INSULATION LEVEL  - The impulse voltage breakdown of cables and equipment.
INSULATION RESISTANCE  - The resistance offered by an insulating material to the flow of current resulting from an impressed DC voltage.
INSULATOR  - a) A nonconducting substance such as porcelain, plastic, glass, rubber, etc; b) a device made of such material used for separating or supporting conductors to prevent undesired flow of electricity.
INTERAXIAL SPACING  - Center to center conductor spacing in paired wire; center to center spacing between conductors in a flat cable.
INTERCOM WIRE  - Wire used to connect communication instruments, telephones, telegraphs, etc.
INTERLACED TAPES  - Two or more tapes, generally of different composition, applied simultaneously in such a manner that a portion of each tape overlays a portion of the other tape.
INTERSTICE  - A space that intervenes between things.
IPCEA  - Abbreviation for Insulated Power Cable Engineers Association. Now ICEA.
IPE  - Abbreviation for irradiated polyethylene tape.
IRE  - Abbreviation for Institute of Radio Engineers, now IEEE.
IRON CONSTANTAN  - A combination of metals used in thermocouple, thermocouple wires, and thermocouple lead wires. The iron wire is positive. The constantan is the negative wire.
IRRADIATION  - In insulations, the exposure of the material to high energy emissions for the purpose of favorably altering the molecular structure by cross-linking.