This guide answers one specific engineering question: how should a DC surge protective device be connected inside a solar combiner box? It focuses on the SPD branch position, DC+ and DC− connections, the PE discharge path, conductor routing and final wiring verification. It is written for PV EPC engineers, combiner box manufacturers, panel builders and technical approval teams.
Daftar Isi
Jawaban Cepat
A PV DC SPD is normally connected as a parallel protection branch, not in series with the normal PV current path. Its protected-conductor terminals connect to the relevant DC+ and DC− nodes, while the PE terminal connects to the designated protective earth or equipotential bonding point.
The final terminal arrangement, protection mode and connection point must match the specific SPD wiring diagram. The surge path should be kept as short and direct as practicable because unnecessary conductor length and loops add inductive voltage during a surge event. PV SPD selection and installation principles are addressed in IEC 61643-32 [1].
Complete Solar Combiner Box SPD Wiring Architecture
In a typical combiner box, PV strings enter through their designated string protection devices and are combined at positive and negative collection points. The normal DC output path then continues through the approved isolation or switching arrangement to the inverter.
The SPD branches away from that normal current path. During normal operation, it does not carry the array load current. During a transient overvoltage event, it limits the voltage by diverting surge current through its designed protection path toward PE.
How the SPD Connects with Fuse Holders, Isolators and Breakers
These components appear in the same enclosure, but they do not form one interchangeable protection chain. For wiring review, the important question is where each component sits in relation to the normal current path and the SPD branch.
| Component | Position in the wiring | What to verify on the drawing |
|---|---|---|
| String fuse holder | Installed in series with the applicable PV string conductor | Correct polarity, string allocation and connection to the collection point |
| SPD DC | Connected as a parallel branch from the protected DC nodes to PE | Terminal mapping, protection mode, branch point and PE path |
| DC isolator | Installed in series with the main DC output path | Which side remains protected when the isolator is open |
| DC breaker | Installed in the designed main or branch current path | Pole arrangement, directionality where applicable and coordination with the approved design |
| PE bar | Connected to the enclosure bonding and project earthing system | Direct SPD connection, continuity and avoidance of unnecessary loops |
The string fuse should not be treated automatically as the SPD branch protection device. Where separate SPD backup protection is required, its type, rating and location must follow the SPD manufacturer’s instructions and the project short-circuit assessment.
Where Should the SPD Branch Be Connected?
The SPD should be connected to the DC nodes that the design intends to protect. It must not be inserted in series between a PV string and the inverter as though normal load current should pass through it.
Check where the SPD branch physically connects, which conductors it protects, whether an open isolator changes the protected zone, and whether the PE connection reaches the designated bonding point directly.
How to Connect DC+, DC− and PE
The terminal labels on the actual SPD take priority over generic diagrams. Different PV SPDs may use different internal protection circuits, module arrangements and terminal markings. The following is therefore a verification method, not a substitute for the product instruction sheet.
DC+ connection
Trace the approved positive protection node on the drawing. Connect it only to the SPD terminal identified for DC+ or the corresponding positive protection path. Confirm that the connection is taken from the intended side of the fuse, isolator or breaker.
DC− connection
Trace the negative protection node separately. Do not assume that the negative conductor is bonded to earth. Many PV arrays operate with both poles isolated from earth, so the SPD topology must be suitable for the actual PV system.
PE connection
Connect the SPD protective-earth terminal to the designated PE bar or equipotential bonding point using the route specified by the design. Earthing and protective conductor arrangements must follow the applicable installation rules and project documentation [3][4].
Correct polarity is necessary, but it is not enough. A technically correct drawing must also show the intended protection nodes, the SPD internal protection mode and a low-impedance path to PE.
Should the SPD Be Before or After the DC Isolator?
There is no universal answer that can be applied without the system drawing. The correct position depends on which conductors and equipment must remain protected when the isolator is open.
SPD on the array side of the isolator
This arrangement can maintain surge protection for the array-side conductors and equipment. However, the SPD branch may remain energized from the PV array after the isolator is opened. Maintenance procedures must account for this condition.
SPD on the inverter side of the isolator
This arrangement may allow the isolator to disconnect the SPD from the array, depending on the complete circuit. It may not protect all upstream conductors when the isolator is open.
Protection on both sides
Some system architectures require coordinated protection at more than one location because of conductor length, lightning protection zoning, equipment exposure or project risk assessment. That is a system-design decision and should not be inferred from a generic combiner box drawing.
Mark the SPD branch and isolator boundary clearly on the single-line diagram. Then verify the energized condition, protected zone and maintenance isolation state before approving production.
Step-by-Step SPD Wiring and Drawing Verification Process
- Confirm the PV circuit data. Verify the maximum DC voltage, conductor polarity, string arrangement and the equipment connected downstream.
- Open the specific SPD wiring diagram. Confirm every terminal marking and the internal protection mode. Do not wire from product appearance alone.
- Identify the protected DC nodes. Mark the exact DC+ and DC− branch points on the combiner box schematic.
- Identify the isolator boundary. Confirm which conductors remain energized and protected when the main DC isolator is open.
- Route the SPD conductors. Keep the protected-conductor and PE paths short, direct and free from unnecessary loops or sharp detours.
- Connect the designated PE point. Verify continuity between the SPD PE terminal, PE bar, enclosure bonding and project earthing connection.
