Fuse

What is a fuse

A fuse is a one-shot overcurrent protection device consisting of a calibrated metal element enclosed in an insulating body. When current through the element exceeds the rated value for a defined time, the element heats above its melting point, vaporises, and interrupts the circuit. After operation, the fuse must be replaced.

For solar applications, specialised photovoltaic DC fuses (designated gPV) are used. These are rated for the unique conditions of solar DC: high voltage (1000 V or 1500 V DC), bidirectional fault current possible, and arc-interrupting capability designed for DC without zero crossing.

Indian solar installations use DC fuses primarily in combiner boxes for string-level protection, sized at typically 1.5 times module Isc. AC-side fuses are used in specialty applications; most AC protection uses MCBs and MCCBs. Our string sizing and overcurrent protection guide covers how fuse sizing fits into the broader string design.

Why fuses matter

DC string protection. Combiner boxes protect each parallel string against reverse current and fault current.

DC equipment cost. DC-rated breakers are scarce and expensive. Fuses are cost-effective.

Predictable behaviour. Fuse interrupting characteristics are well-documented.

Standard sizes. Industry-standard form factors simplify replacement.

Safety. Without fuse protection, a faulted string can absorb reverse current from healthy strings, overheating cables.

How a fuse works

1. Current flows through the calibrated fuse element.
2. The element has a defined I²t (energy) characteristic for melting.
3. Continuous current below rated value causes no significant heating.
4. Continuous current above rated value heats the element above its melting point.
5. The element melts and vaporises within the fuse body.
6. Arc that forms between the broken ends is extinguished by the fuse’s arc-quenching material (often silica sand for HRC fuses).
7. Circuit is permanently open.

For DC fuses, arc extinction is more challenging because DC has no zero crossing. gPV fuses use sand-filled bodies and specialised contact designs to extinguish the arc.

Technical deep dive

Fuse classifications

gPV fuse ratings (typical)

Fuse selection for solar strings

Use SurgePV’s string sizing calculator to cross-check fuse and string sizing together for a given module and inverter combination.

Voltage rating must equal or exceed maximum string Voc at cold temperatures.

Reverse current rating must accommodate reverse current from N-1 parallel strings, where N is the total number of parallel strings in the combiner box.

Form factors

Real-world applications

Combiner box string protection in utility plants.

Inverter input fuses for parallel string protection.

DC isolator integrated fuses.

Specialty AC fuse applications.

Battery DC main fuse for protection against short circuit.

Common fuse problems

Repeated blowing indicating persistent fault.

Wrong size (too small) blowing on normal Isc, especially high temperature events.

Wrong voltage rating leading to arc not extinguishing.

AC fuse used on DC, catastrophic failure.

Connector heating where fuse is loose.

Best practices

Use only gPV-rated fuses for solar DC.

Size fuse at 1.5 × Isc with rounding to standard.

Verify voltage rating against cold-temperature Voc.

Use sealed fuse holders rated for environment.

Document fuse ratings on the single-line diagram.

Replace blown fuses only after fault diagnosis.

Maintain spares on site.

Comparison: fuse vs breaker

Indian market context

Major Indian solar DC fuse brands include Mersen, Eaton Bussmann, Schurter, Phoenix Contact, OBO, Schneider, ABB.

Indian utility-scale solar plants almost universally use gPV fuses in combiner boxes.

Residential rooftop systems with 1 or 2 strings often omit fuses since reverse current is below module reverse current rating.

C&I rooftop systems with 3 or more parallel strings typically include fuse protection.

BIS certification under IS 13703 for low voltage fuses. Installers sourcing pre-wired DCDB and ACDB enclosures with integrated fuse protection can skip building combiner assemblies from scratch on site.

Standards and certifications

Common mistakes

Using AC fuse on DC circuit.

Undersizing fuse, causing nuisance blowing.

Oversizing fuse, defeating protection purpose.

Wrong voltage rating allowing arc to persist.

Repeated replacement without diagnosing fault.

Skipping fuse on combiner box to save cost.

Using fuses without proper holder, leading to overheating.

Key takeaways

Fuses are essential overcurrent protection devices for solar DC strings. Specialised gPV fuses handle the unique requirements of solar DC: high voltage, bidirectional fault current and DC arc extinction. Combiner boxes with 3 or more parallel strings require fuse protection. Sizing at 1.5 × Isc with proper voltage rating, sealed fuse holders and root-cause diagnosis on every blowing event are the disciplines that keep solar plants safe.

Further reading

For how Fuse plays out in real projects, these guides go deeper:

• Solar String Sizing & Overcurrent Protection
• Inverter Battery Connection Diagram and Sizing Guide for Hybrid Solar
• Solar Shading Analysis Software Ireland: 2026 Guide
• Solar Shading Analysis Software Singapore: 2026

Sources

• IS 13703, Indian Standard for low-voltage fuses.
• IEC 60269 series, including IEC 60269-6 for PV fuses.
• UL 2579, US standard for low voltage fuses for PV systems.
• IEC 60364-4-43, Protection against overcurrent.
• CEA (Technical Standards for Connectivity of the Distributed Generation Resources) Regulations, 2013 (as amended 2019), Central Electricity Authority.
• Manufacturer documentation from Mersen, Eaton Bussmann, Schurter, Phoenix Contact, OBO, Schneider.
• IEC 62548, PV array design requirements.
• IEEE Standard Dictionary of Electrical and Electronics Terms.