A cable (1) comprises a conductor (3), an insulating layer (2) which forms a self-supporting ceramic layer when exposed to elevated temperatures experienced in a fire, and an additional heat transformable layer (4). The additional layer (4) can be another layer which forms a self-supporting ceramic layer when exposed to fire, or it can act as a sacrificial layer which decomposes at or below the temperature that the insulating layer forms a ceramic. The addition layer can enhance the strength of the layers before, during or after the fire, the structural integrity of the insulting layer (2) after the fire, the resistance of the layers to the ingress of water after the fire, or the electrical or thermal resistance of the layers during and after the fire.