Lightning Arrester for Solar Panels: How to Select, Size and Place It

Not automatically. DEHN’s rooftop PV white paper says installing PV modules does not raise the risk of a lightning strike. So the need for an air terminal (a rod that catches strikes) comes from an IEC 62305-2 risk assessment of the building. Surge protective devices are treated differently. IEC 61643-32 calls for them unless a risk analysis shows they are not needed.

This page explains what the word “arrester” means. It shows how to calculate the zone an air terminal protects. It also shows how to pick SPDs (surge protective devices) for the DC and AC sides. For the SPD-first view of the same topic, see solar lightning protection.

What does “solar lightning arrester” mean?

People use the phrase for two different devices. An air terminal is part of external lightning protection. That system also has down conductors (cables that carry the strike down) and an earth termination (the part that sends the current into the ground). A surge protective device, or SPD, is fitted to a circuit. It limits overvoltage, which is a short, high voltage spike, on the equipment behind it. An air terminal catches direct strikes. An SPD does not. An air terminal also does nothing for surges that arrive on cables. A project specifies each one separately.

IEC 62305-1:2024 sets the general rules for protecting structures from lightning. IEC is the International Electrotechnical Commission. The edition your local authority has adopted decides which version applies to your project.

Do solar panels need a lightning arrester?

DEHN makes lightning and surge protection products. Two statements from DEHN’s 2022 white paper on rooftop PV answer this:

• A PV system alone does not create a need for lightning protection. Whether a building needs an external system depends on its location, construction and use. The IEC 62305-2 risk analysis settles it.
• A PV system can still carry a large amount of lightning interference into a building through its cables. That is why the same paper treats surge protection separately. It also says rooftop PV must not interfere with existing lightning protection.

The white paper also reports a rule from Supplement 5 of the German DIN EN 62305-3. A system designed to lightning protection class III meets the usual requirements for PV systems. That is a German supplement, so check what your own authority accepts.

For a retrofit, get the records of the existing protection system before you add the array. The new modules, rails and cable routes must sit inside the protected volume. They must also keep the required clearances.

Lightning protection levels and rolling sphere radius

Air-terminal design starts from the lightning protection level (LPL). This is also called the class of LPS (lightning protection system). DEHN’s Lightning Protection Guide reproduces the values from Table 5 of IEC 62305-1 (guide Tables 5.1.1.1 and 2.7.1):

A smaller sphere means a stricter level, because the sphere can then reach more points on the structure. The guide links current and radius with r = 10 × I^0.65. Here r is in metres and I is in kA. For 10 kA that is 10 × 10^0.65 = 44.7 m, which the table rounds to 45 m. The guide is dated December 2014. So confirm the values against the IEC 62305 edition adopted for your project.

Lightning arrester design calculation

The guide describes three ways to place air terminals: the rolling sphere method, the mesh method and the protective angle method. In the rolling sphere method, you imagine a ball of a set radius rolling over the building. Any point the ball can touch needs protection. The guide calls this method the universal one. It recommends it especially for complicated shapes. DEHN’s white paper cites subsection 5.2.2 of IEC 62305-3 for the protective angle and rolling sphere methods.

Protective angle method

The protective angle depends on the level and on the height of the rod above the surface you protect. These values come from DEHN’s guide (Table 5.1.1.4). It gives the angle and the horizontal protected distance for each height:

Worked example: one 3 m rod on a level III roof

1. Level III, rod height 3 m: the table gives 74° and 10.46 m.
2. Check: 3 m × tan 74° = 3 × 3.487 = 10.46 m.
3. So the rod protects a circle of radius 10.46 m around its base, measured on the roof. That circle is 20.9 m across.

For the same rod, the rolling sphere construction gives a different distance. Let a sphere of radius r rest on the surface and touch the rod tip. It then sits at a horizontal distance of √(h × (2r − h)) from the rod, where h is the rod height. With r = 45 m and h = 3 m that is √(3 × 87) = √261 = 16.2 m. The two methods do not give the same number (10.46 m and 16.2 m here). The designer uses the method the project requires. The designer applies it to the actual roof. That means rolling the sphere over every edge, rail and obstruction, not over one rod alone.

ESE terminals

An early streamer emission (ESE) terminal is a rod with a device meant to attract lightning better. The aim is to extend its range of protection beyond that of a conventional rod. NIST’s 2000 overview of ESE systems says the scientific basis for that better performance is far from certain. NIST is the US National Institute of Standards and Technology. This page does not give an ESE radius formula. The formula comes from the French standard NF C 17-102, which is not freely available. Ask the supplier which standard and which authority acceptance its coverage claim relies on. Do not treat a vendor radius as IEC or NFPA (National Fire Protection Association) performance.

Rod placement, shadows and separation distance

An air terminal near the modules casts a shadow. DEHN’s white paper says the core shadow of a rod fades to diffuse (soft) shade at 108 times its diameter. That is 108 × 10 mm = 1.08 m for a 10 mm rod, and 108 × 16 mm = 1.73 m for a 16 mm rod. Put the rod far enough from the modules to avoid a hard shadow on a cell. Then check the result in a shadow analysis of the array.

DEHN’s guide also says you must take the separation distance into account when you set rod height. The separation distance is the clearance between the external system and the PV metalwork. It prevents uncontrolled flashover (a spark jumping across the gap). DEHN refers to chapter 5.6 of its guide and IEC 62305-3 for the calculation.

