Most service calls fall into a few patterns: "generation is low", "the inverter shows an error", "it keeps tripping", and "nothing at all". A structured method finds the cause quickly and safely, and avoids the expensive habit of replacing the inverter first. The golden rule: look at the data before you open a box.
The method
- Ask and read data. When did it start? After rain, after cleaning, after a storm, after the DISCOM changed the meter? Pull up the app's daily curve and the inverter's error log.
- Find the pattern. Is it every day or only sometimes? At a particular time? On one string or all?
- Look. Walk the roof and open the DB covers (not the live terminals): shade, dirt, damaged cables, burnt connectors, red SPD windows, tripped devices.
- Measure voltages, currents and insulation, with the right instruments and safe isolation.
- Split and isolate. Halve the suspect circuit, test each half, and repeat.
- Fix, re-test and record the cause and the readings.
Low generation
Read the daily curve
The shape of the generation curve tells you a lot:
| Curve shape | Likely cause |
|---|---|
| Smooth bell, but lower than usual everywhere | Soiling, haze, high temperature, or a whole string missing |
| Notch at the same time every morning or afternoon | Shading from a tank, parapet, tree, LA mast or new building |
| Flat top at noon | Clipping at the inverter's AC limit (normal if the DC/AC ratio is high), or export limiting |
| Sudden drops to zero around noon, then restarts | Inverter tripping on grid over-voltage |
| Zero until late morning, then normal | Insulation fault on wet mornings, clearing as the array dries |
Compare strings
If the inverter shows per-MPPT or per-string data, compare them. Identical strings should be within a few percent.
- One string low by about one module's share (say 10% on a 10-module string): a module with failed bypass diodes, a module shorted, or a heavily shaded module.
- One string at zero: blown fuse, open connector, broken cable, or an open isolator pole.
- One string voltage much lower than another: count the modules; a reversed module or a module short.
Worked example
A 6.5 kWp system (two strings of 6 × 545 Wp on two MPPTs) used to make about 28 kWh on clear April days and now makes about 25.5 kWh. The app shows MPPT 1 at 13.1 A and 230 V at noon; MPPT 2 at 13.0 A and 192 V.
MPPT 2's voltage is 38 V lower: almost exactly one module's Vmp at operating temperature (about 38 V). One module in string 2 is contributing nothing. On the roof, a thermal camera shows one module with a hot junction box: its bypass diodes have failed short. The loss of about 2.5 kWh a day matches one module out of twelve (28 ÷ 12 ≈ 2.3 kWh). Replacing the module under warranty restores the output.
Inverter error messages
Names differ by maker; the common families are:
Grid over-voltage / grid voltage out of range
The inverter has measured AC voltage above its trip limit. - Log the voltage at the meter at noon (a recording meter for a day is ideal). - Calculate the rise in your AC cable (module 6, lesson 4) and check terminal tightness. - If the grid itself is high, write to the DISCOM with your readings and ask for a transformer tap or feeder check. - Use the inverter's reactive-power or volt-watt functions if the DISCOM permits. - Never widen the trip limits beyond the DISCOM's settings.
Isolation fault / low insulation resistance ("PV ISO", "Riso low")
At start-up the inverter measures the array's insulation to earth; if it is below its limit, it will not connect. - Typical pattern: errors on wet mornings after rain, clearing by 10–11 am. - Isolate the strings and measure insulation resistance of each, + to earth and − to earth (lesson 3). - Split the bad string at its connectors and test each half to find the section. Common culprits: a connector lying in water, a cable pinched under a clamp or cut on a sharp frame edge, water in a junction box, a monkey-bitten cable.
No grid / islanding
The inverter cannot see the grid: check the ACDB MCB and RCCB, the main DB, a loose neutral, and whether there is an actual outage.
Over-temperature / derating
The inverter is reducing output to protect itself: check for sun on the inverter, blocked vents, dust-clogged heat sinks or a failed fan.
Arc fault (inverters with arc detection)
Treat it seriously: inspect every connector and joint on the string for scorching. Do not simply reset and leave.
Tripping
RCCB trips
- Only on wet mornings, on a large array: natural leakage through module capacitance plus moisture can exceed a 30 mA device's threshold. Check the inverter manual's RCD recommendation (type and sensitivity, often 100 mA or 300 mA for larger arrays) and whether the inverter has internal residual-current monitoring.
- At random times, also when the inverter is off: the fault is elsewhere in the building wiring, not the solar.
- Wrong RCCB type (Type AC instead of Type A or B): replace with the type the manual specifies.
MCB trips
- MCB rated too close to the inverter's maximum output current, or heating in a crowded DB in summer: check that the rating is at least 1.25 × maximum output current and that the DB is ventilated.
- Loose terminals heating the MCB: look for discoloured plastic; re-terminate and torque.
SPD failed
After a storm, check SPD windows. A shorted SPD can trip the RCCB or blow its back-up fuse. Replace the cartridge with the same type and rating.
Key points
- Read the data first: the app's daily curve and the error log reveal most faults.
- Compare strings; a shortfall of one module's voltage points to a failed module or bypass diodes.
- Grid over-voltage trips need voltage logging, cable rise calculations and DISCOM help, never wider trip limits.
- Isolation faults on wet mornings mean moisture or insulation damage: test each string and split to locate.
- RCCB trips on large arrays may be capacitive leakage; follow the inverter manual's RCD guidance.
Check yourself
- Output drops to zero around noon on sunny days and restarts after a few minutes. What is the likely cause?
- An inverter shows "Riso low" every morning after rain and runs normally by 11 am. What do you test, and how do you locate the fault?
- Two identical strings show 400 V and 362 V at the same time. What does the difference suggest?
Answers
- The inverter is tripping on grid over-voltage when export is highest; check grid voltage at the meter and the AC cable rise.
- Measure insulation resistance of each string (+ to earth and − to earth) when the fault is present, then split the bad string at its connectors to find the damp or damaged section.
- About 38 V less, roughly one module's Vmp: one module is not contributing (failed bypass diodes, short, or severe shading).