8 a create an air gap 11 between the insulating element 5 a and the tube 1 which is necessary to avoid electric short circuits between the tube 1 and the insulating element 5 a and or the electric heating element 4 which is arranged within the insulating element 5 a in an embodiment the distance elements 8 a and or the base 13 of the safety element 2 a are made of a material which is inflexible enough to give enough stability to the insulating element 5 a in order to avoid relative movement of the insulating element 5 a and the distance element 8 a relative to the tube 1 but even if the distance elements 8 a would allow enough movement of the insulating element 5 a the material of the distance elements 8 a which encases the insulating element 5 a would act as an insulation between the insulating element 5 a and the inner wall of the tube 1 as already described in fig2 the insulating element 5 a is encased by the distance elements 8 a on the narrow sides of the tube 1 so that in case of a relative movement the distance elements 8 a would come into direct contact with the inner wall of the tube 1 instead of the insulating element 5 a or the electric heating element 4 a further modification shown in fig3 is that the boundary area 6 shows no rib elements protruding from the boundary area 6 in the direction of the center of the tube 1 the fixation between the outer surface of the tube 1 and the inner surface of the boundary area 6 is formed by a friction bond rather than by.