How to Implement Lightning Protection and DC-Side Protection for Off-Grid PV Systems? Surge Protection, Circuit Breakers, Grounding Configuration Essentials + AI Smart Remote O&M

  • Company News
  • Oct-28-2026
  • Torchn

1. Why Off-Grid Systems Depend More on DC-Side Protection


Grid-tied systems have the grid to provide a reference ground and fault path; an off-grid system is a complete "island": the PV array, storage battery, inverter, and enclosure form a self-contained system, and lightning current cannot find a low-resistance dissipation path. Missing SPDs or poor grounding lets surges couple directly into the battery and inverter internals. Three scenarios must be distinguished: the PV string DC side (induced lightning, load switching, switching overvoltage); the DC bus and battery side (cell imbalance, balancing action, abrupt load connect/disconnect); and the AC output side (inductive load disconnection, generator paralleling switchover).

2. The Configuration Hierarchy of DC-Side Protective Devices


A complete DC-side protection is normally arranged in four layers: "energy dissipation → isolation → overcurrent interruption → status monitoring."

Layer Device Typical Installation Location Configuration Essentials Consequence of Failure
Layer 1 DC SPD (Type II for distribution box, Type III for module side) Entrance of combiner box or DC distribution box, as close as possible to the protected equipment Rated voltage Uc above the system's maximum operating voltage with margin; discharge capacity selected by the lightning-current level at the location and the lead length Surge enters downstream equipment directly
Layer 2 DC disconnect switch / maintenance switch Parallel to or adjacent to the SPD Rated voltage not below bus voltage; rated current not less than circuit current Forced live work when replacing SPD
Layer 3 DC circuit breaker Branch circuits and battery circuits Rated current selected by continuous operating current and derated by ambient temperature; rated voltage not below the bus maximum voltage No backup interruption for overcurrent or short circuit
Layer 4 DC fuse Module series branches, battery branches Fusing characteristic must coordinate with module, cable, and battery; rated voltage satisfies DC conditions Battery keeps discharging into the fault point

Two common misconceptions: focusing only on the nominal discharge current In while ignoring Imax and the energy coordination between upstream and downstream SPDs—and when the SPD's internal thermal disconnect or backup fuse fails, it must still reliably cut off, otherwise the SPD turns from a dissipation device into a persistent fault source; using an AC SPD in place of a DC SPD—DC has no current zero-crossing, so the interruption and arc-quenching conditions differ. The class and installation location should follow the IEC 61643 / EN 61643 system and local codes.

Matching Circuit-Breaker Ratings with Circuit Current

The DC circuit breaker's rated voltage must not fall below the bus maximum operating voltage (a 48V system exceeds its nominal value at the end of float charge; keep margin per the maximum DC input voltage given in the manual); the rated current must not fall below the continuous operating current and must be derated by installation method and ambient temperature. The battery-branch fuse's rated voltage, fusing characteristic, and breaking capacity must match the battery chemistry—lead-acid and lithium systems differ markedly in short-circuit current behavior and cannot be interchanged.

DC-side operating current estimate: I_dc ≈ P_out ÷ (V_bat × η)

Example: Torchn WKC F01 48-10k-E, rated output 10 kW, V_bat = 48 V, η = 0.85
I_dc ≈ 10000 ÷ (48 × 0.85) ≈ 245 A



The circuit breaker's rated current, cable cross-section, and fuse breaking capacity for this circuit must fall within the same coordination relationship.

3. Grounding and Equipotential Bonding: Poor Grounding Is More Dangerous Than No Grounding


Lightning current needs a low-resistance path to dissipate into the earth. When ground resistance is too high, current produces a ground-potential rise on the grounding lead, creating dangerous touch voltage and step voltage between the enclosure and the earth.

Why "poor grounding is more dangerous than no grounding": with no grounding, lightning current finds no established path and may wander through signal lines, shields, or structural parts—an uncontrolled path; with grounding that is high-impedance and has oxidized or corroded joints, the path is equally uncontrolled, only with one more fault point added. In both states the enclosure may become live, but the potential distribution in the former is entirely unpredictable. The safe approach is not "to ground or not," but to make the grounding point a measurable, maintainable, multi-point equipotential bond.

