PV DC Overvoltage Protection Box: Selection, Standards & Installation Guide (2025)

DC SPD Selection: Imax, Iimp, Uc and Type 1 vs Type 2

The DC surge protective device (SPD) is the single most important component inside a PV DC protection box. Its job is to clamp transient overvoltages from direct or nearby lightning strikes and divert the surge current to earth before it reaches the inverter DC input. Selection comes down to four parameters defined in IEC 61643-31: the SPD type (1, 2 or combined 1+2), the impulse current Iimp (10/350 µs), the nominal discharge current In and maximum discharge current Imax (8/20 µs), and the maximum continuous operating voltage Uc. Getting any one of these wrong will either leave the inverter exposed or cause the SPD to fail prematurely in service.

The decision between Type 1 and Type 2 is driven by lightning exposure and the location of the SPD relative to the array. Type 1 SPDs are specified when a direct strike onto an external Lightning Protection System (LPS) is possible and part of the lightning current can be conducted into the DC circuit, typically on buildings with an external LPS or on free-field arrays with masts. Type 2 SPDs handle induced surges only and are the default for rooftop systems without an LPS or at the inverter end of long DC runs. For most C&I and utility plants a Type 1+2 combined SPD near the array combined with a Type 2 at the inverter is now standard practice.

Parameter	Type 1 (per IEC 61643-31)	Type 2 (per IEC 61643-31)	When to use
Impulse current Iimp (10/350 µs)	≥ 12.5 kA per pole	Not required	Type 1: buildings with external LPS, free-field arrays exposed to direct strikes
Max discharge current Imax (8/20 µs)	Typically ≥ 25 kA	≥ 40 kA per pole (common 40 kA / 60 kA)	Type 2: rooftop PV without LPS, inverter-side protection
Nominal discharge current In (8/20 µs)	≥ 15 kA typical	≥ 20 kA typical	Verify against separation distance and cable length
Voltage protection level Up	≤ inverter DC Uw with margin	≤ inverter DC Uw with margin	Must be below the inverter DC port withstand voltage
Max continuous DC voltage Uc	> 1.2 × Voc(STC) × k_T	> 1.2 × Voc(STC) × k_T	Both types; see warning below
Short-circuit withstand Iscpv	Must exceed array Isc at the SPD location	Must exceed array Isc at the SPD location	Required by IEC 61643-32 for PV applications

Uc must cover cold-climate Voc rise

A common field failure is choosing an SPD with Uc only slightly above the array Voc at STC. PV module Voc rises as cell temperature drops, so Uc of the DC SPD must satisfy Uc ≥ 1.2 × Voc(STC) × k_T, where k_T is the temperature correction factor from IEC 60364-7-712 (approximately 1.02 to 1.25 depending on minimum site temperature). For a 1000 V DC string this typically means an SPD rated Uc ≥ 1150 V DC; for 1500 V systems, Uc ≥ 1800 V DC. Under-rating Uc causes continuous leakage through the MOV, thermal runaway and nuisance disconnection of the internal thermal disconnector.

PV-specific SPDs only

Do not substitute AC Type 2 SPDs on the DC side. PV DC SPDs must be tested to IEC 61643-31 and carry a defined short-circuit withstand current Iscpv per IEC 61643-32. They use a 3+0 (Y) or 1+1 (differential) MOV topology designed to fail-safe on DC, which standard AC SPDs cannot do.

DC MCB and String Fuse Sizing for PV Arrays

DC MCBs and string fuses form a coordinated protection strategy in PV systems, each addressing different fault scenarios. String fuses protect against individual string faults and reverse current, while DC MCBs isolate array strings and coordinate with downstream DC SPD and inverter protection. Correct pole count and tripping characteristic selection depends on your DC bus topology—grounded or floating—and short-circuit current magnitudes from parallel-connected strings.

DC Bus Topology and MCB Pole Configuration

In ungrounded (floating) DC systems, both positive and negative conductors must be protected; a single ground fault may not immediately trip protection, so 2-pole MCBs are required to isolate the entire string circuit. In grounded systems (negative or positive conductor grounded through a low-impedance path), a single pole MCB on the ungrounded conductor can interrupt the fault loop, though local regulations and utility guidelines may mandate 2-pole protection regardless. Verify your regional standard—IEC 60364-7-712 recommends 2-pole for enhanced safety in most scenarios.

