Sizing gPV fuses for solar strings: when you need them and which rating

Why strings need fuses at all

In normal operation every string pushes current towards the inverter. If one string develops a fault, such as a short in a module or a cable, the healthy strings in parallel can drive current backwards into it. One or two parallel strings cannot supply enough reverse current to damage a module. With three or more, they can. The module datasheet states how much reverse current a module can take, usually as its maximum series fuse rating.

Step 1: do you need string fuses?

Mersen's guidance gives the maximum number of strings that can be paralleled without fuses:

N ≤ 1 + IRM / ISC STC

Here I_RM is the module's maximum reverse current (its maximum series fuse rating) and I_{SC STC} is the module's short-circuit current at standard test conditions. If you have more strings than N, fuse them. Check against IEC 62548 and your project specification, which may be stricter.

Step 2: choose the current rating

• Lower limit: IEC 60269-6 gPV fuses are rated so that at least 1.4 × ISC STC is needed at a fuse ambient of 45 °C or less. Combiner boxes in Indian sun often run hotter, and above that the fuse must be derated using the manufacturer's curves, which pushes the rating up. For reference, the US NEC uses 1.56 × I_SC.
• Upper limit: the fuse rating must not exceed the module's maximum series fuse rating, or the fuse will not protect the module.
• If the derated minimum ends up above the module's limit, change the design: fewer strings per input, a different combiner location, or ventilation.

Step 3: choose the voltage rating

Open-circuit voltage rises as temperature falls. Mersen's rule is:

Ufuse ≥ 1.2 × M × VOC STC

Here M is the number of modules in series. The 1.2 factor covers the voltage rise at low temperature. Use the module's temperature coefficient and the site's record low if your design basis requires a precise figure. In practice this lands on 1000 V DC or 1500 V DC fuse ranges.

Worked example

A module has I_{SC STC} = 13.9 A, a maximum series fuse rating of 25 A, and V_OC = 49.5 V. There are 20 modules per string and 6 strings into one combiner input.

• Unfused limit N ≤ 1 + 25/13.9 ≈ 2.8, so with 6 strings, fuses are needed.
• Minimum current 1.4 × 13.9 = 19.5 A before derating. A 20 A or 25 A gPV fuse, depending on the derating at your box temperature, stays at or below the 25 A module limit.
• Voltage 1.2 × 20 × 49.5 = 1188 V, so use a 1500 V DC rated fuse. A 1000 V DC fuse is not enough.

This is an illustration. Always use the actual module datasheet, the fuse manufacturer's derating data, and your design standard.

Other practical points

• Floating (ungrounded) arrays: fuse both positive and negative. With one pole earthed, fuse the other.
• Use gPV class fuses only. AC fuses or general DC fuses are not tested for the low fault currents of PV strings.
• Use touch-safe fuse holders rated for the same voltage, and check the holder's own temperature derating.

From Ashima

We supply Mersen HelioProtection gPV fuses:

• HP10M series (10 × 38 mm, 1000 V DC) and HP15M series (10 × 85 mm, 1500 V DC), from 5 A to 32 A,
• Modulostar CUS101 fuse holders,
• DC distribution boxes built to your array.

The full part-number table is on the SPD & DC fuses page.