DC-side surge protection: how string routing decides whether the inverter lives or dies

Elektronik Projekte

The unassuming loop on the roof

A photovoltaic installation on a single family home looks harmless: ten to twenty modules in series, a plus and a minus lead down to the inverter in the basement. What most installers on site never consciously think about: between the plus and the minus line of a string a loop area opens up, and that loop area decides whether a lightning strike 200 metres away takes the 3000 franc inverter with it or not.

In a nearby lightning strike the surrounding magnetic field rises to high values within a few microseconds and falls back just as fast. The di/dt of a lightning current is typically 20 to 200 kA/µs. A loop in the vicinity acts as an induction coil:

Uind=−dΦdt=−A⋅dBdtU_{ind} = -\frac{d\Phi}{dt} = -A \cdot \frac{dB}{dt}

A is the loop area, that is the area spanned between the plus and minus conductors. The key statement of the equation is: the induced voltage is directly proportional to the loop area. Double the area, double the voltage. Half the area, half the voltage. The one knob the installer holds in hand without adding any components is exactly this area.

A loop area of 10 m² with a lightning strike 100 m away can induce several kilovolts, even if the lightning never touches the building itself.

The simplest and most effective measure: route + and – together

The measure most often forgotten in the field costs nothing: route the plus and minus line of every string in parallel, from the last module of the string back to the DC combiner box or directly to the inverter.

Concretely:

A carefully routed string has a loop area in the range of 0.1 to 1 m². A sloppy one can reach 20 to 50 m², which means fifty to a hundred times the induced voltage. That is the difference between “the surge protector clears the event cleanly” and “the surge protector is overrun and the inverter is dead”.

Keep data lines away from the DC strings

Just as commonly overlooked is the second wiring mistake: communication lines routed parallel to the DC strings. RS-485, Modbus, Ethernet to the energy manager, the cable to the smart meter or to the battery — anything shielded or unshielded that runs alongside the same tray becomes a second pickup loop in a lightning event. The induced voltage goes straight into the communication ports and in the worst case takes the inverter’s control electronics with it, or propagates into the home network.

Rule of thumb: at least 200 mm separation between DC strings and data lines, or use shielded cable with the shield bonded on both ends. Cross at right angles rather than running in parallel. Where runs exceed 20 m, fibre optic is the more robust choice because it simply does not have the problem.

The relevant standards are IEC 62305-4 (protection of electronic systems within the lightning protection concept) together with EN 50174-2 (separation distances between power and information technology cabling). For signal-side SPDs, IEC 61643-21 applies. In practice though, clean physical separation of the cable paths usually matters more than any added arrester.

String sizes for single family homes and what they mean for protection

Typical layouts for a single family home today:

System sizeString layout (typical)String open-circuit voltageString current (Isc)
5 kWp1× 12 modules at 420 W~540 V~14 A
8 kWp1–2 strings of 9–10 modules~450 V~14 A
10 kWp2 strings of 12 modules~540 V2× 14 A
15 kWp (with storage)2–3 strings of 10–12 modules~450–540 V2–3× 14 A

Residential inverters typically have a maximum DC input voltage of 1000 V (occasionally 1100 V) and string currents up to 15 A per MPPT. The string size is therefore bounded by the open-circuit voltage at –10 °C.

For surge protection that means: the DC-side arrester has to be rated for the maximum system voltage of the string. A type 2 arrester for 1000 V DC is today’s standard for residential. With several strings per inverter, every MPPT input needs its own protection, because the strings represent independent loops.

Consequences of a near strike without clean wiring

What concretely happens when the loop is large and the protection does not catch the event:

  1. Inverter input capacitors fail first, often with an audible bang and visible burn marks on the board.
  2. MPPT switching transistors (SiC or Si MOSFETs) die through avalanche breakdown.
  3. Communication electronics (RS-485, Ethernet to control system) often go down too, because the surge propagates along the shields.
  4. String fuses trip if present, but they protect the module wiring against short circuit, not the inverter.
  5. The modules themselves are more robust. With repeated strikes the bypass diodes in the junction boxes can short.

On the insurance side, damage without documented surge protection is often treated as contributory negligence. Building insurers today typically require evidence of installation according to the recognised state of the art.

Cost of DC-side protection for a single family home

Rough indication for Switzerland (2026, without installation, list prices before the usual installer discounts):

ComponentPrice range
DC-side type 2 arrester (1000 V DC, plug-in): Dehn®, Phoenix®, OBO®, Citel®CHF 150 – 350 per string
Combined type 1+2 arrester (with external lightning protection present)CHF 400 – 800 per string
AC-side type 2 arrester for the grid sideCHF 100 – 250
Generator junction box with built-in T2 and fusesCHF 300 – 600
Residential inverter (5–10 kW, hybrid with battery interface)CHF 1500 – 3500
Installation of surge protection (electrician)CHF 200 – 500

The relation is decisive: full DC-side protection for a typical single family home is 5 to 15 % of the inverter price. Skipping it risks the entire inverter plus several days of downtime plus insurance friction plus often collateral damage to the house wiring and the loads on it.

