There are many documents showing the terminology and specs used to define and choose a fuse. These are useful for both initial designs (prototyping) and repairs/replacements. Two examples from Littelfuse are shown below.
Click here: FUSEOLOGY Selection Guide
Click here: Fast Fuses Versus Slow: How do You Choose?
Within this data, the causes and effects of temperature variance might be the most useful for anyone having issues with unexpected fuse behavior in an otherwise stable circuit. This is especially true for DIY and home repair projects. A common problem with fuses, for example, is that they will operate (open) well before expected, and this can be a nuisance or even render some devices inoperable.
These issues arise from the relationship between the fuse element, its melting point, and all sources of temperature input. Note that most fuses (not circuit breakers) that are used in daily life have a fuse element, and it doesn’t react directly to the current level (amperage) so much as it does to all combined sources of heat that may cause it to reach its melting point. Resistance to current is a function of the fuse, but outside sources also contribute to the temperature rise of the fuse element material.
Test Temperature
Documentation for a fuse may show a test temperature—25C, for example. This is the baseline temperature at which the current carrying capacity was determined. Check the manufacturer documentation for a re-rating formula or use a standard industry value.
For UL rated fuses, the operating current should be no more than 75% of the fuse rating. An example from Littelfuse is shown below.
Schurter Electronic Components offers advice on the percentage rating along with the UL or IEC rating.
Ambient Temperature
Some sites may describe the test temperature as an “ambient temperature”, but this term should apply to additional temperature inputs that affect the baseline test temperature. Manufacturers may provide a chart showing the effects of ambient temperatures on specific fuses.
The ambient temperature should not be directly connected with a general room temperature. It should be based on the localized area of the fuse where the temperature may be higher or lower than nearby air temperatures for various reasons. PCB enclosures and air flow are factors in localized ambient temperatures, as are heat inputs from conduction, radiation, and convection.
In each application, those additional heat sources must be discovered to determine if they are affecting the fuse performance. Heat may radiate from nearby coils, for example, or rise from an appliance below the PCB enclosure.
Solder Junctions and Wires
Solder junctions always offer some current resistance, and depending on the quality of the soldering, they may be a significant source of heat conducted to the fuse element. Consider the heat buildup and transfer from any traces or wires leading to the fuse, as well. If these are not properly sized, they could be conducting excess heat into the fuse area.
Fuseholders
Fuseholders may also add to the localized ambient temperature by conducting heat to the fuse or preventing heat dissipation around the fuse. There may be a manufacturer recommend fuseholder operating current given as a percentage of the nominal current. This is similar to the fuse re-rating procedure. If 60% is recommended, for example, and the actual operating current is near 100%, not only can the fuseholder fail, but it may be transferring a significant amount of heat to the fuse.
Insulation
The introduction of unwanted physical insulation will also increase the ambient temperature near the fuse. In most cases, this is a simple buildup of dust and grime on the surface of the components and the PCB. This prevents heat from dissipating and may cause unexpected fuse openings.
Pulsed Waveforms
The previous examples of heat introduction into a fuse environment are useful for both DIY/repair as well as initial design. This last example involving pulse waveforms is related more to prototyping and design, but it is important to understand how energy applied in various pulse forms affects the operation of a fuse.
The melting point of a fuse indicates the amount of energy needed to melt the fusing element. The expressions commonly used are I^2t and A^2 Sec (Ampere Squared Seconds). Basically, current and time are the factors, and pulsed waveforms do not all have the same values. Each fuse has its own thermal cycling pattern, too—the dissipation of heat over a given time period—that determines its behavior.
Below are some examples given by Littelfuse regarding pulsed waveforms and the relationship between pulsing and melting points.
Testing
Given the reports and guides regarding fuse behaviors under various temperatures, it may be tempting to conclude that fuse selection can be broken down into exact calculations. However, even within batch productions of one fuse product, there is variance. Combined with the inaccuracy of measuring some of the aforementioned heat sources and the unpredictability of some environments, there is still much room for error.
That is why testing is still important. This is par for the course in prototyping and design, but also true for repairs where not all of the original fuse characteristics are known. It should not be assumed that a fuse rating and a circuit operating current are enough to select a fuse, and that raising or lowering a fuse value is the immediate solution to a problem. Consider changing the environment where the fuse is operating, first, to see if this affects the original behavior of the fuse. Test a sample of fuses within a design to look for any product variance.
To select a fuse for repair or design, start with the main Digi-key fuse category: To select a fuse for repair or design, start with the main Digi-key fuse category: [click here Fuse ] (Product Index > Circuit Protection > Fuses) or by searching from the Digi-key home page: [click here https://www.digikey.com/ ].
For further reading, use any of the links, below.
SCHURTER Electronics Components: Guide to Fuse Selection
EATON: Fuse FAQs
OPTIFUSE: Blog
DIGI-KEY: Fuse Tutorial