Lightning and surge protection for solar on farms and sites

Protect every place where a long cable enters a building or a device. On the DC side, a surge protector goes close to the inverter, and a second one near the panels when the cable is long. On the AC side, a type 2 protector goes in the main board, or type 1 when the building has a lightning conductor. Data cables to the battery and the monitoring need protection too. On an off-grid system, your own earth electrode is the only reference, so earthing is part of the design from day one. For a grid-connected site, the same rules apply to a business battery system.

A storm passes over the farm at night. In the morning, the inverter display stays black and the monitoring shows nothing since 2 a.m. "Is a surge protector worth it, or is that just something installers sell?" That question comes back on every installer forum after such a night.

The inverter itself is rarely the real cost. The real cost is the weeks without production or back-up while you wait for parts, plus the discussion with the insurer. One owner wrote that after a strike, going back on the grid "was a godsend while we waited weeks for replacement parts". On a farm, the same board often also feeds the milking robot, the ventilation computer and the alarm.

Farms are exactly the sites where this matters. On dairy farms in Londerzeel and Lokeren, solar panels, a wind turbine and a battery system replaced an expensive medium-voltage connection. Such sites have a mast, roofs on several buildings and cables running between them. That is why surge protection is a design question, not an accessory.

What does lightning do to solar panels, an inverter and a battery?

A direct strike on your roof is rare. A strike nearby is far more likely, and it can be enough to break an inverter. The lightning current raises the voltage of the earth around the strike, and its magnetic field induces a surge in every cable loop close by.

According to lightning protection maker DEHN, surges from a direct or nearby strike are "by far the heaviest" for electrical installations. Surges from nearby strikes and from switching are much weaker. But on a farm with motors and welding, switching surges come every day.

According to Brussels Environment (Energuide), about 250,000 lightning strikes are counted above Belgium every year. An open farm with high buildings, a mast and long cable runs is more exposed than a unit on an industrial estate.

Is surge protection for solar panels mandatory in Belgium?

The AREI (the Belgian electrical code) requires protection against surges from lightning and switching, "according to the rules of good workmanship". It does not prescribe one device for every installation.

The logic is in two articles. Article 4.5.1 says people and goods must be protected against surges from atmospheric phenomena and switching. Article 1.4.1.2 says standards registered by the NBN count as the rules of good workmanship. So the European installation standards decide what is needed in practice.

The key one is HD 60364-4-443. As the Belgian distributor Stago summarises it, protection is required where a surge can hit human life, public services or commercial and industrial activity. Farms are named in that list, next to hotels, banks and factories. For other buildings, a simplified risk analysis decides. It looks at lightning density, an overhead or underground supply, and a rural or urban setting.

For household solar installations up to 10 kVA, the AREI adds one concrete rule. The metal frames of the panels and their structure must be earthed, unless the manufacturer forbids it. The earth conductor must be at least 2.5 mm² with mechanical protection, or 4 mm² without it.

Type 1, type 2 or type 3: which surge protector do you need?

The type tells you how much energy the device can divert. The European product standard EN 61643-11 defines the three classes.

The types work as a team. DEHN explains that they must be "energy-coordinated": the device upstream limits the energy so the one downstream is not overloaded. Buy them as one designed set, not as loose parts.

Some inverters have room for protection on board. SMA, for example, offers DC surge protection of type 1+2 or type 2 as an accessory for one of its Sunny Tripower ranges. Check the datasheet of your own inverter before you add anything.

Where do the surge protectors go on a farm with solar panels on the barn?

Wherever a cable can bring a surge into a building or a device. That means the DC cable from the roof, the grid supply, the cables between buildings and the data lines.

DEHN gives a simple rule for solar installations. The protectors go as close as possible to the inverter. When the cable between a protector and the device is longer than 10 metres, a second protector is needed. In its examples, a battery within 10 metres of a protector is covered by it; one further away needs its own.

In Belgium, the protector in the board is connected after the main residual-current device (RCD), according to the Stago installation guidelines. The cable from the protector to the earth bar must be short. DEHN recommends 0.5 m in total and never more than 1 m, because every metre of cable adds voltage during a surge.

Do not forget the thin cables. The battery management system, the monitoring gateway and a network cable to the office all run between devices. DEHN advises protection on data cables that come in from outside as well.

Why do long cable runs to barns and sheds raise the risk?

