Blueprint: Coordinating RCD/GFCI, DC fuses, and surge protectors

This blueprint shows how to coordinate RCD/GFCI devices, DC fuses, and surge protectors across solar and storage systems. The goal is clear: fast fault isolation, fewer nuisance trips, and safer operation for PV arrays, inverters, lithium batteries, and loads. The approach reflects field practices from residential rooftops to commercial ESS racks.

Why tight coordination matters

Inverter-based resources limit fault current, which changes how protection should operate. According to Integrating Solar and Wind, fault currents in converter-dominated grids drop because power electronics are current-limited, so traditional protection needs adjustment to remain selective and reliable. Grid codes also set fault behavior and clearing times to keep systems stable; see Grid Codes for Renewable Powered Systems for how clearing times and predictable behavior are specified and updated as systems evolve. Finally, segmentation into protection zones enhances selectivity. A similar idea is used at high voltage level with HVDC breakers to isolate faulted sections while keeping the rest running, as noted in Floating offshore wind outlook. We can apply the same zoning concept in low-voltage PV+ESS to ensure faults get cleared locally.

Device roles and operating principles

RCD/GFCI on the AC side

Residual current protection reduces shock risk and detects ground faults that overcurrent devices may miss.

• Typical trip thresholds: GFCI (North America) 5–6 mA, usually within 25–30 ms for personal protection; RCD (many regions) 30 mA within 300 ms at IΔn, faster at higher residual currents.
• Types: Type A (AC and pulsating DC), Type F (enhanced immunity to harmonics and inrush), Type B (sensitive to smooth DC; used around VFDs/EVSE and some power electronics).
• Selectivity: Use time-delayed “S-type” 100–300 mA upstream, and 30 mA downstream RCBOs per circuit to avoid wide-area trips.

Practical point: Hybrids and microinverters can produce DC leakage components. Type A often suffices; Type F or Type B can be justified by inverter datasheets and local code.

DC fuses for PV strings and batteries

DC arcs sustain differently from AC. Use DC-rated fuses with sufficient voltage and DC breaking capacity.

• PV strings: Use gPV fuses sized per array short-circuit current and code. A common rule is fuse ≥ 1.56 × Isc for conductor and OCP sizing in PV circuits (check local code), and voltage rating ≥ maximum system Voc at lowest temperature.
• Batteries (LiFePO4): Size fuses to continuous charge/discharge current with margin (often ≥125% of maximum continuous current). Use fast aR/semiconductor or class-T style fuses near the battery to limit let-through energy into busbars and inverters.
• Selectivity: Downstream string fuses should clear string faults without opening upstream main DC fuses. Check I²t characteristics to maintain grading.

Surge protective devices (SPDs) for AC and DC

Lightning and switching surges stress semiconductors. Coordinate Type 1/2/3 SPDs to clamp energy and limit residual voltage.

• Type 1 (service entrance): Withstand partial lightning current (Iimp, 10/350 µs), typical per pole 12.5 kA. Use where external LPS is present or in high exposure sites.
• Type 2 (distribution/combiners): Nominal discharge current In typically 5 kA (8/20 µs); Imax 20–40 kA. Use at PV combiners and AC subpanels.
• Type 3 (point-of-use): For sensitive electronics; coordinate with upstream Type 2.
• Key ratings: Uc/MCOV above 1.1 × maximum continuous voltage; Up low enough to protect inverter and battery insulation but high enough to avoid leakage trips. PV SPDs need DC voltage ratings aligned to array Voc,max under cold conditions.

The coordination blueprint

Build protection in layers and zones so downstream devices act first and upstream devices backstop.

• Zone 1: PV array and strings. Each string gets a gPV fuse. A Type 2 DC SPD sits at the combiner. Keep SPD leads short and parallel. The combiner output has a main DC disconnect.
• Zone 2: Battery pack. Place a high-rupturing-capacity DC fuse within 200–300 mm of the positive terminal. Add a DC breaker for isolation. Include a Type 2 DC SPD on the DC bus if cable runs are long or the site is surge-prone.
• Zone 3: Inverter DC and AC. The inverter sits between DC and AC zones. On AC, install a Type 1 SPD at the service (if the site has LPS or high exposure), then Type 2 at the distribution board, and Type 3 near sensitive loads.
• Zone 4: Final circuits. Use RCBOs (RCD+MCB) at 30 mA per circuit. Upstream use a selective 100–300 mA RCD to avoid blackouts.

Timing and current grading:

• Residual current: Upstream S-type RCD adds time delay so downstream 30 mA trips first.
• Overcurrent: Downstream fuse/breaker curves should be below upstream device curves in the relevant fault range. For DC, verify I²t let-through so semiconductor devices receive less energy than their withstand.
• Surge: Place SPDs in a cascaded manner from Type 1 to Type 3, with coordinated Up values to keep the clamping envelope inside equipment basic insulation levels.

Worked example: 10 kW PV + 15 kWh LiFePO4 ESS

System snapshot: Rooftop PV 10 kW, three strings in parallel, Voc,max 580 V in cold weather, Isc,string 11 A; hybrid inverter 10 kVA; battery 51.2 V nominal, 280 A peak discharge (continuous 200 A); service 230/240 V or 120/240 V split-phase.