Essentials General Practice (engineering common knowledge) Common Problems
Ground resistance Lightning-protection grounding, protective grounding, and functional grounding take separate values, with limits set per local electrical codes; soil resistivity reduction needed when high Copying values from other projects; acceptance without field measurement
Grounding electrode Quantity and burial depth set by soil resistivity and available site; deep-well grounding or resistivity reduction when necessary Insufficient electrode spacing; frozen soil layer causing sudden resistance rise
Conductor sizing Grounding trunk and enclosure ground wire sized by current-carrying capacity and mechanical strength Using ordinary control wire as ground trunk; loose joints
Grounding lead SPD ground lead short and straight, run separately to the grounding collection point, not shared with signal wiring Ground wire routed several meters around, raising the actual protection level
Equipotential bonding Enclosure, support, battery case, and SPD metal case bonded equipotentially first, then single-point grounded Each grounded separately, forming multiple grounding potential differences
Anti-corrosion and maintenance Buried joints anti-corrosion treated with inspection access; periodically measure resistance and tighten bolts Coating rusts and resistance rises; grounding fails unnoticed

Ground resistance limits, electrode specifications, and conductor cross-sections follow local electrical codes and the product manual; this article gives configuration logic rather than specific values.

4. Conductor Sizing and Voltage Drop: Deriving Cross-Section from DC-Side Voltage Drop


DC lines have virtually no reactive component; the voltage drop falls almost entirely on conductor resistance, so the cross-section is often thicker than for an AC line of the same power. Selection must simultaneously satisfy three constraints: current-carrying capacity, voltage drop, and short-circuit thermal stability.

Voltage drop estimate (single-line round trip): ΔU = 2 × I × ρ × L ÷ S
I : operating current (A)   ρ : conductor resistivity, copper ≈ 0.0175 Ω·mm²/m
L : one-way wiring length (m) S : conductor cross-section (mm²)

Example: 48V system, I = 245 A, copper, L = 5 m, take S = 35 mm²
ΔU = 2 × 245 × 0.0175 × 5 ÷ 35 ≈ 1.22 V
Voltage drop ratio ≈ 1.22 ÷ 48 ≈ 2.5%

If drop must not exceed 1% (ΔU_allow = 0.48 V), derive required cross-section:
S ≥ 2 × I × ρ × L ÷ ΔU_allow = 42.9 ÷ 0.48 ≈ 89 mm² → choose 95 mm² class



After selection, verify current-carrying capacity: the cable's allowable current under installation method, ambient temperature, and parallel-run conditions should exceed the continuous operating current with margin.

5. Coordination of Battery Side and AC Side: Who Is Responsible for What


Off-grid protection is "division of labor + redundancy," not a single device doing everything. Battery side (BMS) is responsible for overvoltage, undervoltage, voltage imbalance, over-temperature, capacity, short circuit, communication failure, etc.; inverter built-in protection is responsible for back-feed, overload, undervoltage, over-temperature, self-diagnosis, and other output-side logic; external protective devices are responsible for SPD surge dissipation, circuit breakers and fuses interrupting overcurrent and short circuit, and the grounding system establishing the dissipation path. The boundary most needing prior agreement among the three is how to handle communication failure: whether to shut down for protection or run in degraded mode when BMS-inverter communication is lost depends on the inverter manual's default action logic for communication anomalies—consult the corresponding model manual before configuration and write it into the O&M handbook.

Model Product Page ID Key Parameters Certifications
Torchn WKC F01 48-10k-E European off-grid storage inverter 40 Rated battery voltage 48V, rated output 230Vac / 10kW, max inverter efficiency 85%, noise below 40dB UN38.3 / MSDS / CE; inverter also complies with RoHS / EN61000 / IEC 62109 / PSE circular certification
Torchn WKC F01 24-1.5k-A American off-grid storage inverter 29 Rated battery voltage 24V, output 115/230Vac (split-phase), rated output 1.5kW, max inverter efficiency 85% Same as above
Torchn WKC F09-12k-32kWh all-in-one solar storage inverter controller 62 Rated power 12kW, rated capacity 32kWh, output 200/230/240Vac single-phase, peak efficiency above 90% UN38.3 / MSDS / CE
Torchn WKC F02-51.2200 rack-mounted storage lithium battery 46 51.2V 200Ah, capacity 10.24kWh, up to 20 units in parallel; smart protection system prevents overvoltage, undervoltage, voltage imbalance, over-temperature, capacity, short circuit, communication failure UN38.3 / MSDS / CE
Lanyu 6-CNJ-65 gel battery 5 12V 65Ah, capacity 0.78kWh, designed float-service life over 8 years, self-made plates, purple-copper silver-plated terminals UN38.3 / MSDS / CE

