Surge Protection in Business Premises: When You Need It and How It Relates to the Lightning Rod

The typical scenario looks like this: the hall has a lightning rod, an inspection report with no defects, and after a June thunderstorm half the cameras are dead, along with one switch port and the air-handling control unit. The lightning need not have hit the building at all. It was enough that it struck two hundred metres away and the voltage was induced into the cables running into the building from outside.

The external lightning protection did its job — the air-termination system, down conductors and earthing are there to limit the effects of a direct strike on the structure. The electronics inside are the responsibility of the second part of the same system: internal protection, meaning bonding, adequate separation distances and surge arresters. And that is precisely what is usually missing or half-finished in halls like this. Let’s go through how external and internal protection differ, when a company should look into arresters, what the T1, T2 and T3 types mean, why photovoltaics is a chapter of its own, and what you can tell from an inspection report.

The lightning rod and surge protection are two parts of one system

What people commonly call a lightning rod is technically external lightning protection: the air-termination system, down conductors and earthing. Their task is to intercept the discharge at a defined point and conduct the lightning current into the ground so that it does not spread uncontrolled through the structure — they are therefore meant to limit the effects of a direct strike on the building and reduce the risk of damage. The risk is reduced, but it does not disappear; even a properly designed and maintained lightning rod cannot rule out fire or damage.

The lightning protection system does not end there, though. The second part is internal protection: equipotential bonding, maintaining an adequate separation distance between the air-termination system and internal installations, and surge arresters. Its purpose is to limit the effects of the lightning current on the installations inside the building. The air-termination system, down conductors and earthing on their own therefore usually will not protect sensitive electronics — without properly designed SPDs and the related measures, the interior stays uncovered.

What is more, conducting the lightning current to earth does not in itself prevent a transient surge from getting inside along the power, data or antenna wiring. Quite the opposite — in a building with a lightning rod you have to expect that part of the lightning current will find its way into the internal wiring when the air-termination system is struck, whether galvanically through the bonding or by induction into cables running in parallel.

Surges are limited by surge arresters, referred to in documentation by the abbreviation SPD (surge protective device). Their job is to cut the transient surge down to a level that the connected equipment can withstand according to its overvoltage category.

Two sentences worth remembering follow from this:

• External protection without arresters and bonding does not mean your company’s electronics are protected.
• Arresters without an air-termination system do not replace protection of the building against a direct strike.

And a third: a power strip labelled “surge protection” for 400 CZK is at best the last, supplementary stage. On its own it will not stand in for arresters in the main and sub-distribution boards — it has neither the discharge capacity for that, nor anything ahead of it to reduce the energy of the discharge.

When a company should address surge protection

There is no universal rule along the lines of “every company buys the same arrester”. The solution is designed around the building, the incoming supply, the design of the lightning rod, the type of system earthing, and around what happens if operations stand still for two days.

Whether lightning protection is installed on a particular building at all follows from construction requirements — currently from Decree No. 146/2024 Sb. (Czech construction requirements decree) — and from a risk assessment under the ČSN EN 62305 series of standards. Since the end of 2025, the 3rd edition of this series applies, i.e. ČSN EN IEC 62305 ed. 3; the 2nd edition remains valid in parallel until 31 October 2027. For projects to be carried out in 2026 or 2027 it is therefore worth assessing the risk under ed. 3 straight away. Neither the decree nor the standard will tell you which device to buy, though; that is a matter for the design of the specific installation.

With an older building, there is no obligation to rebuild the installation every time a new standard comes out. Grounds for an assessment are, however, a refurbishment, a change of use, new technology, missing or outdated documentation, a defect found during an inspection — and of course a lightning strike or a series of unexplained electronics failures.

What T1, T2 and T3 mean

Surge protection is not a single device but a set of mutually coordinated stages. Each has a different role and different test parameters.

There are also combined T1+T2 arresters on the market, for which the manufacturer declares both tests — from DEHN typically the DEHNventil and DEHNshield ranges, from OBO Bettermann the V50 and V25 devices, from Phoenix Contact FLASHTRAB. Pure T2 surge stages are represented by DEHNguard, OBO V20 or VALVETRAB; separate ranges exist for data and signal circuits, for example PLUGTRAB. These are examples of categories, not recommendations — every range contains dozens of variants depending on voltage, type of system earthing, number of poles and discharge capacity, and swapping one for another is not a detail.

So the figure “40 kA” on the packaging tells you nothing on its own — not without knowing which waveform it refers to. For protecting electronics, the voltage protection level Up is equally important, i.e. the value the arrester actually cuts the surge down to, as is the maximum continuous operating voltage Uc, which must sit comfortably above the mains voltage.

Where the design breaks down in practice

Most surge protection that does not work fails not because the wrong device was bought. The mistakes are usually in the installation and in how things follow on from each other.

