Type 2 Spd Selection Guide Uc In Imax Up Korlen

Surge protective devices are small, cheap, and almost always the difference between a near-miss and a destroyed inverter, PLC or server rack. Yet most selection mistakes come down to misreading four rated values on the label. This guide walks through each one, shows how they interact, and uses Korlen’s KNY-40 Type 2 SPD as a worked example with real data.

1. What a Type 2 SPD is (and where it goes)

IEC 61643-11 defines three SPD classes by where they sit in the surge-protective chain:

• Type 1 — installed at the incoming service entrance to handle direct lightning current (8/20µs, often paired with a lightning air terminal).
• Type 2 — the general-coordination protector for distribution boards; absorbs induced surges and the residual from a Type 1 device. This is the device almost every building needs.
• Type 3 — point-of-use protectors (socket strips, equipment-side) for sensitive electronics, always downstream of a Type 2.

A Type 2 SPD is therefore the standard board-mounted protector. In a typical install you put one at the main incoming board and another at sub-boards feeding exposed or sensitive loads. Our PV protection guide shows the same device sitting at the AC feed of a solar inverter.

2. The four parameters that decide selection

Read the label in this order — each value answers a different question.

Uc — maximum continuous operating voltage

Uc is the highest AC (or DC) voltage the SPD can carry forever without degrading. Rule of thumb: pick Uc at least 10–20% above the system’s highest continuous voltage. For a 230/400V TN network, a 275V variant is marginal; 320V or 385V is the safer choice for grids with chronic overvoltage. Undersizing Uc is the single most common cause of premature SPD failure.

In — nominal discharge current (8/20µs)

In is the current the SPD can absorb repeatedly (the “every-storm” rating). A higher In means longer service life. For most commercial and light-industrial sites, 20kA is a solid baseline; exposed or high-lightning-risk sites justify 20kA or more.

Imax — maximum discharge current

Imax is the single largest surge the SPD can survive once (the “worst day” rating). It is always ≥ In. The gap between In and Imax tells you the margin for an extreme event. Korlen’s KNY-40 rates Imax at 40kA, double its 20kA In.

Up — voltage protection level

Up is the residual clamping voltage the SPD lets through during a surge. This must stay below the impulse-withstand voltage of the equipment you are protecting (a typical protected device tolerates 1.5–2.5kV). Up is what actually saves your load, so it is the number to check against your gear — not just the current ratings.

Two more values matter in practice: tA (response time, the faster the better — KNY-40 is 25ns) and the short-circuit withstand / backup rating (the SPD must be backed by a device that clears a follow current if the varistor short-circuits).

3. Step-by-step selection

1. Fix the system voltage. 230/400V TN-C-S → start at 275V Uc but prefer 320/385V for margin. 120/240V split-phase → 150V class.
2. Set Uc above the worst-case continuous voltage. If the local grid floats high, go to the next Uc step rather than the minimum.
3. Choose In from lightning exposure. Urban/suburban with Type 1 upstream: 20kA In is fine. Open/exposed site, no Type 1: 20kA In minimum, consider higher.
4. Confirm Up is below your equipment withstand. For 1.5–2.5kV-rated loads, a 1.5–1.8kV Up (L-N) is appropriate.
5. Add a backup device. A dedicated SPD disconnector (Korlen LT-SSD40) or an upstream MCB/RCCB sized to the SPD’s short-circuit rating.
6. Verify the standard. IEC/EN 61643-11 compliance, plus any regional mark your market requires.

4. KNY-40 verified specification

Korlen’s KNY-40 Type 2 SPD is offered in three Uc variants so the same module fits 275V, 320V and 385V networks. The verified ratings:

5. SPD in a PV / DC system

Solar sites need surge protection on both sides of the inverter. The AC side takes grid and induced surges (Type 2, as above). The DC side — long PV cabling acting as an antenna — needs a DC-rated SPD and DC-rated overcurrent protection. Korlen’s KNB2-63DC DC MCB provides the DC-side overcurrent break; pair it with a DC SPD at the combiner for exposed arrays.

6. Backup protection and isolation

An SPD is not a standalone device. If its varistor fails short, a follow current must be cleared safely. Two things are required:

• A backup overcurrent device. A dedicated SPD disconnector (LT-SSD40) or an upstream KNBL1-32 RCCB / MCB sized to the SPD’s short-circuit rating. Note an SPD does not provide earth-leakage protection — that still needs an RCCB.
• An isolation point for maintenance. A KNH2-100 isolator switch gives a visible break so the SPD can be replaced with the circuit made safe.

7. Common mistakes & selection checklist

Common mistakes:

• Choosing the lowest Uc “to match the nominal voltage” — it fails on the first overvoltage day.
• Installing a Type 2 with no backup device — a failed module can take the board down.
• Mixing Type 1 and Type 2 without coordination — the Type 2 must sit downstream of the Type 1.
• Trusting Imax alone — Up is what protects the load, check it against equipment withstand.

One-page checklist:

1. System voltage known → Uc ≥ 10–20% above it.
2. Lightning exposure assessed → In ≥ 20kA for most sites.
3. Up below load impulse-withstand (1.5–2.5kV class).
4. Backup disconnector / RCCB sized to SPD short-circuit rating.
5. Isolator present for safe replacement.
6. IEC/EN 61643-11 marked for the market.

FAQ

Do I need a Type 2 if I already have a Type 1?

Yes. A Type 1 handles the direct strike at the service entrance; the Type 2 downstream absorbs the residual and the induced surges on internal wiring. They are complementary, not interchangeable.

Can one SPD protect both AC and DC?

No — AC and DC SPDs are rated differently (DC has no zero-crossing, so arc clearing is harder). Use an AC-rated Type 2 at the inverter output and a DC-rated SPD at the PV combiner.

How do I know it has failed?

KNY-40 modules have a visible status window; pair them with the LT-SSD40 disconnector so a failed module isolates instead of faulting the circuit. Inspect at least annually, or after a major nearby strike.