A string fuse failure is an open or damaged fuse that protects a single PV string. The string then stops contributing power. Fuses that are wrongly sized for the string current can be damaged. So can fuses that carry more current than their rating, for example when strings are paralleled outside the inverter. In monitoring data it can appear as a sudden output drop roughly equal to one string's power. On inverters with monitored fuses it raises a string current error warning. Thermal imaging can also find heated or overheated string fuses in combiner boxes. To confirm, check each fuse on the fuse boards with a multimeter. If no fuse is open but the alarm stays on, review the inverter's string configuration settings. Before replacing a fuse, open the AC and DC disconnects and disconnect the strings. Wait for stored energy to discharge, and check with a clamp meter that no current is flowing. Replace the open fuse, then check that the string input current does not exceed the fuse rating.
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affectsc1 verified · fact String protection fuse
sec=h269 h=2f21dd0defb6tbl=44.r4 h=84bd9c214225tbl=37.r5 h=b91e58f0f23dtbl=35.r5 h=b91e58f0f23dverified by: review-workflowverified on: 2026-10-23
caused_byc2 drafted · lead Undersized component
Fuses sized incorrectly for the single-string current can be damaged; fuses protect against return currents from a faulty string.
sec=h111 h=ea078ce30773review note: support check: partial - The passages say incorrectly sized fuses can damage the fuse and that fuses protect against return currents. They don't say a string fuse failure is caused by an undersized component. They also don't say the sizing is for single-string current in a way that links to this cause.; review: partial - The passage says fuses sized wrongly for single-string currents can damage the fuse, and that fuses guard against return currents, but it never says the fuse is too small, so the 'undersized' cause goes beyond the text.
c3 drafted · lead Installation error
Strings paralleled outside the inverter can push input current above the fuse rating.
tbl=44.r4 h=84bd9c214225tbl=37.r5 h=b91e58f0f23dtbl=35.r5 h=b91e58f0f23dreview note: support check: partial - The passages say to check whether input current exceeds the fuse rating if parallel strings were made outside the inverter. They don't call this an installation error or state outright that it causes fuse failure.; review: partial - The W017 rows tell you to check that string current does not exceed the fuse rating when strings were paralleled outside the inverter, which implies the cause, but they never state it as a cause or call it an installation…
presents_asc4 verified · fact String current measurement error
Reported as warning W017 String Err. on models with monitored string fuses.
tbl=44.r4 h=84bd9c214225tbl=37.r5 h=b91e58f0f23dtbl=35.r5 h=b91e58f0f23dverified by: review-workflowverified on: 2026-10-23
c5 verified · fact Abrupt production drop
Output falls suddenly by roughly the power of one string.
sec=2.5 h=e99cf005beedverified by: review-workflowverified on: 2026-10-23
c6 verified · fact Hotspot in IR image
Heated or overheated string fuses in combiner boxes show as thermal abnormalities.
sec=5 h=fc0f2dc6c73ffig=29 h=39c8e0698b73verified by: review-workflowverified on: 2026-10-23
detected_byc7 verified · fact Fuse inspection
Check fuse state on the fuse boards with a multimeter.
tbl=44.r4 h=84bd9c214225tbl=37.r5 h=b91e58f0f23dtbl=35.r5 h=b91e58f0f23dverified by: review-workflowverified on: 2026-10-23
c8 drafted · lead Infrared thermography
sec=5 h=fc0f2dc6c73ffig=29 h=39c8e0698b73review note: support check: partial - IR imaging detects heated string fuses in combiner boxes, but the passages do not say it detects a fuse failure (open or blown fuse).; review: partial - The IEA passages say IR imaging detects heated or overheated string fuses in combiner boxes, but they never say it detects a failed (open or blown) fuse.
c9 verified · fact Performance monitoring
A sudden power drop about equal to one string's output points to a string fault such as a blown fuse.
sec=2.5 h=e99cf005beedverified by: review-workflowverified on: 2026-10-23
mitigated_byc10 verified · fact DC fuse replacement
sec=h269 h=2f21dd0defb6tbl=44.r4 h=84bd9c214225sec=4.8.5.2 h=6a584bf20b9atbl=37.r5 h=b91e58f0f23dtbl=35.r5 h=b91e58f0f23dverified by: review-workflowverified on: 2026-10-23
c11 drafted · lead Safety isolation
Open AC and DC disconnects, disconnect strings, wait for stored energy to discharge and confirm zero current with a clamp meter before changing fuses.
sec=h269 h=2f21dd0defb6sec=4.8.5.2 h=6a584bf20b9areview note: review: partial - Every step in the note (open the AC/DC disconnects, disconnect the strings, wait for stored energy to discharge, confirm zero current with a clamp meter) is in the passages, but only as safety prerequisites for replacing a fuse, not as…
c12 drafted · lead Inverter settings review
If no fuse is open but the alarm persists, check the configured presence or absence of input strings.
tbl=44.r4 h=84bd9c214225tbl=37.r5 h=b91e58f0f23dtbl=35.r5 h=b91e58f0f23dreview note: review: partial - The rows do say to check the configured presence or absence of input strings if no fuse is damaged and the alarm persists, but that step deals with a false alarm when no fuse has failed, not with string fuse…
indicates c4indicates c4indicates c5indicates c5