Solar & DC

PV SPD Minimum Ucpv Calculator

Derive the minimum continuous operating voltage Ucpv a PV surge protective device must have, from the string open-circuit voltage corrected for cold temperature, and screen the SPD type and connection. Under-rating Ucpv lets the array voltage age or fail the SPD; over-rating weakens the protection level.

ⓘEnter the whole string Voc at STC. The calculator applies the cold-temperature rise and the IEC 61643-31 safety factor.
Minimum Ucpv
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String cold Voc
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Quick rule 1.2 × Voc,STC
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SPD type & connection
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How Ucpv is derived

Voc(cold) = Voc,STC × [ 1 + (β/100) × (Tmin − 25) ] Ucpv ≥ Voc(cold) × 1.1 (IEC 61643-31) Quick rule: Ucpv ≥ 1.2 × Voc,STC

Ucpv is the maximum continuous DC voltage the SPD withstands without conducting. It must sit above the highest voltage the array ever presents — the cold open-circuit voltage — with a 1.1 margin so the SPD is not stressed in normal service. The 1.2 × Voc,STC rule of thumb is a quick equivalent that folds temperature and margin into one factor; use the higher of the two results.

Type and connection

  • Type 2 — the usual choice, protecting the array and inverter from induced and switching surges where there is no external lightning protection system.
  • Type 1 (or 1+2) — required where the building has an external LPS or high direct-strike exposure, to handle partial lightning current.

PV SPDs commonly use a Y (3+0) connection — each DC pole to earth through a varistor plus a spark gap between the mid-point and earth — so a single earth fault does not defeat the protection. Follow the SPD maker’s wiring for the array earthing arrangement.

⚠Match Ucpv to the string, and mind the distance. An SPD more than about 10 m of cable from the equipment it protects needs a second SPD nearer the load. Coordinate the SPD with an upstream DC disconnector for safe end-of-life disconnection.

Frequently asked questions

Why not just use Voc at STC?

Because the array voltage rises in the cold and the SPD needs headroom above the worst case. Sizing Ucpv on the STC value risks continuous stress and premature ageing on cold, sunny days.

Where do PV SPDs go?

At the array/combiner and at the inverter DC input, with a second device where cable runs exceed roughly 10 m. Both AC and DC sides of a PV system need their own SPDs.

Type 1 or Type 2 for my site?

Type 2 without an external lightning protection system; Type 1 (or a combined Type 1+2) where an LPS is present or the exposure is high. The Ucpv requirement is identical for both.

Select the PV SPD

Match the Ucpv, type, and connection to a DC surge protective device for the array and inverter.

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Related products: SPD · PV combiner box · Fuse

Standards referencedIEC 61643-31 (PV SPDs) · EN 50539-11 · IEC 60364-7-712 (PV systems) · IEC 62305 (lightning protection).

Estimate for guidance only. The final Ucpv, SPD type, connection, and positions must be confirmed by a qualified person against IEC 61643-31, the array design, and the site lightning risk.