Akash Hirpara A surge protective device (SPD) is a component that limits transient overvoltages from lightning or switching by diverting surge current to earth. PV systems use DC SPDs tested to IEC 61643-31 on array and inverter inputs, and AC SPDs to IEC 61643-11 or UL 1449, with Type 1, 2, or 3 ratings by location.
SPD Types
| Type | Location | Use |
|---|---|---|
| Type 1 | Service entrance | Withstands direct lightning |
| Type 2 | Combiner / Inverter | Induced surges, branch protection |
| Type 3 | Equipment-specific | Fine protection, sensitive loads |
Key Takeaways
- SPDs protect against voltage surges from lightning and grid disturbances.
- Type 1 (service), Type 2 (distribution/inverter), Type 3 (sensitive equipment).
- Combiner boxes typically include Type 2 SPDs.
- Inverters often have integrated Type 2 SPDs.
- High-lightning regions justify Type 1 at the service entrance.
Where SPDs Fit in the Electrical Design
An SPD only does its job if it’s placed where the surge actually enters the system. On the DC side, that’s usually inside the combiner box, which is why a Type 2 device is bundled into most combiner box designs by default rather than added as an afterthought.
On the AC side, the surge path runs through the AC disconnect and into the main service, so Type 1 and Type 2 protection is typically staged across both points rather than concentrated at one.
This is also where SPDs get confused with a different class of protection: an SPD clamps a voltage transient, while an arc fault detector is watching for a degraded connection arcing inside a conductor, the two devices solve unrelated failure modes and neither substitutes for the other.
Where the site also carries a dedicated lightning protection system, the SPD scheme has to be coordinated with the LPS grounding and bonding design rather than specified in isolation, since a poorly bonded down-conductor can push surge current back into the electrical system the SPD is meant to protect.
In practice, SPD type, location, and clamping voltage are called out on the project’s single-line diagram, alongside conductor sizing and overcurrent protection, reviewers checking a permit set will expect to see them there rather than inferred from a bill of materials.
Related Reading
SPD placement is one of the details that shows up on review checklists precisely because it’s easy to specify vaguely and hard to verify from a photo, our breakdown of solar SLD software covers how single-line diagrams document surge protection alongside grounding and overcurrent devices, which is the artifact an AHJ or utility reviewer will actually check against.
Getting that documentation right the first time is largely a permit-set coordination problem rather than a component-selection one, which is why it tends to fall to whoever is already drafting the site’s electrical drawings.
Frequently Asked Questions
3 commonly searched questions about SPD (Surge Protection Device).
Where should SPDs be installed in a solar PV system?
What is the difference between Type 1 and Type 2 SPDs?
Does the NEC require surge protection for solar?
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