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Temperature Rise: Limits, Ambient Assumptions and the Parts People Forget
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Temperature rise testing checks that no part of the equipment becomes hot enough to be a burn hazard, to damage insulation, or to ignite surrounding material. The limits are stated as rises above ambient rather than absolute temperatures, because the equipment is rated for an ambient range and must remain safe at the top of it.
The ambient assumption
Unless the manufacturer declares otherwise, the reference ambient is conventionally 40 °C for this family of equipment. A measurement taken in a 22 °C laboratory therefore has to be compared with the limit using the rated ambient, not the laboratory temperature. Reporting a 30 K rise as acceptable because the part measured 52 °C in a cool room is the single most common arithmetic error in thermal testing.
Which parts
Windings, transformer cores, wiring insulation, terminals, the enclosure surface, and any part the user can touch. The last one has the lowest limit and is the one most often skipped, because the thermocouple is placed inside the case where the interesting temperatures are rather than on the handle where the user is.
Test conditions
Worst case means the highest rated supply voltage, the maximum rated load, and the least favourable mounting. A bench instrument tested in free air on an open bench will run cooler than the same instrument in a rack, and the rack is a legitimate installation the instructions usually permit.
