Lightning and switching surges are among the leading causes of inverter and monitoring failures in the field. A large PV array is a wide, elevated antenna: the loop formed by string cables, the mounting structure and the earth conductors couples induced voltages of several kilovolts into the DC circuits even for strikes hundreds of metres away. Surge protective devices (SPDs) limit those overvoltages to a level the inverter can survive, but only if the right device is selected, rated for the system voltage, and installed with short leads.
This guide walks through the selection of DC and AC SPDs for utility, commercial and industrial PV systems, with the parameters to check on a datasheet and the standards that apply.
Know the Standards Before the Datasheet
Three standards cover most designs. IEC 61643-31 specifies SPDs for the DC side of PV installations; IEC 61643-11 covers low-voltage AC SPDs; and the IEC 62305 series (parts 1 to 4) defines lightning protection levels and the zone concept. In North America, UL 1449 (4th edition) and NEC 690 and 285 apply. IEC 60364-7-712 gives the installation rules for PV supply systems, and IEC 62548 covers array design. SPD datasheets should state the test class: Type 1 (T1, 10/350 µs impulse current Iimp), Type 2 (T2, 8/20 µs nominal discharge current In) or Type 1+2 combined.
Decode the Key Parameters
| Parameter | Meaning | Selection rule |
|---|---|---|
| Ucpv (max continuous DC voltage) | Highest steady voltage the SPD tolerates | ≥ 1.2 × array Voc at the lowest site temperature (e.g., ≥ 1,500 V class for 1500 Vdc systems) |
| Up (voltage protection level) | Residual voltage during surge | Must be below the inverter's impulse withstand (often ≤ 2.5-4 kV for the DC input) |
| In (8/20 µs) | Nominal discharge current | ≥ 20 kA for T2 on exposed arrays; 15-20 kA is typical on protected sites |
| Iimp (10/350 µs) | Lightning impulse current for T1 | Needed when the building has an external LPS or direct strike risk |
| Isccwpv | Short-circuit withstand | ≥ array short-circuit current (with bifacial gain) |
| Uc (AC) | Max continuous AC voltage | ≥ 1.1-1.15 × nominal line voltage; higher on weak grids |
Voltage margin is critical: a 1,000 Vdc-rated SPD on a 1,500 Vdc array will fail short-circuit under normal operation. Choose the SPD Ucpv from the maximum Voc at the lowest temperature, calculated as Voc_stc × [1 + β_Voc × (T_min − 25)] × N_series.
Placement: Where SPDs Go in a PV Plant
Apply the lightning protection zone (LPZ) concept. Install SPDs at every point where a conductor crosses from one zone to another:
- Array-side (DC combiner box or string inverter DC input): a Type 2 DC SPD at the combiner output or at each inverter DC input, with a Type 1 or 1+2 if the array is within 3 m of an LPS down conductor or on a building with an LPS.
- Inverter DC side: many modern inverters (Sungrow SG-series, Huawei SUN2000) integrate Type 2 DC SPDs; confirm the rating and replaceable cartridges.
- Inverter AC side: Type 2 AC SPD at the LV switchboard or inverter AC terminals, rated for the grid system type (TN-S, TT, IT).
- MV transformer: surge arresters (MOV type, Class 1 or 2 per IEC 60099-4) on the MV side.
- Communication and monitoring lines: RS485, Ethernet and sensor lines need data-line SPDs (IEC 61643-21) at the data logger and weather station.
The 10-Metre Rule and Lead Length
If the distance between the SPD and the equipment is more than 10 m, the voltage can double at the equipment by oscillation, and a second SPD is needed near the equipment. Equally critical is the lead length: every metre of connecting wire adds roughly 1 kV per 1 kA/µs of surge due to inductance (about 1 µH/m, V = L × di/dt). Keep the total connecting length to under 0.5 m and use a V-wiring or Kelvin connection so that the conductor to the equipment is the shortest path. Use conductors with at least 6 mm² copper for T2 and 16 mm² for T1 earth connections.