Configuration	DC Bus Type	MCB Poles	String Fuse Requirement	Typical Application
1P MCB + Neutral Ground	Grounded (negative to ground)	1P protecting positive string	DC fuse per string (IEC 60269-6)	Small residential, code-permitted regions
2P MCB + Floating Bus	Ungrounded (floating)	2P protecting both positive and negative	DC fuse per string (IEC 60269-6)	Residential, C&I, utility-scale (recommended)
2P MCB + Optional Ground	Grounded or hybrid	2P for redundancy and safety	DC fuse per string (IEC 60269-6)	High-reliability systems, export markets

Short-Circuit Current Coordination: Isc-String vs Inom

Each string generates a short-circuit current I_sc-string (typically 110–130% of nameplate Isc) under fault conditions. When multiple strings are in parallel, the array combines these currents; this combined current must not exceed the DC MCB's rated breaking capacity (I_cn). The string fuse rating (I_nom) is selected to protect individual strings: for a string Isc of 12 A, a 16 A fuse is typical (per IEC 60269-6); the MCB protecting all strings must tolerate the sum of parallel string faults. Always size DC MCB breaking capacity ≥ array I_sc + 25% margin.

Breaking Capacity Must Match Array Architecture

A 6 kW array (6 strings × 2 kW) with I_sc = 12 A per string generates ~72 A combined short-circuit current if all strings short simultaneously. The DC MCB must have a rated breaking capacity I_cn ≥ 72 A. Using a standard 16 A or 20 A MCB rated only for 1.5 kA will result in nuisance tripping or inadequate fault isolation. Consult your inverter datasheet and module supplier for exact Isc and string count.

Tripping Characteristic (B, C, D Curves) for DC PV Circuits

DC MCBs use tripping curves (instantaneous magnetic thresholds) calibrated for DC fault speed and load inrush. Type B MCBs trip at 3–5× rated current and are suitable for sensitive electronics and low-impedance circuits. Type C (5–10× rated current) suit standard PV arrays and mixed AC/DC circuits. Type D (10–20× rated current) accommodate high inrush or heavy cable runs. For PV DC circuits, Type C is standard; Type D may be needed if string cables exceed 100 m or system impedance is very low. Verify that your DC MCB is rated for DC operation per IEC 60898-1 or equivalent; AC-only MCBs will not extinguish DC arcs reliably.

Type B (3–5 × I_n)Low-trip threshold; semiconductors, sensitive loads; uncommon in PV.

Type C (5–10 × I_n)Industry standard for PV; balances nuisance-trip immunity and fault response.

Type D (10–20 × I_n)High inrush tolerance; used for very long cables or specialized topologies.

String Fuse Selection per IEC 60269-6

DC string fuses are rated per IEC 60269-6 and must disconnect a single string in reverse-bias or short-circuit conditions without nuisance trips during normal array operation or transients. For a string with I_m = 10 A (module current) and I_sc-string = 12 A, a 16 A fuse is typical; for 15 A strings, use 20 A fuses. The fuse must clear before the string diodes thermally fail. Coordinate fuse speed class (gG = general purpose; aR = high-breaking capacity) with SPD discharge duration: if a Type 2 SPD discharges 10 kA in 1–2 µs, the fuse need not respond to that transient. Array-level current limiting and voltage clamping by the SPD ensure fuses operate primarily on sustained faults, not surge events.

IEC 60364-7-712 System-Level Requirements

IEC 60364-7-712 (Low-voltage DC systems) mandates that DC circuits be protected by a combination of switching devices (MCB or contactor) and overcurrent or overvoltage protection. In PV arrays, this means: string fuses (overcurrent per array string), DC MCB (system isolation), DC SPD (overvoltage), and string diodes (reverse current). Each protects a different fault mode; omitting any component creates a gap.

IEC 60364-7-712, IEC 61643-31 and CE/TÜV Compliance

A PV DC protection box sold into the EU, UK or Australian markets must be built around three overlapping standards: IEC 60364-7-712 for the PV installation itself, IEC 61643-31 for the DC SPD, and IEC 60947-2 for the DC isolator or MCB. Component-level certificates (TÜV, VDE, KEMA) cover the devices inside the enclosure, while CE marking under the Low Voltage and EMC Directives covers the assembled box placed on the EU market. North American projects follow a separate track built around UL 1741, UL 1699B and the NEC, which is why the same enclosure is rarely accepted on both continents without a dual build.