Length of the connecting conductors. With a steep impulse waveform, the inductive voltage drop on the leads runs to kilovolts per metre. An arrester with a Up of 1.5 kV connected via half a metre there and half a metre back will therefore “show” the equipment a substantially higher voltage than its catalogue figure. The requirement to keep the connection as short as possible comes from ČSN 33 2000-5-53 ed. 3 (clause 534) — total connection length up to 0.5 m, 1 m at most — and is handled by what is known as V-connection, where the arrester sits directly in the path of the current and the leads are practically eliminated. A few needless loops of conductor in the board can render even an expensive device worthless.

Coordination of the stages. There has to be enough impedance between T1 and T2 for the stages to share the energy. In practice this means roughly ten metres of cable between them, or a decoupling inductor — unless the manufacturer explicitly declares the two devices to be mutually coordinated. Two arresters side by side on one rail without this arrangement do not give you “double protection”.

Type of system earthing. Wiring in a TN system differs from a TT system, where a 3+1 arrangement with a spark gap between N and PE is usually used. A device selected on voltage and pole count alone, with no regard to the earthing system, may be unusable.

Backup fuse and residual current device. An arrester needs overcurrent protection as specified in the manufacturer’s data sheet; if the upstream protection is equal or smaller, a separate backup fuse is not needed. And if the SPD sits downstream of a residual current device, the passage of the discharge current will at the very least trip the RCD — and at worst destroy it. That is why arresters are placed upstream of the RCD, or a surge-current-resistant RCD is chosen.

Data gets forgotten. The classic case is a server on a UPS with nice surge protection on the power supply, and next to it an unprotected metallic Ethernet run going out to an outdoor camera under the gutter. The surge arrives along the data cable, the switch port goes — and occasionally the whole switch with it. The answer is an arrester on the data line, and for longer outdoor runs fibre, which eliminates the problem by design.

Photovoltaics is a special case

A PV system adds long DC cables across the roof, an inverter and a new connection to the main distribution board. Surges therefore have to be dealt with on both the AC and the DC side, and both belong in the design.

On the DC side, an arrester designed specifically for photovoltaics is used — an ordinary AC device with a similar-looking voltage rating is no substitute. A DC arc does not extinguish itself, and that places different demands on the arrester’s construction. The choice depends on the maximum string voltage, the array configuration, the inverter location and the relationship between the panel mounting structure and the air-termination system.

That relationship is exactly where misunderstandings tend to arise. The often-repeated rule that a panel should be “at least 60 cm from the lightning rod” is misleading — what decides is the calculated separation distance s under ČSN EN IEC 62305-3 ed. 3 (the older ed. 2 running in parallel until 31 October 2027), which depends on the design of the air-termination system, the number of down conductors and the length of the route. Where it cannot be maintained, the PV mounting structure is bonded to the lightning protection. That, however, changes the brief for the arresters: instead of a T2 alone, a T1 or a combined device may be called for, because part of the lightning current now has to be accounted for.

An arrester built into the inverter does not protect the whole PV system or the rest of the building — it protects the inverter input. Where the individual stages go and how they are coordinated belongs in the design. We look at the wider picture around running a rooftop PV system in more detail in the article on PV inspections and their intervals.

What gets checked during an inspection

Arresters are not a separate agenda — they are checked as part of the inspection of the electrical installation or the lightning protection, according to the agreed scope. Both the electrical installation and the lightning rod count as reserved technical electrical equipment; the requirements for their inspections and checks are set out in Government Regulation No. 190/2022 Sb., following on from Act No. 250/2021 Sb.

The word “scope” matters here. An order worded solely as “lightning rod inspection” need not cover arresters in sub-distribution boards, the DC side of a PV system or data lines. If those are to be included, they belong in the order explicitly.

With SPDs, the inspection engineer checks above all:

• whether the type and parameters match the design, the earthing system and the design of the lightning rod,
• the placement of the individual stages and their energy coordination,
• the backup fuse and behaviour in the event of arrester failure,
• the connection to the protective conductors and to the main bonding,
• the length and routing of the connecting conductors,
• the state of the visual indicators and of any remote fault signalling,
• visible damage, loose connections, signs of overheating,
• protection of the PV system, antennas, data lines, cameras and BMS, if they fall within the system being assessed.

A green window on the module means one thing only: the disconnector has not tripped yet. It is not confirmation that the system is correctly designed — a varistor degrades gradually, through a series of smaller surges, and the thermal fuse disconnects it only at the very end. From that moment on there is a plastic dummy in the board until someone notices the indicator. That is exactly why remote fault signalling makes sense on critical distribution boards.

After a direct lightning strike or unexplained damage to electronics, there is no point waiting for the scheduled date. The sensible thing is to have the air-termination system, earthing, arresters and the connected installation looked at all at once — if you don’t know what you actually have in your boards, we’ll assess it on site and you will get a list of specific defects with what to address immediately and what can be scheduled. Intervals for regular inspections of the air-termination system are covered separately in the article on how often a lightning rod inspection is repeated, and we have described what the individual findings in the report mean in our guide to reading an inspection report.

