The context or reason for this question is the following:
Reminder: the question is about the fuse, not about the TRIAC. The context may help to understand the question.
A 230V output is controlled by a DPAK2 TRIAC that is protected with a fuse of type F6.3A/250V 5x20mm glass type. The TRIAC can take 8A.
A test was made with a load (heater) of 1000W (about 4 Amps). The circuit worked fine for several hours. Then the heater was switched to 2000W (about 8 Amps). The fuse did not blow, but the TRIAC did. The reason for that is that at 8A, the TRIAC has to dissipate too much heat and there is no specific cooling. Possible 6A is also too much, but that is not the question.
Thermal fuses are rated for speed with T, M or F, and are rated for a given current. This is in fact misleading. The fuse does not break above the given current - il requires a lot of time for a glass fuse rated at 4 Amp to blow at 4.5 Amp. In fact the rating T, M and F are "specified" for currents that are 10 (ten) times the nominal current.
According to my analysis of the F6.3A glass fuse, at 8A the fuse will blow after about 200s. Which leaves enough time for anything else to overheat and blow itself.
So this made we realise that glass fuses, ceramic fuses and other fuses exists possibly because they have "better" ratings with regards to overcurrent protection.
Are there fuses that would allow a 4Amp current to flow indefinitely, but blow pretty fast (say about one second) for a current slightly exceeding 4 Amps, for example 4.5 Amps.
A comparison between fuse technologies that are commonly available for PCBs would be nice but the principal information I am looking for is the existance (or not) of fuses that do what most people would expect them to do: blow in a short time as soon as the current exceeds the current rating.