Isc Normal, Voc Normal, and the Knee Has Gone Soft

Correct answer: A) Increased series resistance somewhere in the string — a connection, splice or termination that has degraded

The two endpoints of an IV curve and the shape between them answer different questions. Short-circuit current is about how much light is reaching working cells. Open-circuit voltage is about how many cells are contributing. Both are normal here, so the light is arriving and the cells are all in the circuit. What is left is what happens to the power on its way out, and that is the knee.

Resistance in series with the string does not stop current flowing and does not remove any voltage at open circuit, because at open circuit no current flows through it to drop anything across. It only takes its share once the string is delivering, and it takes more as the current rises. That bends the corner of the curve inward and flattens what should be a sharp transition, which is precisely what a fill factor measures. An 8% shortfall in fill factor with both endpoints intact is very close to a signature.

This is also the fault that the other tests in the bag cannot see. A voltage check at open circuit reads normal, because nothing is flowing. A current check reads normal, because the resistance is small compared with the load. The joint announces itself in three places only: in the heat it makes, in a millivolt drop measured across it while current flows, and here, in the shape of a curve. An IV tracer is the instrument that turns a suspicion into a measurement of the whole string at once.

The O&M point: the reference curve is the reason any of this works. A rounded knee means nothing without something sharp to compare it against, and the comparison only holds if both traces were corrected to the same conditions. A commissioning trace, kept, corrected and findable years later, is one of the most valuable things a site can own, and it costs nothing at the time. It is also, routinely, the thing nobody captured.

Why the other options are wrong

B) Soiling or uniform shading across the string

Soiling and uniform shade reduce the light arriving at the cells, and current follows light almost proportionally, so the first thing they move is short-circuit current. The curve would sit lower rather than change shape at the corner, and voltage would be barely affected. Here the current endpoint matches the reference exactly, which rules this out before the shape is even considered.

C) Two modules bypassed by their diodes

Bypassed modules take their cells out of the voltage sum while the rest of the string carries current past them, so the signature is a lower open-circuit voltage with the current endpoint untouched. That is the opposite endpoint from the one that would have to move here, and open-circuit voltage matches the reference. Worth keeping the two signatures side by side, because they are the two easiest to confuse and they point at completely different repairs.

D) A low-resistance shunt path developing inside one module

A shunt path gives current somewhere to go instead of leaving the string, and it does so most visibly at low voltage, so it tilts the part of the curve that should be flat near short circuit rather than rounding the knee. The two defects both cost fill factor, which is why the shape matters and the single number does not: fill factor tells you something is wrong, and where the curve deforms tells you what.

References

• NABCEP OMAT JTA v.2026.5 — Domain 4, Task 1.D: "IV curve characteristics (e.g., short circuit, open circuit, maximum power point [MPP], fill factor)"
• NABCEP OMAT JTA v.2026.5 — Domain 4, Task 3.C: "IV curve tracer", listed among the electrical test equipment. No standard is cited; the answer follows from where a series resistance can and cannot take voltage, and the correction of both traces to the same conditions is the same discipline as any other comparison against a nameplate or a reference.