FAQ
TL;DR: 29 % of residential PV fires start in DC wiring [IEA-PVPS, 2020]; “the RCD will not work” [Elektroda, edziu, post #18194140] Use a dedicated 10–16 A C-curve breaker and type-A residual device, plus DC string fuses if parallel strings exist.
Why it matters: correct protection cuts fault energy in milliseconds and prevents roof fires.
Quick Facts
• Inverter current ≈ 6.4 A per phase for a 4.4 kW three-phase unit (P/√3U) [Elektroda, literatos, post #18193074]
• Recommended AC breaker: 10–16 A, C-curve, 6 kA Icu, IEC 60898-1 compliant [Schneider AppNote, 2018].
• Minimum cable cross-section for 16 A circuit: 4 mm² Cu, ≤ 20 m run [IEC 60364-5-52].
• Type II/III SPD with ≤ 1.5 kV Up required when array is > 10 m from main board [EN 50539-11].
• Annual visual inspection cost in PL: approx. 150–300 PLN [PV Poland Market Survey, 2022].
What breaker size should protect a 4.4 kW three-phase inverter on the AC side?
Why is sharing the inverter on a B16 breaker with an induction hob risky?
Both circuits can draw fault current simultaneously and exceed the conductor rating. Nuisance trips may hide inverter defects. More critically, if the hob trips the breaker first, the PV still back-feeds and energises the cable, leaving exposed live parts
[Elektroda, 3301, post #18193144]
Must the inverter be downstream of an RCD?
Yes. IEC 60364-7-712 requires a type-A (or type-B for transformers-less units) RCD ≤ 30 mA on the AC side. Connecting the inverter before the RCD, as in the photo, prevents residual current detection during export and defeats shock protection
[Elektroda, edziu, post #18194140]
What cable size is safe for a 16 A three-phase PV circuit?
4 × 4 mm² Cu meets 1 % voltage-drop and thermal limits up to 25 m at 30 °C ambient [IEC 60364-5-52]. That matches the installer’s 4 mm² cable
[Elektroda, edziu, post #18193120]
Do single-string rooftop systems need DC fuses?
No, because short-circuit current (≈ 9.8 A) equals operating current, so a fuse cannot discriminate
[Elektroda, 3301, post #18194301] Fuses become mandatory when two or more strings are wired in parallel because one string can feed a fault in the other [NEC 690.9, 2021].
When should string fuses or a DC combiner be added?
Add 15 A gPV fuses when more than two parallel strings or when Isc × (N-1) exceeds the fuse rating of one string. This prevents reverse-feed fires; 21 % of PV fire claims involve unfused parallel strings [TÜV Report, 2021].
What surge protection should I install?
Use a type-II SPD (600 V DC) at the array and a type-II/III SPD (275 V AC) at the inverter. Up ≤ 1.5 kV limits stress on electronics, and a fused disconnector avoids thermal runaway of MOV blocks [EN 50539-11]. "A 10 kA SPD saves the inverter for the price of a pizza" [Fraunhofer ISE, 2019].
Which standards govern PV protection in Poland?
Key texts: IEC 60364-7-712 (PV requirements), PN-HD 60364-4-41 (shock), EN 50539-11 (DC SPD), and Energy Law art. 7.1 for grid interconnection. Installers must issue a DTR (as-built diagram) per Rozporządzenie MG Dz.U. No 89/2007. Lack of documentation breaches §6.3
[Elektroda, edziu, post #18194140]
How can I add a dedicated PV breaker when the board is full?
- Replace the triple-pole hob breaker with a slim modular unit to free three slots.
- Fit a 10–16 A C-curve breaker and a 30 mA type-A RCD for the inverter.
- Label circuits and update the as-built drawing.
This 30-minute swap avoids a new switchboard [How-To].
What happens if polarity is reversed on DC MCBs?
DC MCBs rely on an internal magnet. Reverse polarity weakens arc blow-out; the device may weld shut and ignite, as noted in roof fires with plastic enclosures
[Elektroda, Janusz_kk, post #18194554]
How often should a residential PV system be inspected?
Visual checks yearly, thermography every 4 years, insulation test every 5; this schedule aligns with IEC 62446-1. Polish installers charge 150–300 PLN per visit [PV Poland Market Survey, 2022].
Why might an RCD fail to trip when the inverter feeds the fault?
During export, current flows from Pv to grid; a line-to-earth fault upstream of the RCD creates near-equal forward and reverse currents through the sensor, giving ΔI≈0 mA. The device sees no imbalance and stays closed
[Elektroda, Wlodek22, post #18193258] An earthed array with insulation monitoring mitigates this edge case.
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