Electrical devices containing PTC elements

Claim 1

An elongate electrical device which has a substantially constant cross-section along its length and which comprises (a) a first electrode which is connectable to a source of electrical power; (b) a second electrode which is connectable to a source of electrical power; (c) a first PTC element which is composed of a PTC composition having a useful T s of 0° to 280° C. and which surrounds and physically contacts substantially the whole of the surface of said first electrode; (d) a second PTC element which is composed of a PTC composition having a useful T s of 0° to 280° C. and which surrounds and physically contacts substantially the whole of the surface of said second electrode; (e) at least one relatively constant wattage (RCW) element which surrounds said said first and second electrodes and said first and second PTC elements and which makes physical contact with each of said first and second PTC elements; and (f) at least one current-directing (CD) element between said first and second electrodes and between said first and second PTC elements; wherein when said first and second electrodes are connected to a source of electrical power while the whole device is below its operating temperature, current passes between the electrodes and either immediately or after an initial period of not more than 5 seconds the path of maximum current density passes through each of said first and second PTC elements and at least one RCW element, with the resistance of that current path being greater than the resistance of the current path which would be adopted if the CD element between said first and second electrodes and between said first and second PTC elements were replaced by an element of the same composition as that RCW element.