Why do thermal protectors have a different value than separate overloads?
Also, are thermal protectors ever field chosen and installed?
My response is not to be taken as all inclusive - just one person's anecdotal experience. Note to nit pickers - yes, I am certain there are many exceptions to my comments.
About the only motors I have seen with integral thermal protectors are single phase like tablesaws, aircompressors, oil-fired furnace guns, or blower motors, or submersible pumps in service station fuel tanks. An exception is 3ph submersible pumps is refueling systems. The submersible pumps have thermal cutouts to shut them off if the pump suctions are dry - the pumps are fluid lubricated and the motors are fluid cooled.
I suspect the single phase tablesaw (and others) type of applications have motors with integral thermal protectors cause it is a dollar cheaper that having to put in a thermal overload switch.
In either case, separate overload or integral, the point is to prevent a fire not really save the motor.
How did the code panel select the currents for integral thermal portectors? Probably the same method use for some of the other numbers given (25 ohms comes to mind). I'd guess either blindfolded dart boad, or ouiji board.
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