That math comes out in favour of protection in essentially every case.

When does type 2 suffice, when do you need type 1?

The distinction follows the question: is there external lightning protection (air terminations, down conductors, earthing per IEC 62305) on the building, and is there a direct strike risk to the array?

In Switzerland the corresponding risk analysis is typically done by the electrical planner or solar installer in coordination with the cantonal building insurance (VKF recommendations).

Field notes: old building without a foundation earth electrode

The NIN today requires a foundation earth electrode for new buildings. Old buildings, and Switzerland has plenty of them, do not have one, and digging one in afterwards is rarely proportionate. A common scene in such retrofits:

The electrician bonds the main equipotential bus, out of necessity, to everything that looks metallic: heating pipes, water pipes, gas line. That is not wrong as a main equipotential bond, but as the sole earth for the DC-side surge protection of a PV installation it is not enough. The effectiveness depends too much on corrosion and joint quality of those pipes, and the geometry was never designed for surge currents.

A pragmatic solution is an external rod electrode, as close to the inverter as possible, with the SPD earth line wired directly onto it. The location choice is not romantic but physical: somewhere the soil stays moist year round. A well watered patch under an apple tree, present in many Swiss gardens anyway, works better than a dry south facing flowerbed. The root ball keeps the soil moist, the shade prevents summer dry-out, and damp soil has a markedly lower specific resistance than dry soil. Lower earth resistance means the arrester can route the surge current away without driving the potential at the house connection up dramatically.

Just as important as the electrode itself is the earth lead from the SPD to it. It has to be short, direct, and lead straight out of the building. An earth lead that snakes through the house, past the distribution board, the data cabinet or the battery BMS, acts as an antenna itself: the lightning surge current generates a strong magnetic field on its path that couples into everything the wire runs past. Put bluntly, you would be inviting the lightning you were trying to keep out of the house back in through the rear door. Ideally the SPD earth lead leaves the building by the shortest route to the rod, separated from all other cables.

For such an arrangement to be acceptable three conditions have to be met: the implementation is documented, the main equipotential bond stays in place, and the external electrode is tied into the rest of the building’s earth points so no dangerous potential differences can develop.

Anyone with a similar setup: do not improvise, agree the approach in advance with the responsible inspection body. A short pre-clearance is much cheaper than a finding at the final inspection.

The relevant standards at a glance

StandardContent
IEC 62305-1 to -4Lightning protection generally: risk management, external and internal lightning protection, protection of electrical and electronic systems
IEC 61643-31 (replaces EN 50539-11)Specific requirements for SPDs in PV applications: test requirements, surge current capability, behaviour under DC
IEC 61643-11 / -12General requirements for SPDs in low voltage networks
IEC 61643-21SPDs for telecommunication and signalling networks (RS-485, Ethernet, bus lines)
EN 50174-2Separation distances between power and IT cabling
NIN 2025 (Switzerland)Low voltage installation standard: mandates surge protection for new installations under specific conditions
SNR 464022Swiss normative reference for PV installations
VKF fire protection guidelinesAssociation of cantonal fire insurances: lightning protection requirements by building class
VDE-AR-E 2100-712 (Germany, often referenced)Application rule for the DC-side dimensioning with SPDs

Important: NIN 2025 mandates surge protection for new installations as the default, with only narrow exceptions (e.g. occasional outbuildings). Planning a PV installation today without an SPD is in most cases planning a non-compliant installation.

Practical checklist for the residential install

Conclusion

The most effective surge protection on the DC side of a PV installation costs nothing and is still forgotten regularly: route + and – of each string consistently together so the loop area stays minimal. The electronic protection, type 2 and where needed type 1+2, comes on top and is today’s state of the art, a standards requirement and an insurance condition all at once.

For an 8 kWp residential installation the total cost of full surge protection is in the CHF 400 to CHF 1200 range. Given an inverter that costs between CHF 1500 and 3500 and an installation budget of CHF 15,000 to 25,000, the proportion is clear.

Keywords: PV surge protection, DC SPD Type 2, PV lightning protection residential, minimise loop area, IEC 62305, IEC 61643-31, NIN 2025, protect solar inverter, foundation earth retrofit, PV protection concept consulting, electrical engineering Switzerland, swissEmbedded PV monitoring

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Sources and further standards