Solar photovoltaic installations present a fusing challenge that does not exist in conventional AC electrical systems. DC current does not cross zero, strings of modules can drive current in reverse through a fault, and the operating voltages and temperatures involved place demands on fuse-links that standard gG or gM types are simply not designed to meet.
This guide explains the fusing requirements specific to PV systems, covers the IEC 60269-6 gPV standard in practical terms, and walks through the calculations you need to size and select string fuses correctly — from a residential rooftop installation to a utility-scale array.
Why PV systems need specialist DC-rated fuses
The first question many engineers ask is: can I just use a standard DC-rated fuse in a PV combiner box? The answer is no — and understanding why is the foundation of correct PV fuse selection.
The DC arc interruption problem
When an AC fuse clears a fault, the alternating current passes through zero 100 times per second (at 50 Hz). Each zero crossing gives the fuse an opportunity to extinguish the arc inside the cartridge. In a DC circuit there is no zero crossing. A DC arc, once established, will sustain itself and must be actively quenched by the fuse’s internal design — typically through a longer element path, specialised silica sand filler, and a carefully engineered element geometry.
A standard gG fuse installed in a DC circuit will not reliably interrupt a DC fault. It may appear to operate — the element melts — but the arc can continue conducting indefinitely, causing the fuse body to overheat, ignite surrounding materials, or fail to isolate the fault.
The reverse current problem
In a PV array with multiple strings connected in parallel at a combiner box, a fault on one string can cause current from the remaining healthy strings to flow backwards through the faulted string. This reverse current can exceed the module’s rated reverse current tolerance (typically 1.35 × Isc per module string), damaging cells and creating a sustained fault condition.
A gPV string fuse must be capable of carrying the string’s normal operating current in the forward direction AND of clearing the maximum reverse current from the parallel strings — a dual requirement that no standard fuse type addresses.
Rule of thumb: string fuses are required whenever two or more strings are connected in parallel. A single-string system with no parallel connection has no reverse current path and does not require string fuses — though DC cable and inverter protection is still required.
IEC 60269-6 and the gPV utilisation category
The international standard for PV fuse-links is IEC 60269-6, which in the UK is published as BS 88-6. It defines the gPV utilisation category — a full-range fuse specifically engineered for DC solar applications.
A gPV fuse must demonstrate:
- Reliable DC arc interruption at rated voltage — typically 600 V DC, 1000 V DC, or 1500 V DC
- Ability to carry continuous current at elevated temperatures typical of combiner box enclosures
- A time-current characteristic that permits normal array operating currents (including irradiance peaks) without nuisance operation
- Reliable clearing of reverse currents before module damage occurs
- Compliance with specified I²t values to coordinate with module reverse current ratings
Do not use fuses marked only gG or DC (without the gPV designation) for PV string protection. Only gPV fuses have been tested and certified to the full IEC 60269-6 requirements.
AC vs DC fusing in a PV system
| Property | AC Fusing (Grid-Side) | DC Fusing (PV String/Combiner) |
|---|---|---|
| Current type | Alternating — arc self-extinguishes at zero crossing | Direct — no natural zero crossing; arc is sustained |
| Arc interruption | Easier; conventional gG fuses handle it | Much harder; requires specialist DC-rated fuse design |
| Applicable standard | IEC 60269-1 / BS 88-2 (gG, gM) | IEC 60269-6 / BS 88-6 (gPV) |
| Utilisation category | gG or gM depending on load | gPV mandatory for PV string protection |
| Voltage rating | 400 V AC (3-phase) typical | DC voltage rating must exceed Voc(STC) × 1.2 minimum |
| Reverse current | N/A in normal operation | Must withstand reverse current from parallel strings |
| Temperature derating | Standard BS 88 derating curves apply | Additional derating for enclosed combiner boxes; higher ambient |
Where fuses are required in a PV system
| Location | Fuse Type | Standard | Required When? |
|---|---|---|---|
| PV string (at combiner box) | gPV | IEC 60269-6 | Two or more strings in parallel — reverse current risk |
| Combiner box output (DC main) | gPV or DC circuit breaker | IEC 60269-6 | Protects DC cable to inverter; required if cable not otherwise protected |
| Inverter AC output | gG or gM | IEC 60269-2 | Protects AC cable from inverter to grid connection point |
| AC distribution (grid side) | gG | IEC 60269-2 / BS 88-2 | Standard LV distribution protection |
| Battery storage DC link | gPV or specialist DC fuse | IEC 60269-6 or manufacturer spec | Required — battery systems present sustained DC fault energy |
Step-by-step: sizing a gPV string fuse
The sizing procedure for gPV string fuses is defined in IEC 60364-7-712 (Electrical installations — Solar photovoltaic power supply systems). Follow these steps:
Step 1: Obtain the module datasheet values
You will need three parameters from the PV module datasheet:
- Isc(STC) — short-circuit current at standard test conditions (1000 W/m², 25°C)
- Voc(STC) — open-circuit voltage at STC
- Maximum reverse current rating (Imr) — the maximum current the module can tolerate flowing in reverse; typically stated as 1.35 × Isc(STC) but always verify from the datasheet
Step 2: Calculate the design string current
In ≥ 1.25 × Isc(STC)
The factor of 1.25 accounts for the possibility that irradiance can exceed STC conditions (e.g. reflected or concentrated light from cloud edges). This is the minimum fuse current rating.
Step 3: Calculate the maximum reverse current
Irev(max) = (n − 1) × Isc(STC) × 1.25
Where n = number of strings in parallel. This is the current that will flow in reverse through a faulted string when all other strings are healthy and generating at peak irradiance.
