Not only for string combiner box but 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 a correct rating of fuses or fuses burnt frequently at a site in the period of a 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
Now when inner Box temp of SMB is 65degC (fuse holder) then as per derating datasheet of say Bussmann fuse, fuse current carrying capacity will become from 12Amps to 9.5Amps at 65degC.
Particularly at this instance, fuses will blow, also in the summer season, this will happen frequently at the site.
So in case of Bussmann fuses, they will define the selection of fuses as per below formula:
Fuse rating = Isc X 1.56 times
Fuse rating = Isc(8.68Amps) X 1.56 times(very safer side) = 13.54Amps so selected fuse say 15Amps
Conclusion: Before selecting fuse rating, a solar designer has must know about the following information:
- Maximum Irradiation and ambient temperature of site in summer
- Selection of SMB based on internal temperature rise calculation of each component.
Author: Mr.Amit Upadhyay Deputy Manager-Design & Engineering at Mahindra Susten