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Electrical devices which comprise two planar electrodes and a conductive polymer element, at least a part of which is a PTC element. The conductive polymer element has an intermediate portion of increased resistance, resulting from the presence of insulating or high resistance portions within the co

Electrical devices which comprise two planar electrodes and a conductive polymer element, at least a part of which is a PTC element. The conductive polymer element has an intermediate portion of increased resistance, resulting from the presence of insulating or high resistance portions within the conductive polymer element, so that when a hot zone is formed in the PTC element, it is located at or near the intermediate portion, away from the electrodes.

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An electrical device which comprises (1) a laminar conductive polymer element, at least a part of which is a PTC element, and (2) two substantially planar electrodes which lie either side of the laminar conductive polymer element and which can be connected to a source of electrical power, said elect

An electrical device which comprises (1) a laminar conductive polymer element, at least a part of which is a PTC element, and (2) two substantially planar electrodes which lie either side of the laminar conductive polymer element and which can be connected to a source of electrical power, said electrodes being electrically connected to opposite faces of said conductive polymer element so that when the electrodes are connected to a source of electrical power, they cause current to flow through said PTC element; said device being such that, if the portion thereof between the electrodes is divided into parallel-faced slices, the thickness of each slice being about 1/5 of the distance between the closest points of the two electrodes and the faces of the slices being planes which are perpendicular to a line joining the closest points of the two electrodes, then there are at least two Type A slices, each of which (a) comprises a part of the PTC element which, when the current through the device is increased rapidly from a level at which the PTC element is in a low temperature, low resistance state to a level whch converts the PTC element into a high temperature high resistance state, increases in temperature at a rate x, and which (b) is free, within the periphery of the conductive polymer element, of portions having a resistivity at 23°C. higher than said conductive polymer and extending through the thickness of the slice, and at least one Type B slice which (a) comprises a part of the conductive polymer element which, when current through the device is increased rapidly from a level at which the PTC element is in a low temperature, low resistance state to a level which converts the PTC element into a high temperature high resistance state, increases in temperature at a rate y which is greater than x; and (b) comprises, within the periphery of the conductive polymer element, at least one first portion composed of a conductive polymer and at least one second portion comprising a material having a resistivity at 23°C. higher than said conductive polymer; each of the slices adjacent an electrode being a Type A slice.