Double Insulated Tools - hdbk10920172

DOE-HDBK-1092-98

Flexible cord sets shall be listed as an assembly by a national recognized testing laboratory (See

Section 2.5). Flexible cord sets used on construction sites shall contain the number of conductors

required for the circuit plus an equipment grounding conductor. The cords shall be hard use or extra-

hard use as specified in the NEC.

8.3.4 DOUBLE INSULATED TOOLS

The NEC references the use of Double Insulated Tools in NEC Sections 250-42(f) Ex. 4, 250-45(b)

Ex. 3, 250-45(c) Ex., 250-45(d) Ex. 2, 422-8(d)(2), 625-9(a), 625-9(b) Ex., 625-9(d) Ex., 680-22(a)

Ex. 4, 680-28, and 680-30. UL Standard UL 1097 "Double Insulation Systems for Use in Electrical

Equipment" provides the requirements for equipment marked "Double Insulation" or "Double

Insulated". Since the end product standard take precedence, the end-product UL Standard should also

be consulted when there are questions pertaining to products that require double insulation.

Double insulation is a system comprised of two insulation systems (basic and supplementary) that

are physically separted and are not subjected to temperature, contaminants and other deteriorating

factors at the same time.

Basic insultion 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 insultion 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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