DC Surge Protectors for Solar Arrays: Choosing and Placing an SPD
A nearby lightning strike doesn't have to hit your array to damage it. The surge induced in long runs of PV cable can reach the charge controller or inverter and take out its input stage in an instant. A surge protective device (SPD) gives that energy a path to ground before it reaches the electronics. It's not a guarantee, but on long or exposed array runs it's inexpensive insurance. Here's how to pick one that matches your array and where it belongs.
What a DC SPD does
An SPD sits between your PV conductors and ground. At normal voltage it does nothing and draws essentially no current. When a surge drives voltage above its threshold, it conducts briefly, diverting the surge current to ground and clamping the voltage the equipment sees. Most DC SPDs for solar use metal oxide varistors (MOVs), sometimes combined with spark gaps, inside replaceable DIN-rail modules.
SPDs are most useful against induced surges from nearby strikes. A direct strike to an array is a different scale of event, and no small SPD will guarantee survival. Our grounding and lightning basics article covers the bigger picture.
The ratings on an SPD datasheet
| Rating | What it means | How to use it |
|---|---|---|
| Ucpv / Uc (max continuous operating voltage) | Highest DC voltage the SPD can see continuously without conducting | Must be above your array's cold-corrected open-circuit voltage |
| Up (voltage protection level) | Voltage the SPD lets through during a surge | Lower is better; should be below what your equipment can withstand |
| In (nominal discharge current) | Surge current it can handle repeatedly | Higher means more durable |
| Imax | Maximum single surge current | Higher means more headroom for big events |
| Type / Class | Type 1 handles direct-strike waveforms; Type 2 handles induced surges | Type 2 for most small systems |
| Status indicator / remote contact | Shows when the module has failed | Visible window or a contact for alarm wiring |
Voltage rating: the most common mistake
Buy an SPD rated for your PV voltage with margin. If you choose one with a Ucpv too close to your array's Voc, it may conduct on a cold morning and fail. Use your cold-weather Voc, which can be noticeably higher than the label figure. Our winter Voc guide shows the calculation. Make sure you're buying a DC PV SPD; AC SPDs aren't designed for DC.
Configuration
PV SPDs typically protect positive-to-ground, negative-to-ground, and sometimes positive-to-negative. Two-pole and three-pole modules are common. Match the configuration to your system and your controller or inverter's recommendations; some equipment makers specify how SPDs should connect to their inputs.
Where SPDs go
A surge travels along conductors. The goal is to divert it before it reaches the equipment, with the shortest possible path to ground.
- At the array end of long cable runs, especially ground mounts far from the building. A combiner box is a convenient place. See our combiner box guide.
- Where cables enter the building or the equipment enclosure.
- Close to the charge controller or inverter PV input. Many builders place one SPD here even on short runs.
- On the AC side if the inverter connects to building wiring. AC surge protection at the panel is a separate device and a job for a licensed electrician.
Long runs (rule-of-thumb distances vary by source and standard) benefit from SPDs at both ends, because a surge can be induced anywhere along the cable.
Grounding makes or breaks an SPD
The SPD can only divert surge current if it has a low-impedance path to ground. That means:
- A ground conductor from the SPD to the equipment grounding system that's as short and straight as possible. Long leads and sharp bends raise impedance at surge frequencies.
- Equipment grounding of the array frame and racking, bonded correctly. See our grounding hardware guide.
- For ground mounts and outbuildings, a grounding electrode system as your local code requires, bonded to the building's system where code requires.
- No loops in the PV cables and ground conductor, which can pick up induced voltage. Run positive, negative and ground together.
Grounding and bonding are code matters. Rules for grounding electrodes, bonding between buildings and conductor sizes vary by location. For anything tied to a building's electrical system or the grid, have a licensed electrician design and install it, and pull permits where required.
Is an SPD worth it for a small build?
Long cable runs from a ground mount; exposed hilltop or open-field sites; regions with frequent thunderstorms; expensive controllers or inverters.
Small, short-run arrays mounted on a building with a short cable to the controller, in low-lightning areas.
Even in the weaker case, many builders add one because SPDs cost relatively little compared with a charge controller or inverter. Some controllers have basic surge protection built in; check the datasheet, but don't assume it's equivalent to a dedicated SPD.
Shopping red flags
The DIN-rail SPD market has the same quality spread as the DC breaker market. A few things separate a part worth trusting from one that just looks the part:
- No datasheet. A real PV SPD has a datasheet listing Ucpv, Up, In and Imax, plus a wiring diagram. A product listing with only a voltage and a photo isn't enough.
- AC and DC ratings mixed up. Some listings show an AC voltage in the title for a part sold as "solar." Look for the DC PV voltage on the module itself.
- No replaceable module or status window. Not a deal-breaker on its own, but it makes the SPD harder to maintain and to check after storms.
- Vague certification claims. Look for the specific standard the SPD is tested to in the datasheet, and a listing mark you can verify where your local code requires one.
Also think about the enclosure. An SPD module at the array end lives outdoors, so it needs a weatherproof box with proper cable glands, the same as a combiner. Indoors, it can share a DIN rail with your PV disconnect breaker, which keeps the surge path short.
Inspection and replacement
MOVs degrade each time they absorb a surge and eventually fail. Most PV SPD modules have a window that changes color when the module has reached end of life, and many let you pull the module out and replace it without rewiring the base.
- Check indicator windows after every major storm and during routine maintenance.
- Replace failed modules with the same type and rating.
- Look for scorch marks or melting around the SPD and its terminals.
- Recheck terminal torque, especially on the ground conductor.
- If your SPD has a remote contact, wire it to a monitor or alarm so you know when it fails.
Don't forget the disconnect. Many SPD instructions call for an upstream fuse or disconnect so a failed SPD can be isolated safely. Follow the SPD maker's instructions.
FAQ
Do I need a surge protector on a small solar array?
It's most valuable on long or exposed cable runs and in lightning-prone areas. For short runs on a building in low-lightning areas, it's optional but inexpensive insurance.
Can I use an AC surge protector on the DC side?
No. Use an SPD designed and rated for DC photovoltaic circuits, with a continuous voltage rating above your array's cold-corrected open-circuit voltage.
Where should a solar SPD be installed?
Close to the equipment you want to protect and where cables enter the building, with short ground leads. Long runs benefit from SPDs at both the array and equipment ends.
How do I know if my SPD has failed?
Most modules have a status window that changes color when they've reached end of life. Some have remote contacts for alarms. Check them after storms.
The bottom line
Pick a DC PV-rated SPD with a voltage rating above your cold Voc, mount it close to the equipment and at the building entry, keep its ground lead short, and check its indicator after storms. It won't stop a direct strike, but it gives induced surges somewhere to go besides your charge controller's input stage. For anything tied to building wiring, bring in a licensed electrician.