How to recognise a good quotation

A supplier should not start by picking a particular box. First they need to know what the system earthing and main protection are, how the lightning rod is built, how much room there is in the boards, where the cable routes run, whether there is a PV system on the roof — and which equipment brings the company to a stop when it fails.

A usable quotation states:

• the type and exact designation of every arrester,
• the reason it was chosen,
• the Iimp, In, Imax, Up and Uc values,
• the wiring arrangement and backup fuse,
• the modifications needed to the distribution board,
• the AC, DC and data line solutions separately,
• replaceable modules and fault signalling,
• as-built documentation and the follow-up inspection.

A warning sign is a quotation built solely on a high kA figure, with no inspection of the board and not a single mention of the lightning rod. The second warning sign is a quotation that deals with powering the server and says nothing about the cable to the outdoor camera.

What it costs to add

The price is not determined by the number of modules but by the state of the board and the extent of the installation. As a rough guide: adding a T2 stage to a board with free space and tidy bonding comes to a few thousand crowns. A combined T1+T2 with modifications to the board tends to be in the thousands to low tens of thousands. A site with several boards, a PV system, cameras and data routes gets into the tens of thousands.

If the board is old, full or structurally unsuitable, the main item will not be the arrester but the rebuild. So ask for an itemised quotation that separates materials, board modifications, labour, documentation and the inspection — otherwise there is no comparing it with a second one. We have gone through everything that feeds into the price of inspecting distribution boards in the article on distribution board inspections.

What to prepare before ordering

Gather the electrical and lightning rod inspection reports, the as-built design, the PV documentation and photos of the boards — a phone will do, with the doors open. Write down the equipment the company cannot work without: server, tills, gates, cooling, security, control system, machines.

And put three questions to the supplier:

1. Which routes can a surge take into the building?
2. Which stages are you proposing and why these in particular?
3. Does the solution also cover the PV system, data, antennas and outdoor equipment?

Anyone who answers without hesitating knows what they are doing.

Frequently asked questions

Is surge protection needed even if we have a lightning rod?

It is precisely in a building with a lightning rod that you should look at how part of the lightning current can get into the electrical wiring. The air-termination system with down conductors and the internal protection including arresters are designed as one connected system, not as two independent jobs.

Is an arrester worth it in a building without a lightning rod?

Yes. Surges also arrive along the distribution network, from a nearby strike or from switching your own equipment. An arrester does not, however, replace protection of the building against a direct strike where the risk assessment shows it is needed.

Is an extension lead with surge protection enough?

Not as the only protection. A socket-outlet product corresponds roughly to a T3 stage and works only in conjunction with arresters in the board that reduce the discharge energy ahead of it.

How often are arresters replaced?

There is no universal interval. What you watch is the visual indicator, the manufacturer’s instructions and the condition found during the inspection. A check is also in order after a lightning strike or after an event in which electronics were damaged.

Are arresters checked during every lightning rod inspection?

That depends on the agreed scope and the building’s documentation. So state explicitly in the order the external lightning protection, the arresters in the distribution boards, the DC side of the PV system and the data lines you want covered.

Sources for this article

• Lightning rod inspections – a SOHE service.
• Lightning rod inspection: how often under ČSN EN 62305 – intervals for regular inspections of the air-termination system and what affects them.
• Distribution board inspections – what gets checked in a board and which defects turn up most often.
• PV inspections: obligation, intervals, risks – checks on rooftop photovoltaics including the DC side.
• Act No. 250/2021 Sb. – occupational safety in connection with the operation of reserved technical equipment.
• Government Regulation No. 190/2022 Sb. – reserved technical electrical equipment and the requirements for ensuring its safety, including inspections and checks.
• Decree No. 146/2024 Sb. – construction requirements, from which the installation of lightning protection on buildings follows.
• We cite technical standards without a link because they are not freely available: the ČSN EN 62305 series (lightning protection and risk calculation), ČSN 33 2000-4-443 and ČSN 33 2000-5-53 ed. 3 (clause 534) (protection against overvoltages and surge protective devices in low-voltage installations) and the ČSN EN IEC 61643 series (arresters for power supply systems, data circuits and photovoltaic installations).

This text is informative in nature and does not replace legal advice. The specific regime for lightning and surge protection needs to be set according to actual operations, the manufacturer’s documentation, the environment of use and a risk assessment.

Not sure whether your lightning rod is backed up by working surge protection, or whether all you have in the board is green windows? We will go through the air-termination system, earthing, arresters and the connected installation, and you will get a list of specific defects — lightning rod inspections. Ask us for a no-obligation quote or send photos of your distribution boards to info@sohe.cz and we will come back to you with a proposed scope.