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A Type 2 DC surge protection device (SPD) is wired in parallel between the DC conductors and earth: its positive terminal connects to DC+, its negative terminal to DC−, and its earth terminal to the protective earthing busbar. Get that connection wrong, or install the SPD in series with the load, and the device cannot do its job — and in some cases it can damage itself or the equipment it is meant to protect. This guide shows the standard Type 2 DC SPD wiring arrangement for photovoltaic (PV) and other DC systems, walks through installation step by step, and lists the wiring mistakes that cause most field failures.
The scope here is the SPD itself: how its terminals are connected, where it sits in the circuit, and how it is mounted and verified. For the wider picture of how an SPD shares a solar combiner box with string fuses, a DC MCB and the inverter feed, see the site guide to solar combiner box wiring diagrams. For help choosing the Type 2 DC SPD model in the first place, the Type 2 DC surge suppressor selection guide covers ratings and application.
A Type 2 DC SPD behaves like a voltage-controlled switch connected between each DC conductor and earth. Under normal operation it is effectively invisible: almost no current flows through it. When a lightning-induced or switching surge raises the voltage above its clamping level, the SPD becomes conductive, diverts the surge current to earth, and clamps the voltage that the protected equipment sees. When the surge passes, it returns to its high-impedance state. This is why the SPD must be connected in parallel with the protected circuit — never in series.
A typical three-terminal Type 2 DC SPD for a two-wire PV circuit is wired as follows:
| SPD terminal | Connect to | Purpose |
|---|---|---|
| + (DC+) | Positive DC conductor from the array or battery | Voltage limiting between DC+ and earth |
| − (DC−) | Negative DC conductor | Voltage limiting between DC− and earth |
| ⏚ / PE | Protective earth (PE) busbar or grounding point | Surge current path to earth; must be short and low impedance |
Terminal markings and the number of protected poles vary by manufacturer and by SPD configuration. Always follow the wiring diagram printed on the specific device and its data sheet.
Conceptual wiring diagram only, not a site drawing or to scale. The SPD is tapped in parallel; load current continues past it to the inverter.
Type 2 DC SPDs are installed on the DC side of photovoltaic systems, typically at one of two locations: inside a solar combiner box at the array, or at the DC input of the inverter. In larger systems the SPD may be fitted at both points, with the more exposed location carrying the heavier surge duty.
The physical rule is simple: the SPD should sit as close as practical to the equipment it protects, and its connecting leads must be as short as possible. Long leads add inductance, and inductance raises the effective protection level — a device with an excellent voltage protection rating (Up) can still let a damaging voltage through if it is wired with metre-long tails. Most installation instructions ask for each SPD lead to be kept under roughly 0.5 m; where the SPD and the protected equipment are far apart, a second SPD closer to the equipment is normally the better answer than a single device with long leads.
Because the DC side of a PV system behaves differently from an AC mains circuit, the SPD must be a DC-rated model with a DC voltage rating matched to the array. The difference between AC and DC surge protection is not cosmetic — voltage, discharge capability and fault behaviour all differ. See the comparison of AC SPD vs DC SPD before selecting a device for a solar application.
No wiring diagram can compensate for an SPD that does not match the circuit. Check these values against the SPD data sheet before mounting anything:
The meanings of these parameters are explained in detail in the guide to Iimp, In, Imax and Up SPD specifications.
A DIN-rail Type 2 DC SPD installation is a small, tidy job when the materials are ready:
Working on a live DC circuit is dangerous: PV strings are energised whenever light hits the panels and cannot be simply switched off at a load. Plan the work so the relevant strings are isolated, covered, or otherwise made safe before terminals are opened.
Open the DC isolator or disconnect the relevant strings and confirm the DC conductors are at zero voltage before opening the enclosure. On PV installations, cover the modules or otherwise remove the light source if disconnecting a live string is unavoidable.
Clip the SPD onto the DIN rail inside the enclosure, close to the busbar or terminal block it will protect. Position it so the leads to DC+, DC− and earth can run as short and straight as possible. Make sure the module is fully engaged on the rail and does not rock when pressed.
If the SPD data sheet requires backup protection — and most DC SPDs do — install the specified gPV fuse or DC MCB on the supply side of the SPD tap. The backup device isolates the SPD if it fails to a short-circuit condition and protects the branch during a fault. Selecting the backup rating is covered in the guide to SPD yedek sigortası veya devre kesici seçimi.
Connect each SPD terminal to the corresponding point on the DC circuit:
Check polarity against the device markings, tighten each terminal to the torque stated on the SPD or data sheet, and keep the conductors as short and direct as the layout allows. Do not coil spare cable length near the SPD: loops add inductance to the surge path.
The earth connection is part of the surge current path, so it must be as short and low-impedance as the DC connections. Connect the SPD earth terminal to the protective earth busbar of the same enclosure or system, and confirm that the earthing system itself is sound. A floating or high-resistance earth leaves the surge nowhere to go and defeats the SPD.
Mark the SPD and its backup protection on the enclosure schedule so a future maintenance engineer can identify the circuit. Note the installation date and the SPD model in the maintenance log, because DC SPDs age with every surge they absorb and need periodic checks.
The single most common field error is installing the SPD in series with the protected load — that is, cutting the DC conductor and passing all the load current through the SPD. A Type 2 DC SPD is not designed for this. In series it becomes a permanent obstruction in the power path and can overheat or fail in normal operation, and it still provides no useful clamping because the surge sees the wrong circuit topology.
The correct arrangement is always parallel: the SPD is tapped across the DC+ and DC− conductors (and to earth) while the load current continues to flow past it, through the circuit’s own protection and switching devices. If a wiring diagram shows the SPD in the main power path, treat the drawing as wrong.
General installation mistakes that apply across AC and DC systems are covered further in the article on common surge protective device installation mistakes.
Before closing the enclosure and restoring power, run through a short verification list:
After installation, the SPD should be inspected periodically rather than installed and forgotten. How to check a device that may have absorbed surges — including the red/green status window and when to replace the module — is covered in the guides to the SPD failure indicator and to backup protection on this site.
Always in parallel with the DC circuit it protects. The SPD is connected between DC+, DC− and earth while the load current flows through the circuit’s own conductors and protection devices. Connecting an SPD in series is a wiring error.
On a standard two-wire DC SPD: the + terminal to the positive DC conductor, the − terminal to the negative DC conductor, and the earth terminal to the protective earth busbar. Follow the terminal diagram printed on the device, since configurations with different numbers of poles exist.
Only if the SPD data sheet explicitly states it needs none, usually because the device has a tested internal disconnector and is certified for that arrangement. Most DC SPDs list a maximum backup gPV fuse or DC MCB rating, and the manufacturer’s instructions should be followed.
As close as the layout allows. Long leads add inductance and raise the voltage the equipment actually sees. Where the SPD must be remote from the protected equipment, add a second SPD at the equipment rather than lengthening the first device’s leads.
A polarised DC SPD connected the wrong way can conduct continuously instead of only during a surge, which leads to overheating and early failure. Always verify the + and − markings before energising the circuit.
A correctly wired Type 2 DC SPD is a low-maintenance insurance policy: parallel connection, short leads, solid earthing, matched ratings and the specified backup protection. The diagram on the device data sheet is authoritative; this guide explains the logic behind it so that a review of any proposed wiring can be done with confidence.
Bir GA&DA projesi için, send the technical team your DC system voltage, surge exposure, and the equipment to be protected to request the matching Type 2 DC SPD documentation, wiring details and backup protection coordination.
