Field reference for utility scale solar technicians. 7 entries, taken from the FieldWatt app.
Reference notes from FieldWatt, the offline field toolkit for utility scale solar commissioning and O&M technicians. It works with no cell signal, because the sites do not have any.
Get FieldWatt on the App StoreA DC combiner on a 1500 volt plant carries lethal voltage whenever the sun is up, on both the line and load side, regardless of inverter state. Opening the combiner disconnect does not de energize the string side terminals. Fuses can be energized from both directions. Arc flash risk applies above roughly 400 volts DC.
Qualified person only. Open the combiner disconnect, verify absence of voltage with a DC rated meter on every circuit you will touch, and wear DC rated PPE per the site arc flash study. Strings cannot be turned off; work them as energized or cover the modules.
Follow the site specific switching and LOTO procedures.
Symptoms of a blown string fuse: the string reads near zero current while neighbors under the same irradiance carry normal current, and the combiner output current runs low by one string. With the disconnect open, a blown fuse shows string Voc across the fuse holder (source on one side, bus at a different potential on the other) while a good fuse shows near zero volts across it. Find why the fuse blew before replacing it: ground fault, damaged cable, reversed polarity from construction, or a fuse undersized against NEC 690.9.
Never pull or insert fuses under load. Open the disconnect, verify absence of current, then handle fuses. Replace with the exact class and rating from the design documents.
Verify replacement fuse rating against the module maximum series fuse rating and the design documents.
Identical strings on the same combiner should measure nearly identical open circuit voltage and, under stable sun, nearly identical current. A string more than 5 percent off its neighbors in current, with at least 600 watts per square meter of irradiance, indicates a fault: shading, soiling, a failed module, a bad connector, or a blown bypass diode. A string low in Voc by roughly one module voltage usually has a shorted bypass diode or a failed module. Meaningful current and voltage curve tracing needs at least 500 watts per square meter.
Measure with a DC rated clamp meter and voltmeter. Compare like to like under the same conditions and record irradiance with every reading.
Record module temperature and irradiance so comparisons hold up later.
The combiner is the natural sectioning point for insulation resistance testing: lift or isolate strings and test each circuit to ground, comparing against the IEC 62446 minimum for the system voltage. Test dry when possible; a circuit that fails wet and passes dry has a moisture path that will return. Halve the search space each test to localize a fault fast.
Isolate the circuit under test from the inverter and from parallel strings before applying test voltage. Surge protective devices must be disconnected or they will read as faults and may be damaged.
Use the test voltage class appropriate to the circuit per IEC 62446 1.
Scan combiners under load with an infrared camera at least annually and after any fuse event. A fuse holder, lug, or breaker running visibly hotter than its neighbors at similar current indicates a loose or corroded connection on its way to failure. Compare like to like: same current, same sun, same enclosure.
Scan through the open door only with PPE appropriate to the site arc flash study, or use an infrared window if fitted.
Trend images over time; absolute temperatures depend on load and ambient.
Loose terminations are the leading cause of combiner fires. Verify torque on line and load lugs, fuse holders, and ground bars with a calibrated torque tool to the values on the enclosure label or in the manufacturer sheet, and mark verified terminations with a paint pen. Aluminum conductors relax and deserve rechecking in the first year.
Torque work is dead work: isolate and verify absence of voltage first.
Use manufacturer torque values, not generic tables, whenever the label is legible.
Most combiner surge protective devices carry a status window or remote contact. A failed SPD can leak to ground and trip inverter insulation monitoring, so check SPD status early when chasing insulation faults. Replace failed cartridges with the identical part; SPD ratings are matched to the 1500 volt system.
Isolate before replacing cartridges. Confirm the ground lead is intact and torqued; an SPD without a solid ground bond protects nothing.
Verify replacement part numbers against the design documents.
Reference notes from FieldWatt, the offline field toolkit for utility scale solar commissioning and O&M technicians. It works with no cell signal, because the sites do not have any.
Get FieldWatt on the App StoreThese notes are a field aid, not a substitute for the governing codes, the stamped drawings, the authority having jurisdiction, or manufacturer manuals. Verify against the current documentation for your installed equipment.