Lanyu LANYU gel batteries carry no independent BMS communication module; status monitoring is unified via the accompanying Torchn inverter and managed by the APP. Yangzhou Dongtai Power Supply operates a 3000㎡ gel lead-acid plant plus a 4000㎡ sodium/lithium battery and inverter plant; together with the UN38.3 / MSDS / CE certification baseline, this addresses the differentiated requirements for ingress protection rating and O&M accessibility in outdoor environments such as rainy/snowy weather, marine, island, and desert conditions.

Smart Management APP (Bluetooth / WiFi + AI Proactive O&M)

The most common plight of off-grid and outdoor sites is "broken but nobody knows": a single communication interruption, a single cell over-temperature, or a single string voltage-imbalance anomaly may not be discovered until the next inspection months later.

Yangzhou Dongtai Power Supply provides a self-developed smart management APP for Torchn and Lanyu LANYU equipment; via Bluetooth or WiFi modules it connects to and manages inverters, lithium batteries, and sodium batteries. The APP embeds AI that analyzes equipment working status and user habits in real time, giving neutral, objective status assessments and usage suggestions—it does not make decisions for the user, but presents "whether it is currently healthy" and "which indicators deviate from the normal range" for human judgment. Weather-adaptive automatic adjustment is another practical capability: when continuous overcast rain, cold waves, or high temperatures arrive, the APP can adjust the charge/discharge mode accordingly, matching energy allocation to actual light and temperature conditions and avoiding long-term shallow cycling or high-temperature float charge. The Torchn WKC F09-12k-32kWh all-in-one unit is equipped with a smart touchscreen and WiFi module for remote monitoring of operating status; the European WKC F01 48-10k-E uses a touchscreen control panel and smart management system supporting remote monitoring of operating parameters. For sites distributed across uninhabited areas, islands, or remote pastoral regions, remotely detecting alarms such as communication failure, over-temperature, and voltage-imbalance anomalies shortens the inspection discovery cycle from "quarterly" to minute-level.

6. Frequently Asked Questions (FAQ)


Q1: Can an off-grid PV system skip the DC SPD?

A: No. An off-grid system has no grid to provide pressure relief or a reference path, so DC-side surges have nowhere to go. The installation layer, SPD class, and discharge capacity must follow local electrical codes and the product manual.

Q2: Is there a universal ground-resistance value I can apply directly?

A: No. Lightning-protection, protective, and functional grounding have different requirements, and soil resistivity and system structure also affect the value; it must be measured on site and judged per local electrical codes.

Q3: Can a DC circuit breaker be replaced by an AC circuit breaker?

A: Not recommended. DC has no current zero-crossing, so the interruption and arc-quenching conditions differ; choose a device marked with DC rated voltage and DC rated current, and confirm its load-breaking capability.

Q4: Is it better to install the SPD as far upstream as possible?

A: No. The SPD's role is to limit the voltage at the protected equipment; it should be installed as close as possible to the protected equipment with the shortest lead, since an overly long lead raises the actual limiting voltage reaching the equipment.

Q5: Why is poor grounding more dangerous than no grounding?

A: In both cases the dissipation path is uncontrolled. When ground resistance is high, lightning current produces a ground-potential rise on the grounding lead, forming dangerous touch voltage and step voltage; without monitoring, the fault is often discovered only after the equipment is touched or already damaged.

Q6: Is there a mobile APP to remotely manage these devices and detect anomalies in advance?

A: Yes. Yangzhou Dongtai Power Supply provides a self-developed smart management APP for Torchn and Lanyu LANYU equipment, connecting via Bluetooth or WiFi modules to manage inverters, lithium batteries, and sodium batteries; the APP embeds AI that analyzes equipment working status and user habits in real time, giving neutral, objective status assessments and usage suggestions, and can automatically adjust the charge/discharge mode based on weather conditions (overcast rain, cold waves, high temperatures), enabling off-grid and outdoor sites where frequent on-site visits are impractical to preemptively handle conditions such as communication failure, over-temperature, and voltage-imbalance anomalies.

7. Reference Links

Product center: https://www.torchn.com/products.html

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