Key clauses to specify against

IEC 60364-7-712 — Special installations: PV supply systems. Clauses 712.443 and 712.534 define where SPDs are required on the DC side (typically when the distance between array and inverter exceeds 10 m, or where the structure has an external LPS) and require the SPD to be rated for the array's maximum U_{OC STC} × 1.2.
IEC 61643-31 — Requirements and test methods for SPDs connected to PV installations. Defines the DC-specific Class I (I_imp, 10/350 µs) and Class II (I_n/I_max, 8/20 µs) test sequences and the mandatory DC short-circuit withstand test I_SCPV.
IEC 61643-11 — The AC counterpart; relevant when the same enclosure also carries AC SPDs for the inverter output.
IEC 60947-2 — Low-voltage switchgear: circuit-breakers. Governs DC MCB rated breaking capacity (I_cu/I_cs) and the DC time constant used in testing.
IEC 60898-1 — Applies only to AC MCBs for household use; do not cite it for DC devices.
IEC 60269-6 — gPV fuses for PV strings, including the gPV utilisation category and 1.5 × I_{SC STC} rating basis.

CE marking is not a product certificate

CE is a self-declaration by the manufacturer that the assembled enclosure complies with the applicable EU directives — primarily LVD 2014/35/EU, EMC 2014/30/EU and RoHS 2011/65/EU. It is not equivalent to a third-party certification. For tenders in Germany, France and the Nordics, buyers typically also require TÜV or VDE component certificates for the internal SPD and DC MCB, plus an IEC 61439-2 type-test report for the assembled low-voltage switchgear.

North American and Australian equivalents

UL 1741 — Inverters, converters and controllers for use in independent power systems. Referenced for PV balance-of-system equipment in the US and Canada.
UL 1699B — PV DC arc-fault circuit protection, required by NEC 690.11 on most US rooftop systems above 80 V_DC.
UL 489B / UL 98B — DC-rated moulded-case circuit-breakers and DC disconnects for PV service.
AS/NZS 5033 — Installation of PV arrays in Australia and New Zealand; references IEC SPD and fuse standards but adds local requirements on isolator placement and labelling.
AS/NZS 4777.1 — Grid-connected inverter requirements, relevant when the DC box feeds a hybrid or AC-coupled inverter.

What to ask for in a datasheet

Request the following before placing an order: (1) IEC 61643-31 test report with I_imp/I_max and I_SCPV values; (2) IEC 60947-2 test report for the DC MCB stating I_cu at the rated U_{e DC} and the DC time constant used; (3) IEC 60269-6 gPV certificate for the string fuse; (4) EU Declaration of Conformity for the assembled enclosure citing LVD, EMC and RoHS; (5) IP rating test report to IEC 60529. Exact certificate numbers vary by model and must be confirmed against the specific part number quoted.

Application Scenarios: Rooftop, C&I and Utility-Scale PV

The enclosure specification, component count, and monitoring requirements for a PV DC overvoltage protection box scale directly with system voltage, string count, and installation environment. A 5 kW residential rooftop needs only basic DC SPD Type 2 and string fusing in a compact plastic enclosure, whereas a 2 MW utility ground-mount demands IP65-rated metal cabinets, multi-pole DC MCBs rated for 1500 V_DC, and remote monitoring of surge-counter status. Below we compare the typical bill-of-materials and enclosure choices across three common deployment scenarios.

