Double Insulated Tools - hdbk109220040142

DOE-HDBK-1092-2004

Basic insulation is applied to live parts to provide protection against electrical shock.

Supplementary insulation is independent of the basic insulation and provides protection against

electrical shock in case of failure of the basic insulation. Also of importance is the reinforced

insulation which consists of one or more layers of insulating material that, in itself, provides the

same degree of protection as double insulation.

For example, two layers of insulation separating an armature lamination from an armature

conductor is not double insulation. This is reinforced insulation. To achieve a double insulated

system, one layer of insulation separates the armature lamination from the armature conductor

(basic insulation) and an insulating sleeve provides a second layer between the armature

lamination and the motor shaft (supplementary insulation).

Generally, double insulated equipment is constructed so that double insulation is provided

between all live parts and (1) the accessible surfaces of the equipment, and (2) all inaccessible

parts and surfaces that are conductively connected to the accessible surfaces of the equipment.

Under certain conditions, reinforced insulation systems are acceptable when applied to

brushcaps; brushholders; commutator, and end turns of armature winding switches; power

supply cords; and internal wiring.

Power supply cords for double-insulated tools shall be jacketed and shall not include a

grounding conductor.

"Double insulated" or "double insulation" must be permanently marked on the tool. In addition

the double insulated symbol (a square within a square) may be used.

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