Glossary · DC and AC electrics
String fuse (gPV)
Protects a string from reverse current fed by parallel strings; under DSTU HD 60364-7-712 it is needed with three or more strings in parallel.
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Definition
When several strings are paralleled, a faulty string (a short in a panel or cable) can receive current from all the others. With one or two strings this current stays within what a panel withstands, so no separate protection is needed. From three parallel strings, DSTU HD 60364-7-712:2022 (cl. 712.431) requires overcurrent protection for each string.
1,1 · ISC MAX ≤ In ≤ IMOD_MAX_OCPRThe fuse rating In must be at least 1.1·ISC MAX of the string and no more than IMOD_MAX_OCPR, the maximum protective current allowed by the panel datasheet. Both poles, plus and minus, must be protected (cl. 712.432). The device’s rated voltage Ue must be at least the string’s UOC MAX. PV circuits use gPV class fuses.
Example: a string with ISC STC = 13.9 A has ISC MAX = 17.4 A, so In is at least 19.1 A. Whether the nearest rating fits under IMOD_MAX_OCPR can only be read from the specific panel’s datasheet. If the inverter has a separate MPPT input for every one or two strings, fuses are often not needed at all.
How to count parallel strings. A note to cl. 712.431 of DSTU HD 60364-7-712:2022 lets strings be counted separately for each independent MPPT input when there is no back-feed between inputs. So an inverter with two trackers and two strings on each needs no string fuses, while the same four strings on one input do. Protective devices in the DC circuit must be rated for direct current: rated voltage at least UOC MAX, breaking capacity at least ISC MAX. Ordinary AC devices are not suitable here.
How Solispec applies it
Solispec counts the parallel strings on each MPPT input. With three or more, it adds gPV fuses to the bill of materials — one per string, both poles — and states the conditions in the check: In ≥ 1.1·ISC MAX, In ≤ IMOD_MAX_OCPR from the datasheet, Ue ≥ the input’s UOC MAX. Catalogues rarely list IMOD_MAX_OCPR, so the engine does not assume it and asks you to check the datasheet.
Norms and clauses
- DSTU HD 60364-7-712:2022, cl. 712.431, 712.432
Related terms
- Maximum short-circuit current ISC MAXThe design current of a PV array, I_SC MAX = K_I·I_SC STC with K_I of at least 1.25; cables and protection are sized by it.
- Maximum open-circuit voltage UOC MAXThe open-circuit voltage of a panel or string at the site minimum temperature; inverters, fuses and isolators are selected by it.
- PV stringA chain of panels connected in series: voltages add up, the current is shared; frost and the inverter MPPT window limit its length.
- PV DC cableThe cable between the panels and the inverter; its section is chosen by I_SC MAX at 70 °C and checked for voltage drop.
- MPPT and the inverter voltage windowThe inverter’s maximum power point tracker; its voltage range sets the minimum and maximum number of panels in a string.
- DSTU HD 60364-7-712:2022: the electrics of PV systemsNorms explained
Solispec’s calculation is preliminary: for a design it has to be checked by an engineer. The values and clauses follow the current editions of the norms on the date of the page.