To: Michael Wright Sorry for the slow response, it took a while to dig out the references. European Norm EN 563 "Safety of machinery - Temperatures of touchable surfaces - Ergonomics data to establish temperature limit values for hot surfaces" may have what you are looking for.
It contains contact time versus temperature curves showing the threshold band between burn vs no burn for several surfaces such as metal, coated metal, plastic, ceramic and wood plus conditions and recommendations. You will also find that product standards such as IEC 950 for have specific temperature limits for touchable surfaces. These standards do not get into such things as finned, textured or flocked surfaces. I have found that justifications for such conditions can be made by making a test of touch time to discomfort comparison between flat and special surfaces. It is possible in this way to justify for safety purposes higher surface temperatures than specified in the standards for flat surfaces. Bob Johnson
