I do not believe that OSHA has a limit on temperature for exposed surfaces. The general duty clause requires that you must protect your employees from receiving a burn as a result of touching a hot surface. A generally accepted temperature is 140 degrees F as a maximum temperature for exposed surfaces where an employee could be expected to come into contact with it. Some safety supply companies sell stick on warning labels, made of temperature sensing materials, that turn to a red color to warn employees of excessive temperatures.
One more item on this subject. You do not have to insulate the hot surface. An alternate is to provide barriers to prevent incidental employee contact. May times hot surfaces are inevitable and employees must wear appropriate PPE to protect themselves when in proximity to these surfaces.
~140 degrees F is a temperature that many Industrial Hygenists have determined that an employee can have incidental contact with a surface and not be burned.
I do not know of the OSHA requirements. The temperature appears to differ depending upon the time of exposure and heat conductivity. For example, I think that we require piping insulation for personnel protection about 150 degree F, perhaps it is 140 F. However, I think that the plumbing industry limit is about 120 degree F (or 45 degree C) for hot-water heaters and bath water temperature controls. The exposure time limit decreases somewhat exponentially with increasing temperature.
There is not a magic number for the requirements of surface temperatures. You basically have to provide protection under the general duty clause because you simply can't state an absolute temperature for every individual situation. There is not simply a way of stating that 120 F as the limit to where the human body can accept the transfer of heat, people in AZ would be in real trouble. However, general wisdom would tell someone that a strip of steel at 1000 F is a bit too hot to touch without proper protection.
You will need to look at your particular situation and determine what the risks are to your employees. In my professional opinion, anything over 150 F should not be touched by unprotected human hands. Anything over that, and I would provide first, engineering controls to restrict the access of a person from contact with the surface. Second, if access cannot be restricted, limit the access to as little as possible. Lastly, provide PPE(hot mill gloves, arm guards, face shields, etc.) to the employees who have no other option but contact with the material.
