Short answer: a solar array makes direct current, and a surge on the DC side does not behave like a surge on a 230/400 V AC network — there is no zero crossing to help an arc extinguish, and the array keeps feeding current for as long as the sun is up. So the DC side of a PV installation needs its own surge protective device, a DC SPD, specified by the maximum continuous operating voltage (Ucpv) against the string’s maximum system voltage, the impulse current (Iimp or In) it must survive, the voltage protection level (Up) it leaves for the equipment, and its pole configuration. An AC SPD cannot be substituted for it.
This page sets out how a DC SPD is specified, where it mounts along a PV installation, and what we can quote today from the CHINT range. It is written from the position of an independent trading company, not a manufacturer and not an appointed distributor of any brand: certificates are stated per exact ordering code, never for a whole range.
Why the DC side cannot borrow the AC device
On an AC network a surge current crosses zero 100 or 120 times a second, and that zero crossing is what lets a protective gap or a varistor recover. A PV string has no zero crossing at all. Worse, the array is a current source: unlike a transformer-fed AC board, the string does not stop pushing energy into a fault while the modules are illuminated. Both facts feed straight into how the SPD is chosen and how it is wired.
| Question | AC SPD (230/400 V board) | DC SPD (PV string or array) |
|---|---|---|
| Reference voltage to clear | Voltage to earth of the 50/60 Hz supply | Maximum system voltage of the array (a DC figure, e.g. 1000 V or 1500 V class) |
| Arcing | Natural zero crossing assists extinction | No zero crossing — the device and its isolation distance must handle sustained DC arc |
| Source behaviour | Fault current limited by the network impedance | Array keeps supplying while lit; a DC breaker or fuse does the string isolation |
| What the datasheet leads with | Uc (AC), Imax, Up, Type 1/2/3 | Ucpv (DC), Iimp or In, Up, and the earthing configuration |
What a DC SPD is specified by
Four numbers decide most DC SPD selections, in this order.
- Ucpv — maximum continuous operating voltage. Set it above the array’s maximum system voltage at the lowest expected cell temperature, because open-circuit voltage rises as the modules get colder. On a 1000 V DC string you do not specify a 1000 V device “because it matches”; you confirm the Ucpv sits above the highest voltage the string can actually reach.
- Impulse current — Iimp (10/350 µs) or In (8/20 µs). A device for a direct-strike or long-lead exposure is specified on the 10/350 µs waveform; a device protecting downstream of a first stage is specified on 8/20 µs.
- Up — voltage protection level. This is what the inverter or the string electronics actually sees. It must sit below the impulse withstand of the equipment it protects, and the length of the connecting leads adds to it — a long, looping lead can undo a good Up.
- Configuration. A common PV arrangement is a Y-configuration (one pole to positive, one pole to negative, one to earth) so a surge on either rail finds a path. The earthing system — and whether the array is floating or one pole is earthed — changes which configuration is correct.
| Parameter | What it protects against | How it is read |
|---|---|---|
| Ucpv | Continuous array voltage | Above the cold-temperature maximum string voltage, at the array’s operating temperature range |
| Iimp / In | Surge energy | 10/350 µs for a first-stage / long-lead duty; 8/20 µs for a downstream stage |
| Up | Residual voltage at the equipment | Below the withstand of the inverter; lead length adds to the effective Up |
| Poles / mode | Both rails or one | Y-configuration for floating arrays; a single-pole device only where the earthing arrangement supports it |
Where the DC SPD sits
In a typical installation there are two places a DC-side SPD belongs. The first is at the array or string, usually inside the combiner box, close to where the strings are paralleled — this limits the surge before it travels down the DC trunk. The second is at the inverter DC input, or on the DC main between the array box and the inverter, where a second stage protects the inverter terminals. A single device at the far end of a long DC run leaves the near end exposed; that is why the lead length and the physical position count as much as the ratings.
The AC side of the same installation is a separate question. On the inverter’s AC output, and at the entry cabinet, a standard 230/400 V SPD applies.
What we can quote today
On the AC side, the confirmed CHINT SPD devices are the NU6-II / NU6-IIG (Type 2 / Class II, IEC/EN 61643-11, 230/400 V, Uc 275/320/385/440 V, Imax 40/65/100 kA, In 20/30/50 kA, Up 1.5–2.4 kV, with published TÜV and CB documents) and the NU6-I+II (Class I+II, Uc 275/385/255 V, Iimp 12.5 kA, In 25 kA, Imax 50 kA, Up ≤1.5/1.8 kV, with published VDE, CB, CE and RoHS documents). For the DC side of the string, pair the surge device with a DC-rated string breaker: the confirmed part is the NB1-63DC, an IEC/EN 60947-2 DC breaker for PV and telecom, In up to 63 A, with pole count setting the DC rating — 1P DC 250 V, 2P DC 500 V, 4P DC 1000 V — at Icu 6 kA at the matching voltage.
| Role | Device | Standard |
|---|---|---|
| AC-side surge protection (entry cabinet) | NU6-I+II (Class I+II) or NU6-II (Type 2) | IEC/EN 61643-11 |
| DC string protection and isolation | NB1-63DC, pole count sets DC 250 / 500 / 1000 V | IEC/EN 60947-2 |
| DC-side surge protection | DC SPD — ordering code and Ucpv/Iimp confirmed against the array voltage before quoting | DC SPD standard for the PV system |
We will not print a DC SPD ordering code or a Ucpv figure we have not verified against the factory documentation. If a project needs a DC-rated SPD, tell us the maximum system voltage, the earthing arrangement and the expected surge duty, and we confirm the exact code before a quotation is issued.
FAQ
Q: Can I use a normal 230/400 V SPD on the DC side of a PV array?
A: No. Its Uc is an AC rating, its arc management assumes zero crossings, and the array is a DC current source. The DC side needs a device rated Ucpv above the array’s maximum system voltage.
Q: How do I set Ucpv?
A: Above the highest voltage the string can reach, which happens at the lowest expected module temperature because open-circuit voltage rises as temperature falls. Take the array’s maximum system voltage, correct it for the cold extreme, and specify a device above that.
Q: One DC SPD or two?
A: Usually one at the array or combiner box and one at the inverter DC input for a long DC run. A single device at the far end leaves the other end exposed, and a long connecting lead raises the effective Up.
Q: Does pole count matter on a DC SPD as it does on a DC breaker?
A: Yes, but for a different reason. On the NB1-63DC the pole count sets the DC voltage rating (1P = 250 V, 2P = 500 V, 4P = 1000 V). On a DC SPD the configuration is chosen to match the earthing arrangement — a Y-configuration covers both rails and earth on a floating array.
Q: Is the NB1-63DC a surge device?
A: No. It is a DC circuit breaker for string protection and isolation to IEC/EN 60947-2. It does not clamp a surge; the SPD does that. The two are specified together but they do different jobs.
Q: What certification comes with the DC breaker?
A: The NB1-63DC documents are stated per exact ordering code — the series is self-described with CCC, CE, CB and TÜV documents. We send the document for the code being quoted, not a family-level claim.
What to send us for a quotation
Array maximum system voltage · earthing arrangement (floating or one pole earthed) · expected surge duty (first stage / long lead, or downstream) · string current and breaker pole count · destination country · quantity. You get unit price, packing data and the certificate document for the exact ordering code. Request a quotation.
Related: SPD and isolators · DC MCB for solar PV: 250 / 500 / 1000 V · SPD Type 1, 2 and 3 explained · SPD selection by grid (TN and TT) · Miniature circuit breakers · How to verify genuine product