Earthing and bonding of the PV frame

The white paper covers the rooftop cases (IEC TR 63227):

• Separation distance kept, or no external system: the metal mounting system is earthed (grounded) for function, with copper conductor of at least 6 mm². The mounting rails are joined together permanently.
• Separation distance not kept: the mounting system connects to the external lightning protection system with conductors that carry lightning current. For a class III system the minimum is 16 mm² copper or equivalent. The rails are again joined together.
• Cable routing: keep power (DC and AC) and data lines together with the equipotential bonding conductors (conductors that keep metal parts at the same voltage) along their whole route. This avoids large conductor loops.

The white paper covers rooftop systems only. A ground-mount array needs its own earthing design from the project’s lightning specialist.

Choosing SPDs for the PV circuits

Voltage, current and protection level

ABB’s PV protection brochure lists the DC-side selection rules, citing IEC 60364-7-71:

DEHN’s white paper adds a rule from IEC 61643-32. Up must be at least 20% lower than the dielectric strength of the PV system (the voltage its insulation can withstand).

Worked example (hypothetical array, Uoc at STC of 850 V): 1.2 × 850 = 1,020 V. A 1,000 V device is below that, so choose a 1,170 V or 1,200 V version. Uoc is the array’s open-circuit voltage. The brochure’s rule uses Uoc at standard test conditions. A voltage rise on a cold morning is a separate check for the string design.

Where each SPD goes

From the DEHN white paper:

• Inverter DC input: a Type 2 SPD on every MPPT input (the inverter’s DC input from the array). Add a second Type 2 SPD at the modules if the cable from the inverter input to the PV generator is longer than 10 m.
• Inverter AC output: an SPD is required if the cable to the next protection point is longer than 10 m.
• Grid connection point, building with external lightning protection: a multipole Type 1 SPD, for example a spark-gap combined arrester. Part of the lightning current can enter the building on the incoming lines.
• Data lines: SPDs for the monitoring and sensor lines, such as RS 485.

Example devices by class

All values below are from the manufacturer’s published datasheets, linked in the first column. They show what each class looks like and are not a recommendation.

The DCB YPV 1200 FM also lists a total 10/350 µs discharge current of 12.5 kA (6.25 kA per path). This is the Type 1 rating in the same device. Datasheets change between product revisions. So read the current sheet for the model you order.

Review the complete protection package

A PV layout change moves the rods, the clearances and the cable routes together. If you expand the array, recheck the protection layout in the same revision. For a design that keeps these documents aligned, see the solar design workflow, the solar safety compliance checklist and the solar three line diagram service. The surge protection device glossary entry defines the terms used above.

Frequently asked questions

Do solar panels need a lightning arrester?

Not automatically. DEHN’s rooftop PV white paper says installing PV modules does not raise the risk of a lightning strike. So the need for an air terminal (a rod that catches strikes) comes from an IEC 62305-2 risk assessment of the building. Surge protective devices are treated differently. IEC 61643-32 calls for them unless a risk analysis shows they are not needed.

What is the difference between a lightning arrester and a surge protective device?

An air terminal (the usual meaning of lightning arrester) is part of external lightning protection. It catches direct strikes. A surge protective device (SPD) limits overvoltage on a circuit. ABB’s PV protection brochure describes two types. Type 1 SPDs are tested with the 10/350 µs waveform, which simulates a direct strike. Type 2 SPDs are tested with the 8/20 µs waveform, which represents indirect strikes.

How do you calculate the protection zone of a lightning arrester?

Use the rolling sphere or protective angle method from IEC 62305-3. Use the radius for your lightning protection level, per DEHN’s Lightning Protection Guide: 20 m for level I, 30 m for II, 45 m for III and 60 m for IV. For a level III system, DEHN’s table gives a protective angle of 74 degrees for a rod 3 m above the roof. That rod protects 3 × tan 74° = 10.46 m horizontally.

What voltage rating should a PV surge protective device have?

ABB’s PV protection brochure gives the rule Ucpv ≥ 1.2 × Uoc stc, where Uoc stc is the open-circuit voltage of the array at standard test conditions. For an array with Uoc of 850 V, that is 1.2 × 850 = 1,020 V. So a 1,000 V device is too low, and a 1,170 V or 1,200 V device qualifies.

Where do SPDs go in a rooftop PV system?

DEHN’s white paper puts a Type 2 SPD on every DC input (MPPT) of the inverter. It adds one at the modules if the cable between inverter and array is longer than 10 m. It requires one at the inverter AC output if that cable is longer than 10 m. If the building has an external lightning protection system, a Type 1 combined arrester goes at the grid connection point.

Can a generic protection-radius formula approve the design?

No. The protected distance changes with the lightning protection level, the rod height and the method used. On a level III system, a 3 m rod gives 10.46 m by the protective angle table and 16.2 m by the rolling sphere construction. So the designer applies the required method to the actual roof geometry.

Sources

• IEC, IEC 62305-1:2024, general principles.
• DEHN SE, Lightning and surge protection for rooftop photovoltaic systems, white paper WPX029/EN/0422, 2022.
• DEHN SE, Lightning Protection Guide, 3rd updated edition, December 2014.
• ABB, OVR PV T1-T2 QS series PV protection brochure, undated.
• DEHN product datasheets for 900 075, 952 565, 900 910 and 952 405.
• NIST, Early streamer emission lightning protection systems: an overview, 2000.