Lurking on the IEEE EMC-PSTC mailing list recently, there was an interesting post about temperature limits on the surfaces & handles of equipment. Safety standards will often limit how hot the surfaces can get, usually with metal surfaces and handles having lower maximum temperature limits than plastic surfaces or handles.
When I first saw this in standards, it seemed kind of backwards to me because plastic will deform & melt at much lower temperatures than metal. However, the standards (and the people who write them) are more worried in this case about people burning their hands and/or dropping the equipment when it is energized and/or being in so much pain the user jumps back into something else that might be live.
With that in mind, it does make more sense for the metal handles to have a lower maximum temperature limit than the plastic handles. Metal conducts heat better than plastic and therefore metal handles will transmit heat more rapidly into the user’s hands than plastic handles.
Dr. Lasky has an excellent post about this and other properties of metal and of alloys of metal in the August 31st (2005) post on his blog. The article is titled “A Layman’s Perspective on the Effect of Alloying”, and it can be found at http://www.indium.com/drlasky/entry.php?id=319
Also, there is an EN (European Norm) standard regarding touch temperatures for machinery. According to the EMC-PSTC posts, the EN standard is EN 563:1994 “Safety of Machinery. Temperatures of touchable surfaces. Ergonomics data of to establish temperature limit values for hot surfaces”. However, another poster on the EMC-PSTC group said that there is a new CENELEC standard being written, BTTF120-1(SEC)35, but that the work is controversial and the draft standard is not in the public domain. No explanations were given as to why, and I have not tried to look up either the cited EN or CENELEC standards myself, so I can’t personally vouch for this information.