Material having a resistivity with a positive temperature coefficient

Electrically conductive, polymer based material comprising:

• (a) a matrix of a thermosetting polymer selected from the group consisting of epoxy resins, polyimides, unsaturated polyesters, silicones, polyurethanes and phenolic resins;
• and
• (b) 1 to 3% by volume conductive particles distributed through the matrix, said particles in the form of fibers of length 1-3 mm selected from the group consisting of carbon fibers, carbon fibers coated with metal or metal alloy, graphite fibers, graphite fibers coated with metal or metal alloy, graphite intercalation compound fibers, metal fibers, ceramic fibers, and ceramic fibers coated with a metal or metal alloy,
• said material having electrically resistivity which is substantially constant from about 20.degree. C. up to a defined temperature, and which increases rapidly above the defined temperature.