. Use a third ground wire.
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larrymoencurly wrote:

"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 'double insulation'.
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