Coordination Between SPD Stages
Cascaded SPDs must be coordinated so that the upstream device carries the bulk of the energy. Between two SPDs from different manufacturers, a decoupling inductor or at least 10 m of cable is required; SPDs from the same manufacturer are often tested and pre-coordinated. For multi-stage protection of a rooftop system, a typical arrangement is T1+2 at the entry of the building and T2 at the inverter, with a total Up of 1.5 kV or lower at the equipment.
Fusing, Backup Protection and End-of-Life Indication
An SPD that reaches end of life should disconnect safely. Use SPDs with thermal disconnectors and remote status contacts connected to the SCADA. DC SPDs need backup overcurrent protection if the manufacturer specifies it, because DC arcs do not extinguish at zero crossing: use a DC-rated gPV fuse (IEC 60269-6) sized to the SPD datasheet, commonly 20-63 A. For AC SPDs, a gG fuse or MCB with the specified rating should be installed upstream. Schedule an annual inspection and replace cartridges after any severe storm or when the status window turns red; in high-lightning areas (flash density above 4 strikes/km²/year) check twice a year.
Earthing Is Half of Surge Protection
An SPD only diverts current to earth; if the earth is poor, the voltage rise on the earth bar defeats the protection. Bond the array frames, cable trays and SPD earth terminal to one equipotential network, with an earth electrode resistance targeted at 10 Ω or lower and often 1-5 Ω for large plants. See our earthing and grounding design guide for the grid layout and soil-resistivity method.
Worked Example: 1500 Vdc String Inverter Plant
Take a 100 MWac / 130 MWp project using 72-cell-equivalent 625 W modules with Voc = 55.5 V and a Voc temperature coefficient of −0.25%/°C. With 26 modules per string and a minimum site temperature of −10°C, Voc_max = 26 × 55.5 × [1 + 0.0025 × 35] = 1,569 V, which exceeds the 1,500 V limit, so the string length must be reduced to 24 (1,448 V). The DC SPD must then have Ucpv of at least 1,500 V, and the next standard class is a 1,500 Vdc-rated device with Up ≤ 4 kV. The inverter's impulse withstand on the DC port is often 8 kV, so a 4 kV Up gives comfortable margin, but the cable length between the SPD and the inverter terminal still must be kept under 10 m.
On the AC side, a 800 Vac string inverter on an IT or TN system needs a Type 2 AC SPD with Uc of at least 920 Vac (1.15 × 800), In of 20 kA and Up below 4 kV. The MV transformer receives metal-oxide surge arresters with a rated voltage (Ur) selected for the neutral grounding scheme, for example 36 kV Ur on a 33 kV effectively grounded system, with an energy class 2 or 3 per IEC 60099-4 depending on the cable length.
For communications, RS485 loops between inverters and the data logger can span kilometres; each building or cabinet entry should have a data-line SPD with a matched voltage rating (typically 6-12 V for RS485), and cable shields should be bonded at one end only to avoid ground loops, unless the standard specifies both ends for lightning protection. Keep an inventory of spare cartridges, at least 5% of installed quantity, and log every replacement against the event date in the O&M system so that trends become visible.
Procurement Checklist
When requesting quotations, specify: system voltage and Voc_max, SPD class and test standard (IEC 61643-31 or UL 1449), Up, In and Iimp, pluggable cartridge design, remote contact, backup fuse rating and ambient temperature range (−40 to +70°C). Econo Solar sources SPDs, combiner boxes and inverters with integrated protection from Chinese manufacturers and can supply a protection schedule alongside your BoM. To receive a matched list, request a quote with your array data.
Frequently Asked Questions
Do I need both Type 1 and Type 2 SPDs on a ground-mount solar farm?
Usually no. Ground-mount plants without a building LPS normally use Type 2 SPDs on DC and AC sides, plus MV arresters, because direct-strike currents are managed by the air-termination and earthing system. Type 1 or 1+2 is used where a lightning protection system is bonded to the PV system or the separation distance cannot be met.
Can I use an AC SPD on the DC side?
No. AC SPDs rely on current zero-crossing to extinguish arcs and are not rated for continuous DC voltage. Always use SPDs certified for DC PV to IEC 61643-31 with the correct Ucpv.
How often should SPDs be replaced?
Inspect at least annually and after major storms. Replace cartridges when the indicator shows end of life; typical service life is 5-10 years depending on the lightning exposure and number of events.