What is the operating ambient temperature of a fuse?

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Operating ambient temperature of a fuse

Ambient temperature is the air temperature of any object or environment where equipment is stored. In a computing context, maintaining an appropriate ambient temperature is crucial to the proper functioning and longevity of any equipment. 

Ambient temperature is a fancy way of saying the “outside air” surrounding the fuse.

Normally, the fuses are tested in "laboratory conditions" by the safety agencies such as

UL and CSA. The lab conditions are almost always set at 25°C or 77°F. Unfortunately, real-world conditions are not those found in a laboratory.

Fuseheat-sensitives devices meaning that it takes heat (via the overcurrent) in order to melt the fuse element inside the fuse. The more heat...the faster it takes to melt the fuse element...the less heat...the longer it takes to melt the fuse element.

If a fuse will be subjected to a higher temperature than 25°C, then the fuse amperage will need to be increased as to compensate for the higher temperature (to avoid "nuisance tripping"). Likewise, if the fuse will be used at a lower temperature, then the fuse amperage needs to be lowered (or else it might never open).

The rule of thumb is that for every 20°C higher or lower in temperature, the fuse should be re-rated by higher or lower 10-15%.

An example of a fuse re-rating when higher ambient temperatures are present:
Normal full-load current: 1 Amp

Normal fuse sizing: 1.5 Amps (135% of full load current taken to the next higher standard rating)
Ambient Temperature: 65°Clow-temperature

Re-rating: Re-rating: 2 Amps (130% of normal fuse rating)

Conversely, when a fuse is intended to be used in extremely low temperature conditions, the fuse must have a lower rating than that in normal conditions.

An example of the fuse re-rating when lower ambient temperatures are present:
Normal full-load current: 1 Amp

Normal fuse sizing: 1.5 Amps (135% of full load current taken to the next higher standard rating)
Ambient Temperature: -15°C

Re-rating: 1.2 Amps (70% of normal fuse rating taken to the next higher standard fuse rating)

Technical Specification of TE-6700 Series 2nd Generation Temperature Elements

Ambient temperature and operating temperature

What is the difference between Ambient temperature and operating temperature, let's know-

Ambient temperature is the temperature of the air around the equipment. This range is typically between 60°F and 75°F (15°C and 24°C ). 

The operating temperature is the temperature of the equipment itself. Since the equipment is (typically) generating heat, the operating temperature will be higher than the ambient temperature.