Because a surge is induced in a loop, and the bigger the loop, the bigger the surge. On a farm, the panels are often on a barn, the inverter in the house and the battery in a shed.

Inverter maker Solis advises keeping the area enclosed by the PV wiring small, to weaken induced surges. On a barn roof with a long run to the inverter, that routing costs nothing extra when it is done right the first time.

A small wind turbine brings its own cable from the mast to the controller. Our article on whether a small wind turbine pays off on a farm covers the turbine itself.

Do you need a lightning conductor on a barn with solar panels?

The AREI does not require one, but a risk analysis under NBN EN 62305-2 can show that it is needed. That analysis weighs the chance of a strike against what a fire or an outage would cost.

Solar panels do not attract lightning more than the rest of the roof. A fire-safety adviser told VRT that lightning usually strikes the highest point, and panels rarely are. On a farm, the highest point is often a silo, a mast or the ridge of a large barn.

When a building has a lightning conductor, the solar installation has to fit with it. DEHN describes two cases:

• Enough separation distance: the panels stay in the protected zone and do not touch the lightning conductor. Type 2 protectors are then enough on the DC side.
• Not enough separation distance, for example on a metal roof: the metal parts are bonded to the lightning conductor. Type 1 protectors are then needed on the AC, DC and data side.

For a ground-mounted installation in a field, DEHN recommends a meshed earthing grid under the array. Meshes of 20 x 20 to 40 x 40 metres have proven themselves.

Why does earthing matter so much, especially off-grid?

A surge protector sends the surge to earth, so it only works as well as the earthing behind it. The AREI asks for an earth resistance "as low as possible".

For homes, the AREI sets concrete limits: below 100 ohm, and extra RCDs when the earth resistance is above 30 ohm. For business installations, the limit follows from the RCDs that are used. The inspector measures the earth resistance at every conformity check.

On a grid-connected site, the grid gives a reference point. On an [off-grid system](https://www.youpower.be/en/products/off-grid), your own earth electrode is the only reference for the whole installation. It also has to stay effective in island mode, when the inverter forms the grid. A Walloon homeowner described how the inspector checked exactly that, and measured his earth electrode at 20 ohm.

Our article on the components of an off-grid system shows how the inverter, the battery and the generator fit together.

What does your insurer look at after lightning damage?

First whether your policy covers overvoltage damage, and then whether the parts can be repaired or must be replaced.

According to Assuralia, the federation of Belgian insurers, solar panels are included in a standard fire policy. If you add them later, report them to your insurer. After damage, an expert decides whether the panels or the inverter can be repaired, or whether whole parts must be replaced. That decision sets the payout.

Brussels Environment (Energuide) adds a clear piece of advice: make sure your fire policy also covers damage caused by overvoltage. A direct strike and a surge from a strike nearby are not always treated the same way in a policy.

Remote monitoring helps here. In our monitoring software you see after a storm whether every inverter and charger is still running, and when one stopped. Our article on service, warranty and maintenance for a business battery explains what to agree with your installer.

Battery safety goes further than surges: fire, placement and permits. Our article on whether a business battery is safe covers those.

When is extra lightning protection not worth it?

When the installation is compact and the cables are short.

What should you ask your installer about surge protection?

Five questions show quickly whether surge protection is part of the design or an afterthought.

• Which type of protector goes on the DC side, on the AC side and on the data cables, and where exactly?
• How long is the DC cable from the panels to the inverter, and is there a protector at both ends?
• Does the building have a lightning conductor, and is the separation distance to the panels respected?
• What is the measured earth resistance, and how is earthing kept in island mode?
• Which protectors can you check yourself after a storm?

Many protectors have a small window that changes colour when the module is spent. Ask where they are and what the colours mean.

What happens when you ask us to design a solar and battery system for your site?

We start from your site: the buildings, the loads and what must keep running.

1. Step 1: Analysis. We list the consumption, the peaks and the critical loads. Where data is missing, we can measure first.
2. Step 2: Technical design. Our engineering office calculates the type, number and position of the panels, the inverters, the batteries and, where useful, a generator.
3. Step 3: Installation and commissioning.
4. Step 4: Monitoring and support. You follow production, consumption and the state of charge in real time.

Read more about farm systems in our article on a solar and battery energy system for a farm. Our battery systems for businesses range from small installations to modular systems up to 1 MW.