1. Why each string needs its own fuse
A PV string is a continuous DC supply that can feed fault current into a neighbouring string. The standard defence is one gPV fuse per string, mounted in the combiner box, so a fault in one string is isolated before it stresses the others.
2. The 1.25x Isc rule
The fuse must be rated for no less than 1.25 times the string short-circuit current (per IEC 60269-6 for gPV fuses). With bifacial modules the rear-side gain lifts Isc above the front-only nameplate, so the fuse is sized to the peak, not the average.
3. Worked example
A bifacial module with a front Isc of 15 A and an 8 % rear gain can momentarily deliver about 16.2 A. Applying 1.25x gives 20.25 A, so a 20 A gPV fuse is the minimum safe choice; a 15 A class built for the front-only rating would nuisance-trip under real output.
4. Reverse current between parallel strings
When several strings join one input, a fault in one is fed by all the others at once. The combined reverse current is compared against the module maximum series fuse rating to confirm each parallel string needs its own protective device.
Technical diagram shown at a readable responsive scale.
5. Fuse class matters
Only gPV (photovoltaic) fuses are built for the DC characteristics of a PV array: they must clear high-voltage DC arcs that AC fuses cannot. Using a generic fuse in a combiner box is a field failure waiting to happen.
6. Coordination with the rest of the box
The fuse is one layer. The DC disconnect, the surge protective device and the busbar must all be rated for the same worst-case current and voltage. NEUTRON sizes the internal busbar and terminal block against the projected string count and per-string Isc before release.
This is general technical guidance. Confirm final ratings, installation conditions and applicable local requirements against approved project documentation and a qualified engineering review.
Frequently asked questions
What size fuse do I need for a PV string?
A gPV fuse rated at no less than 1.25x the string Isc, including bifacial rear-side gain; verify the choice against the module maximum series fuse rating.
Why use a gPV fuse instead of a standard fuse?
gPV fuses are engineered to interrupt high-voltage DC arcs that AC fuses cannot clear, which is the dominant fault mode in a PV combiner box.
What is reverse current in a combiner box?
When parallel strings share an input, a fault in one string is back-fed by the others; each string therefore needs its own fuse so the reverse current cannot exceed the module rating.
Does bifacial gain change fuse sizing?
Yes. The rear-side current lifts worst-case Isc, so the fuse must be sized to the peak including rear gain, not the front-only nameplate.
Bring the electrical inputs to the first review.
Share the approved module data, site temperature range, cable route, string configuration and destination-market requirements. NEUTRON can review the equipment envelope against the project inputs.
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Technical note: verify final ratings, standards, documents and configuration against approved project documentation and the applicable local requirements.