Why 40oC Operating Temperature - Electrical - Engineer Boards
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R. Steele

Why 40oC Operating Temperature

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While researching the basis of the 40oC max operating temperature outlined in the normative requirements for industrial control panels (UL508, UL 508A, NFPA 70, NFPA 79) the question struck me as to why is 40oC the magic number?  Why not 35, 41, 45, 50 oC maximum operating temperature?  After doing some research into peer-reviewed documents, I am walking away scratching my head.... and am hoping someone within the brotherhood of electrical engineers knows why 40oC is the basis and can point me in the direction of peer-reviewed publications, books, or other documents so I can learn what is so magic about this value.  Any assistance is appreciated.

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R. Steele, 

Perhaps you will have a better luck finding the information you need in Mech Handbook related to thermal topics. The electronic components housed in the control panel have maximum junction thermal temperature any where from 85c to 110c. Once they reach this temperature they tend to behave badly or be in the thermal shutdown state. 40 degree Celsius would be around midpoint of ambient temperature with convection cooling which is achievable in the enclosure. It also makes mech engr happy.

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