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




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.



Mr.Amit Upadhyay Deputy Manager-Design & Engineering at Mahindra Susten

Article Link:https://www.linkedin.com/pulse/fuse-selection-criteria-smb-string-monitoring-box-case-amit-upadhyay

2 thoughts on “2. Fuse Selection Criteria of SMB (String Monitoring Box) in case of Solar System Design”

  1. Dear Sir,

    I understand everthing.In our SMB make Trinity Touch 8 in/ 1 out .Each input Fuse rating 30Amps then how the fuse blown.please give conclusion

    1. Dear Sir,

      Thank you for reaching out to bookmysolar. Request you to please share the images of the junction box for us to look into the issue. Also, please share the following details:
      1. Panels used and their rating
      2. DC Cable used
      3. Terminal Blocks used and their rating
      4. Connection diagram of the junction box
      5. details of the SPD used.

      Please mail the detals to info@bookmysolar.in

      Team MYSOLAR

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