Two Dead Strings at Opposite Ends of the Roof

Correct answer: D) A single fault in whatever those two strings share, because they are electrically associated even though they sit far apart

Two drawings of the same roof organise it by two different principles, and only one of them is the principle a fault obeys. The site plan groups by geography, because it exists to tell you where to walk. The string diagram groups by electrical association, because it exists to tell you what is connected to what. A fault propagates along the second of those, so the forty metres between the two strings is information about the roof and not about the fault.

Once you read them as one circuit, the question becomes what they have in common, and the list is short: the portion of the run they share downstream of the combiner, the combiner's own output protection, and the MPPT channel itself. Any one of those going open takes both strings to zero and leaves everything else on the array untouched. Two separate faults appearing on the same morning, on the two strings that happen to share an input, is a coincidence you should be reluctant to buy when a single explanation is sitting there.

It also changes where you go first. Reading the site plan alone sends a technician to two ends of a roof to inspect strings that are probably fine. Reading the string diagram sends them to one enclosure. The drawings are not interchangeable and the one you happen to have open decides your afternoon.

The O&M point: plan sets get used differently in service than in construction. During installation the drawings are instructions, read in the direction of building the thing. In service they are evidence, read backwards from a symptom, and the string diagram is the one that answers the question service actually asks: what else is on this circuit. It is also the drawing most often missing from the folder handed over at the end of a job, which is worth discovering before you need it rather than during a fault.

Why the other options are wrong

A) Two independent string faults that happened to occur at the same time

This is what the site plan alone suggests, and it asks you to accept that two faults arrived on the same morning and landed on the two strings that share an input. Coincidences do happen, which is why this is worth ruling out rather than dismissing, but the order of work is to examine the shared element first because it explains both observations at once. Going looking for two faults is the expensive version of this afternoon.

B) Shading heavy enough to take both strings to zero, at both ends of the roof at once

Shading is the reflex explanation for low string output and it does not fit either half of what was measured. Zero current means an open circuit rather than a reduced one: shade cuts production, sometimes severely, and does not usually take a string to nothing. Shade heavy enough to do it at two points forty metres apart at the same moment would be visible from the ground and would be affecting the strings in between as well.

C) The array is mis-wired, since strings at opposite ends of a roof should not land on the same input

Routing strings to inverter inputs by geography is not a rule, and it is often deliberately not done. Inputs get balanced for current, runs get matched for length and voltage drop, and shading groups get kept together so that one shaded area does not drag on several trackers. Strings from opposite ends of a roof sharing an input is a design decision rather than an error, and treating an unfamiliar layout as a mistake is a good way to spend the day on the wrong question.

References

• NABCEP OMAT JTA v.2026.5 — Domain 3, Task 1.B: "Electrical diagrams (e.g., one-line or three-line diagrams, wiring diagram, string diagram)"
• NABCEP OMAT JTA v.2026.5 — Domain 3, Task 1.A: "Site plan and array layout", listed separately from the electrical diagrams. That separation is the point of the question: the two documents organise the same site by different principles, and a fault follows only one of them.