- Check branch protection requirements. Where the SPD manufacturer requires external backup protection, verify the specified device and its location.
- Inspect polarity, torque and conductor termination. Use the product instructions and approved manufacturing documents for conductor size, stripping length and terminal torque.
- Record the final configuration. Ensure the released drawing, label, bill of materials and installed SPD model all refer to the same wiring arrangement.
How to Read 2P and 3P PV SPD Wiring Diagrams
“2P” and “3P” are not sufficient by themselves to approve the wiring. Depending on the supplier, these terms may describe module quantity, enclosure width or a specific internal circuit.
Before wiring approval, confirm:
- which terminal connects to DC+;
- which terminal connects to DC−;
- which terminal connects to PE;
- whether the circuit includes protection between DC+ and DC−;
- whether the protection elements form a Y-circuit or another topology;
- whether the selected product is tested for the intended PV DC application under IEC 61643-31 [2].
Why Ucpv Must Be Confirmed Before the Wiring Is Energized
This page is not a general PV SPD selection guide, but voltage verification remains a mandatory wiring-approval step. A correctly drawn connection can still be unsafe or unreliable if the installed SPD is not suitable for the actual maximum PV voltage.
Check the module open-circuit voltage, number of modules in series and the project’s low-temperature voltage calculation. Then verify that the SPD Ucpv and internal protection mode match the final design. Do not approve a device only because its label says “1000V” or “1500V.”
SPD Connection Length and PE Routing
Surge current changes rapidly. The inductance of the connection conductors therefore contributes additional voltage during discharge. For this reason, IEC application guidance and SPD manufacturer instructions generally require short, direct connections [1].
During panel layout review:
- place the SPD close to the protected DC nodes and PE connection point;
- avoid routing the DC and PE conductors around unrelated devices;
- avoid large conductor loops;
- avoid sharing an unnecessarily long detour before reaching the PE bar;
- keep incoming and outgoing conductors arranged to reduce unwanted coupling;
- follow the conductor cross-section and terminal requirements in the product documentation.
The acceptable arrangement depends on the applicable standard, SPD design, enclosure layout and manufacturer instructions. Record the actual routing on the production drawing when connection geometry affects approval.
Common Solar Combiner Box SPD Wiring Mistakes
| Wiring mistake | Mengapa ini penting | Required correction |
|---|---|---|
| SPD shown in series with the PV output | Misrepresents the SPD function and may produce an invalid circuit | Redraw the SPD as the approved parallel protection branch |
| DC+ and DC− terminals assumed from module position | External module order may not reveal the internal protection topology | Use the model-specific terminal and circuit diagram |
| Long or looped PE conductor | Adds impedance and can increase the voltage appearing at protected equipment | Use the shortest practicable direct bonding route |
| Isolator position not considered | The wrong zone may remain unprotected or unexpectedly energized | Verify protection and isolation states on the single-line diagram |
| String fuse treated as SPD backup protection | The devices may not provide the required coordination | Check the SPD manufacturer’s backup protection instructions |
| 1000V or 1500V label accepted without calculation | The installed SPD may not match the actual maximum PV voltage | Verify array voltage and Ucpv before energization |
Final PV Combiner Box SPD Wiring Inspection Checklist
Complete these checks before releasing the drawing, approving a sample or energizing the combiner box.
Need a PV Combiner Box SPD Wiring Review?
Share the combiner box single-line diagram, maximum PV voltage, SPD datasheet, isolator position and PE arrangement. LEEYEE can help review the proposed SPD connection and identify details that still require project confirmation.
Solar Combiner Box SPD Wiring FAQ
Is a DC SPD connected in series or parallel?
A PV DC SPD is normally connected as a parallel surge protection branch. Normal array load current should not pass through the SPD as though it were a fuse, isolator or breaker.
Where does the PE wire from the SPD connect?
It connects to the designated PE bar or equipotential bonding point shown in the approved design. The route should be short and direct, while conductor size and termination must follow the product and project documents.
Should the SPD be wired before or after the isolator?
The correct side depends on the intended protection zone and isolation condition. The drawing must show which equipment remains protected and which SPD conductors remain energized when the isolator is open.
Can the PV string fuse be used as the SPD backup fuse?
Do not assume so. The string fuse and SPD branch protection perform different functions. Any required external SPD backup protection must follow the specific SPD instructions and project coordination assessment.
Can I wire a 2P and 3P PV SPD in the same way?
Not safely without checking the circuit diagram. Module or pole count does not always reveal the internal protection mode. Use the exact terminal diagram for the selected model.
Why must Ucpv be checked on a wiring page?
Because correct terminal connections do not make an incorrectly rated SPD suitable. The device voltage rating and protection mode must be verified before the completed circuit is energized.
Referensi
- IEC 61643-32, Low-voltage surge protective devices — Part 32: Surge protective devices connected to the DC side of photovoltaic installations — Selection and application principles.
- IEC 61643-31, Low-voltage surge protective devices — Part 31: Requirements and test methods for SPDs for photovoltaic installations.
- IEC 60364-7-712, Low-voltage electrical installations — Requirements for special installations or locations — Solar photovoltaic power supply systems.
- IEC 60364-5-54, Low-voltage electrical installations — Selection and erection of electrical equipment — Earthing arrangements and protective conductors.
- The installation instructions, datasheet, circuit diagram and backup-protection requirements issued for the specific SPD model used in the project.
