Overheating in the sense of PED Article 4 paragraph 1(b) means exceeding the design temperature, for instance in the case of a failure of a safety system, or through operator error.
Overheating is a hazard which cannot be eliminated through a safety system, but the risk can be minimized.
However if the design temperature is chosen to take into consideration the highest temperature in all foreseeable conditions, the hazard of overheating does not exist.
Note:
Design temperature will have to take account of the highest temperature of the material, and not only of the fluid content.