Fuse Selection Criteria of SMB (String Monitoring Box) in case of Solar System Design

In case of electrical system design anywhere as per standard practice we select fuse/breaker size with the help of the below formula:

Fuse/Breaker rating size = Current X 1.2 or 1.3 times

Suppose we selected 280Wp module, as per datasheet Isc is 8.68Amps so Selected Fuse = 8.68Amps X  1.25 = 10.85Amps so selected fuse say 12Amps

But in case of solar system design this formula many times fail to provide correct rating of fuses or fuses burnt frequently at site in the period of summer season.

Before starting we all have to know about this fact that in case of solar modules, irradiation is proportional to module current as well as temperature is proportional to module current (minutely) also, which means when irradiance & temperature will increase than module current will also increase from its rated current.

Reasons for burning of fuses (my own practical experience) :

As noted in datasheet All electrical parameter specified at STC 25degC cell temperature & 1000W/m2 irradiance, in summer season radiation will increase from 1000W/ m2 to 1100 to 1200W/m2 also temperature will increase.

Say at 1100W/m2 and 40deg ambient temperature Isc will become from 8.68Amps to 9.62 Amps