OSHA regulation 29 CFR 1926.404(f)(7)(iv) states that electric power tools must be grounded except when they are double insulated. I quote from the Black & Decker rotary drill manual: "Do not use any adapter plugs with earthed (grounded) power tools".

Could someone please explain why power tools use adapter plugs without the ground pin and how is "double insulation" a substitute for grounding ? Thanks.

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    \$\begingroup\$ They're telling you that if your power tool is earthed (grounded) then you may not use an adapter to defeat that requirement. The adapter is not the plug on the end of the cord put there by the factory, it's something extra you add on. \$\endgroup\$ – brhans Dec 27 '18 at 16:58
  • \$\begingroup\$ Thanks, that clarifies my first question. How is "double insulation" a substitute for grounding ? \$\endgroup\$ – Frost Dec 27 '18 at 17:15

How is "double insulation" a substitute for grounding ?

Imagine that some operator is using a drill (or a sawzall), and he had hit a mains wire in the wall. There are two methods to prevent high voltage from getting to the operator.

I. Divert the threatening voltage.

But where to divert it to? The best place to divert it to is the earth ground. This is why some tools require an earth ground connection.

II. Create strong enough insulation which the threatening voltage can't cross

This is the strategy employed by double-insulated tools. Why double insulated? The standard assumes that any single layer of insulation may have a small local defect. With two isolation layers, the chance that defects on both layers will line-up are small. (See also here.)

Grounded tools are often easier to construct. They can be made lighter, or cheaper, or more performant kilo-for-kilo. But they require that the job site is wired with ground connections, which isn't always the case. Many mid-size corded tools are double insulated, which makes then more versatile.

related: Appliance classes


Metal case tools are earth grounded to bypass leakage currents from moist contaminants to exposed live conductors such as brushes.

  • Defeating this without an earth grounded pin connection, at your own risk means the case can induce a shock from leakage to earth ground ( e.g. damp basement concrete floor and leather shoes)

Plastic cases tools rely on redundant dual series paths of insulation;

  • the plastic case
  • brushless motors (BLDC) with insulated magnet wire and permanent magnet rotors with induction from insulated commutation of stator. The energy passes thru magnetism not the arc of brushes.
  • \$\begingroup\$ The type of motor doesn't matter as long as it is insulated from the chassis. by 2 layers of insulation. Most double insulated equipment has a plastic chassis and a plastic casing over it. There is a huge difference between something that has the earth removed and something that is designed to not require one \$\endgroup\$ – Lamster Jun 8 at 18:06
  • \$\begingroup\$ @Lamster this repeats what I said. “Dual path” = double-insulated. OK? \$\endgroup\$ – Sunnyskyguy EE75 Jun 9 at 15:17

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