Step 4: Verify the fuse will clear before module damage
The fuse must clear Irev(max) before the module’s reverse current tolerance is exceeded. Check the fuse manufacturer’s time-current characteristic to confirm that at Irev(max), the fuse operates within the module’s rated exposure time. For most commercial installations, a fuse rated at 1.4–2 × Isc(STC) achieves the correct balance.
Step 5: Select the DC voltage rating
Vfuse(DC) ≥ Voc(STC) × 1.2 × number of modules in series
This accounts for cold-temperature Voc rise (modules produce higher voltages at low temperatures) and provides the standard 20% safety margin. Always confirm that the fuse’s DC voltage rating meets or exceeds this calculated value. Using a fuse with insufficient DC voltage rating is a fire risk.
Step 6: Check the breaking capacity
The fuse’s rated breaking capacity must exceed the maximum prospective fault current at the point of installation. For a combiner box, this is the sum of the short-circuit currents of all parallel strings.
Worked example
Example: 4-string combiner box, 10-module strings, Jinko 530W module
| Parameter | Example Value | Notes |
|---|---|---|
| Module Isc(STC) | 10.5 A | From module datasheet — short-circuit current at standard test conditions |
| Number of strings in parallel | 4 | Combiner box with 4 strings |
| Isc per string (with safety factor) | 10.5 × 1.25 = 13.1 A | IEC 60364-7-712 factor of 1.25 applied to Isc(STC) |
| Maximum reverse current (Imr) | 3 × 13.1 = 39.3 A | Sum of Isc from remaining 3 strings — the worst-case reverse current into a faulted string |
| Fuse In selection | 16 A gPV | Above 13.1 A load current; must clear before Imr of 39.3 A causes module damage |
| DC voltage rating required | Voc(STC) × 1.2 = 540 V DC min | Module Voc(STC) = 45 V × 10 modules in series = 450 V; × 1.2 safety factor |
| Selected fuse | 16 A gPV, 600 V DC, IEC 60269-6 | Confirm breaking capacity exceeds array short-circuit current |
Result: select a Lawson Fuses gPV fuse, 16 A, 600 V DC, compliant with IEC 60269-6. Confirm breaking capacity ≥ 4 × 10.5 × 1.25 = 52.5 A (prospective fault current at combiner output).
Temperature derating in PV applications
Combiner boxes and string junction boxes are often installed in locations with high ambient temperatures — roof voids, outdoor enclosures in direct sunlight, or plant room equipment rated for standard ambient. gPV fuses, like all fuse-links, must be derated when the ambient temperature exceeds the reference temperature used in their rating (typically 20–25°C).
As a rule of thumb for enclosed PV combiner boxes:
- Add 20–30°C to the outdoor ambient temperature to estimate the enclosure internal temperature
- Apply the fuse manufacturer’s temperature derating curve to determine the effective current rating
- For a combiner box in a 40°C ambient environment, the internal temperature may reach 65–70°C — consult the Lawson Fuses gPV datasheet for the derating factor at this temperature
Warning: failure to apply temperature derating is one of the most common causes of gPV fuse nuisance operation in summer months on commercial rooftop systems. Always check the enclosure thermal specification.
1000 V DC and 1500 V DC systems
Utility-scale and large commercial PV systems increasingly operate at 1000 V DC or 1500 V DC string voltages to reduce cable losses and inverter count. This changes the fuse selection requirements significantly:
- Fuses must be rated at 1000 V DC or 1500 V DC — standard 600 V DC gPV fuses are not suitable
- Breaking capacity requirements increase substantially at higher voltages
- The fuse form factor and fuse-holder must both be rated for the system voltage
- IEC 60269-6 covers all three voltage levels; confirm the fuse marking shows the correct voltage rating
Lawson Fuses supplies gPV fuse-links across all standard PV voltage ratings. Contact our technical team for selection support on high-voltage DC systems.
Battery storage systems
The integration of battery energy storage (BESS) with PV arrays introduces additional fusing requirements. Battery systems present a large and sustained DC fault energy source — in some respects more demanding than a PV array, because the battery can deliver high fault currents for extended periods.
Key points for BESS fusing:
- Use gPV or specialist DC battery fuses as specified by the battery system manufacturer — do not use standard AC fuses on DC battery circuits
- Coordinate the battery fuse rating with the battery management system’s overcurrent trip levels
- Verify that the fuse’s I²t let-through value is within the cable’s and battery terminal’s withstand ratings
- For lithium-ion battery systems, follow the battery manufacturer’s specific fuse selection guidance — cell chemistry and pack design affect the required fuse characteristics
Summary: PV fuse selection checklist
Before finalising any PV fuse selection, verify each of the following:
- Fuse is rated gPV to IEC 60269-6 (BS 88-6) — not gG, not generic DC
- Current rating ≥ 1.25 × Isc(STC) of the string
- Current rating allows clearing of maximum reverse current (n−1) × 1.25 × Isc(STC)
- DC voltage rating ≥ 1.2 × Voc(STC) × modules in series
- Breaking capacity ≥ maximum prospective fault current at the installation point
- Temperature derating applied for actual enclosure operating temperature
- Fuse form factor matches the combiner box fuse-holder
- String fuses used wherever two or more strings are paralleled
About Lawson Fuses
Lawson Fuses has manufactured low voltage HRC fuse-links to IEC 60269 and BS 88 standards since 1938. Our gPV range covers string and combiner protection for residential, commercial, and utility-scale PV systems from 600 V DC to 1500 V DC. Products are ASTA certified and ISO 9001 accredited. For datasheets, time-current characteristics, and technical selection support, visit https://www.lawsonfuses.com/ or call 01661 823 232.