Residential RooftopSystem voltage: ≤1000 V_DCString count: 2–4 stringsEnclosure: Compact plastic box (IP54), wall-mountKey components: DC SPD Type 2 (I_max 20 kA), 10×38 mm gPV fuses, optional 2P DC MCBMonitoring: Visual surge indicator (optional)Standards: IEC 60364-7-712, IEC 61643-31

C&I Rooftop (100–500 kW)System voltage: 1000–1500 V_DCString count: 8–16 stringsEnclosure: Metal cabinet (IP54/IP65), indoor or outdoorKey components: DC SPD Type 2 (I_max 40 kA), multi-pole DC MCB (10 kA breaking capacity), din-rail energy meterMonitoring: Din-rail DC energy meter with Modbus RTU, remote surge alarmStandards: IEC 60364-7-712, IEC 61643-31, IEC 60947-2

Utility-Scale Ground-Mount (≥1 MW)System voltage: 1500 V_DCString count: 16–32 strings per combiner boxEnclosure: Outdoor IP65 metal cabinet, pole or ground-mountKey components: DC SPD Type 1+2 coordinated protection (I_imp 12.5 kA / I_max 40 kA), 4P DC MCB (1500 V_DC, 15 kA), smart meter with 4G/EthernetMonitoring: Smart meter (kWh, I, V per string), surge counter with SCADA integrationStandards: IEC 60364-7-712, IEC 61643-31, IEC 60947-2, IEC 62271-1 (for large combiner stations)

Matching TPKELE Products to Scenario

For residential rooftop applications, TPKELE's DC SPD Type 2 modules (U_c 1000 V) and 2P DC MCB (6–10 kA) fit standard plastic enclosures. C&I projects benefit from our din-rail DC energy meters (Modbus RTU) and metal combiner-box assemblies. Utility-scale installations require coordinated SPD arrays and high-breaking-capacity DC MCBs—contact our engineering team for customised combiner-box solutions with SCADA-ready metering.

In all three scenarios, the SPD voltage protection level (U_p) must remain below the inverter's maximum input voltage, and the DC MCB breaking capacity must exceed the prospective short-circuit current of the array at STC. For utility projects subject to grid-code requirements, additional arc-fault detection or insulation-monitoring devices may be mandated; consult local standards and inverter manufacturer datasheets during the specification phase.

How to Specify and Source a PV DC Protection Box from TPKELE

Sourcing a PV DC protection box requires more than matching a catalogue number to a system voltage. The enclosure must integrate DC SPDs, string overcurrent protection, and a DC isolator whose ratings are mutually consistent and traceable to the same IEC or AS/NZS framework used by your local authority having jurisdiction (AHJ). Working through the five steps below before raising a purchase order will eliminate the most common mis-specifications — undersized U_c, mismatched fuse holders, and enclosures rated IP54 where the installation demands IP65.

2Select SPD class and discharge currentType 2 DC SPDs (IEC 61643-31) suit the majority of rooftop and C&I installations; specify Imax ≥ 40 kA and Uc ≥ 1.2 × VOC,max. Where the AHJ or insurer mandates combined Type 1+2 protection — typically for open-field or coastal sites in high keraunic-level zones — confirm Iimp per IEC 61643-11 Class I criteria. Verify the SPD carries a CE mark and, for Australian projects, check compliance with AS/NZS 61643.31.3Size DC MCBs and string fusesEach string fuse must be rated above the string short-circuit current ISC but below the module reverse-current tolerance (typically 1.35 × ISC per IEC 61730). DC MCBs must carry a breaking capacity sufficient for the maximum prospective fault current at the combiner point; confirm the MCB is DC-rated (IEC 60947-2 or IEC 60898-1 with DC addendum) — an AC-only device must never be substituted. Exact frame sizes should be confirmed against TPKELE datasheet values for the specific MCB series.5Request datasheet and CE certificate before orderingAsk for the full product datasheet (with all electrical parameters at rated temperature), the EC Declaration of Conformity, and — for Type 1 or combined SPDs — the test report reference number. For non-standard string counts or custom busbar configurations, request a factory drawing for approval before production commences. Lead times for custom combiner boxes vary; contact TPKELE directly to confirm current scheduling.

DC Surge Protection

TPKELE DC SPD Series

Type 2 and Type 1+2 DC surge protective devices for PV applications, rated up to 1 500 V DC. CE-marked, compliant with IEC 61643-31. Datasheets and Declaration of Conformity available on request.

View DC SPD Specifications

Need a Custom Combiner Box?

For non-standard string counts, utility-scale busbars, or enclosure configurations outside the standard range, TPKELE's engineering team can provide a factory drawing and quotation. Specify your system voltage, string count, and target IP rating when you enquire.

Send a Custom Enquiry

↑ Back to top