• PV string fuses: gPV 15 A, ≥800 V DC rating, one per string. Fuse size exceeds 1.56 × Isc (1.56 × 11 A = 17.2 A) only if code and cable allow; in many cases, 15 A strings need 15 A fuses while conductors are sized per code ampacity rule. Verify with local standards. Use a main combiner fuse or breaker rated to array current (e.g., 60 A) and 800–1000 V DC.
• PV SPD: Type 2 DC with Uc ≥ 1.1 × Voc,max (≈640 V). Choose next standard rating (e.g., 800 V DC) with Up below inverter DC input withstand. Keep leads under 0.5 m loop if possible.
• Battery fuse: Class-T 250–300 A, ≥80–125 V DC rating, mounted near the pack. This covers 200 A continuous with headroom and limits I²t for inverter semiconductor protection.
• AC SPDs: Type 1 at service if site risk justifies; Iimp 12.5 kA per pole typical. Type 2 at the inverter AC output panel (In 5 kA, Imax 20–40 kA). Use short, straight connections to earth bar.
• RCDs/RCBOs: Upstream selective RCD at 100–300 mA with time delay. Downstream 30 mA RCBOs for circuits feeding outlets and the inverter AC output to loads. If the inverter manual calls for a specific RCD type (A, F, or B), follow that.

Result: A PV string fault blows the affected gPV fuse without tripping the main DC fuse. A battery bus fault opens the class‑T fuse instantly. A ground fault on a final circuit trips the 30 mA RCBO, while the upstream S‑type RCD holds. Surges get clamped at service and again at subpanels, keeping let‑through low for electronics.

Commissioning, testing, and upkeep

• RCD/GFCI: Press the test button monthly. During commissioning, perform ramp tests to verify trip thresholds and times. Record results.
• Fuses: Confirm cold Voc and Isc values while sizing. After any fault, replace all operated fuses. Keep spare gPV and class‑T units on site.
• SPDs: Inspect visual indicators twice a year. Measure bonding continuity. Replace any SPD with a tripped indicator or abnormal leakage.
• Cabling and terminations: Torque to spec and re‑check after 48–72 hours of operation due to creep. Keep SPD leads short and parallel to reduce inductance.

Common mistakes and quick fixes

• Nuisance RCD trips after adding SPDs: Move line‑to‑PE SPDs upstream of the selective RCD or choose SPDs with very low leakage. Use Type F or time‑delayed RCDs where harmonics or inrush are present.
• Underrated DC fuses: Verify DC voltage rating against worst‑case Voc,max and use high DC breaking capacity. Avoid AC‑only fuses on PV/battery DC circuits.
• Poor SPD wiring: Long, looped leads raise let‑through voltage. Route short, straight, and bonded to a common earth bar.
• Lack of grading: Downstream RCBOs or fuses should act first. Review time‑current curves and I²t data for selectivity.

Standards, data points, and further reading

Setpoints and device choices must align with local codes and equipment manuals. Fault clearing time and stable behavior during disturbances remain top priorities for reliable operation. For context on protection and system behavior:

• Converter-dominated grids supply less fault current, requiring adjusted protection schemes, noted in Integrating Solar and Wind.
• Grid codes define fault ride-through behavior and clearing practices; see Grid Codes for Renewable Powered Systems.
• Protection zones improve selectivity; compare the concept to HVDC breaker zoning described in Floating offshore wind outlook.
• Broader integration practices and terminology appear in System Integration of Renewables.
• Background on PV technology and safety resources can be found at Energy.gov Solar Energy.

Non-legal advice: This content supports engineering judgment but does not replace codes or professional design review. Verify all selections against local standards and manufacturer requirements.

Bringing it together

A coordinated stack—gPV string fuses, battery DC fusing near the pack, cascaded SPDs, and selective RCD/RCBOs—delivers fast fault isolation and stable uptime. It also scales cleanly from a small off‑grid cabin to a multi‑inverter ESS. With careful grading, short SPD leads, and documented test results, you reduce surprises and protect high‑value electronics across the PV and storage lifecycle.

FAQ

Do I need a Type B RCD for a hybrid inverter?

Not always. Many hybrids work with Type A or Type F. Use Type B only if the manufacturer requires sensitivity to smooth DC leakage or local code mandates it.

How do I size PV string fuses?

Use gPV fuses with current ratings aligned to string Isc and local code rules (often based on 1.56 × Isc for conductor/OCP sizing). Ensure the DC voltage rating exceeds worst‑case Voc,max in cold conditions.

Can SPDs cause nuisance RCD trips?

Yes, leakage through line‑to‑PE SPDs can add up. Place SPDs appropriately, select low‑leakage types, and use selective/time‑delayed RCDs upstream with 30 mA RCBOs downstream.

My inverter limits fault current. Will protection still clear?

Yes, with grading tuned to lower fault levels and residual protection. As noted by the IEA, reduced fault currents in converter‑based systems require adjusted settings and device choices.

Should I add DC-side residual current protection?

Many systems rely on insulation monitoring and AC-side RCDs. DC residual devices exist but are less common due to cost and application specifics. Follow equipment manuals and local code.

Do I need Type 1 SPD at my service?

Use Type 1 where there is an external lightning protection system or high exposure. Otherwise, a robust Type 2 at the service or main distribution board may suffice per site risk and code.