10.2 String Overcurrent Protection Rule

Quick Answer: Fit string overcurrent protection to every PV array and sub-array where 1.35 × IMOD_MAX_OCPR < (NS − 1) × ISC_MAX. If that inequality is false, this rule does not require an OCPD. IMOD_MAX_OCPR is the module maximum overcurrent protection rating (datasheet reverse-current / series-fuse rating). NS is the number of parallel strings. ISC_MAX is the maximum string current — use the 1.25-corrected value in design unless the manufacturer states otherwise; the handbook writes ISC_MAX.

What the inequality is deciding

Learning outcome 4.2.2 in handbook v1.3 is not “always fuse every string.” It is a comparison between how much reverse current the module is allowed to survive and how much reverse current the other strings can actually push into it. When the neighbouring strings can push more than 1.35 times the module rating, you must fit string overcurrent protection. When they cannot, the module is already able to survive the worst reverse current this array can produce, and this particular rule does not demand a fuse or breaker.

Write the trigger exactly as the handbook states it:

String overcurrent protection shall be fitted to all PV arrays and sub-arrays where:

1.35 × IMOD_MAX_OCPR < (NS − 1) × ISC_MAX

The left side is a withstand figure (module capability, with a 1.35 margin). The right side is the available reverse current from the other strings. Protection is required when available reverse current exceeds that margined withstand — that is, when the left side is strictly less than the right side.

Teach each symbol

IMOD_MAX_OCPR is the module maximum overcurrent protection rating. It comes from the module datasheet, not from the inverter and not from the cable table. Manufacturers often label it maximum series fuse, reverse current rating, or maximum overcurrent protection. Physically it is the largest current the module is declared to survive in reverse (for example when it is shaded, shorted, or reverse-biased while neighbours still generate). A typical 60-cell or 120-half-cut crystalline module might declare 20 A or 25 A; you must read the sheet, not guess from Isc.

NS is the number of strings connected in parallel in that array or sub-array. It is the same count that section 10.1 called N. A “sub-array” is a paralleled group that has its own combining point. If you have two combiners of three strings each, you apply the rule to NS = 3 at each combiner, not to six strings that only meet at the inverter, unless those six are paralleled together before any OCPD.

ISC_MAX is the maximum string current. The handbook writes ISC_MAX. In design you use the 1.25-corrected string current as ISC_MAX unless the manufacturer states otherwise — that is the IDC-MAX from section 10.1 (ISC_STC × 1.25 or ISC_MOD × 1.25). Do not put raw STC Isc on the right-hand side and then wonder why a three-string array “only just” misses the trigger; the 1.25 factor is already sitting inside ISC_MAX for UK CoP practice.

The 1.35 on the left is a margin on the module rating. It stops you operating the module at the exact datasheet reverse-current ceiling. The comparison is therefore slightly conservative: you fit protection a little before the reverse current would equal IMOD_MAX_OCPR exactly. BS 7671 Section 712 uses the same 1.35 relationship for PV string overcurrent; the 2922 paper expects the handbook form of the inequality, not a free rearrangement that flips the inequality sign.

Physical picture: who feeds whom

A PV string is a current-limited source. It cannot produce the kiloamp prospective fault currents of a transformer. What it can do, if several strings share a combiner, is this:

1. One string develops a low-resistance fault (shorted cable, failed module, reverse-biased substring with bypass diodes conducting the wrong way).
2. That faulted string becomes a load hanging on the DC bus.
3. Every other healthy string still generates and can drive current into the fault.
4. The number of healthy parallel sources is NS − 1, not NS. The faulted string does not reverse-feed itself from its own Isc in this model; the neighbours do the feeding.

So two strings cannot reverse-feed more than one string’s Isc into a third. With NS = 2 there is no third string. The reverse current available is one lot of ISC_MAX. With NS = 3, two healthy strings can both feed the faulted one, and the reverse current becomes 2 × ISC_MAX. That is the step that commonly crosses the module’s reverse-current rating.

A single string (NS = 1) has (NS − 1) = 0, so the right-hand side is zero. No parallel neighbour exists to reverse-feed it. This inequality does not require string OCPD on a lone string. You still size that string’s cable for IDC-MAX as forward current; you just do not invent a reverse-current fuse requirement from a parallel source that is not there.

Worked example — IMOD_MAX_OCPR = 20 A, ISC_MAX = 17.25 A

Use the array from section 10.1 so the 1.25 correction is already visible: ISC_STC was 13.8 A, so ISC_MAX = 13.8 × 1.25 = 17.25 A. The datasheet maximum overcurrent protection rating is 20 A.

Left side (same for every NS)

1.35 × IMOD_MAX_OCPR = 1.35 × 20

1 × 20 = 20

0.35 × 20 = 7

20 + 7 = 27 A

NS = 2 (the 10-in-series, 2-parallel array)

Right side = (NS − 1) × ISC_MAX = (2 − 1) × 17.25 = 1 × 17.25 = 17.25 A

Test: is 27 < 17.25? False.

Protection is not required by this inequality for NS = 2. One neighbour can only push 17.25 A into a faulted string; the module is declared good for 20 A and the 1.35 margin still sits at 27 A, which is larger than 17.25 A.

NS = 3 (add a third parallel string of the same modules)

Right side = (3 − 1) × 17.25 = 2 × 17.25

2 × 17 = 34

2 × 0.25 = 0.50

34 + 0.50 = 34.5 A

Test: is 27 < 34.5? True.

Fit string overcurrent protection. Two healthy strings can reverse-feed 34.5 A into the third, and 34.5 A is above 27 A. Each string (or each string into that sub-array) needs an OCPD so that a faulted string is disconnected from the neighbours.

If you had wrongly used raw ISC_STC = 13.8 A on the right-hand side for NS = 3, you would have computed (3 − 1) × 13.8 = 27.6 A. Then 27 < 27.6 would still be true, but a different module (higher IMOD_MAX_OCPR, or a slightly lower Isc) could flip the verdict if you drop the 1.25. Design uses ISC_MAX as the 1.25-corrected figure unless the manufacturer states otherwise.

Table — NS = 1, 2, 3, 4 with these numbers

Keep IMOD_MAX_OCPR = 20 A and ISC_MAX = 17.25 A. Left side stays 27 A.

NS = 4 arithmetic: 3 × 17.25 = 3 × 17 + 3 × 0.25 = 51 + 0.75 = 51.75 A.

The jump from NS = 2 (not required) to NS = 3 (required) is the exam favourite, because it is the first time two strings can reverse-feed a third. Memorise the physical sentence as well as the algebra: two strings cannot reverse-feed more than one string’s Isc into a third.

Once the inequality is true, what you fit

The inequality only answers whether protection is required. The device itself still has to protect the module: its operating current must not exceed IMOD_MAX_OCPR, and it must not nuisance-operate on legitimate ISC_MAX (section 10.3). Place the OCPD at the paralleling point — usually in the combiner — so the unprotected length of string cable is short. A fuse on the inverter input that only sees the combined array does not stop one string reverse-feeding another on the combiner bus.

If the array is later extended from two strings to three, re-run the inequality. A two-string array that lawfully had no string fuses can become a three-string array that must have them; the modules did not change, NS did.