Key Steps in Designing an ESE Lightning Protection System
A look at risk assessment, ESE arrester selection, placement, down conductor design, and grounding for a compliant ESE lightning protection system.
Assessing the Risk
Before you design an ESE lightning protection system, you need to check how exposed the structure is to lightning and what will happen if it gets struck. Start by looking at the following factors:
Lightning density of the site: If your site is in a region of high lightning density, the probability of being struck is naturally higher. Areas with frequent thunderstorms demand a higher level of protection compared to areas with lower lightning activity.
Physical characteristics of the structure: If the building is tall, isolated, or located on elevated ground, it is more likely to initiate an upward leader that attracts lightning. An upward leader is the rising spark from the structure that connects to the lightning coming down from the cloud.
How the structure is used: If the facility is a hospital, data centre, telecom site, industrial plant, or fuel storage area, the consequences of a lightning strike are severe. In these cases, you cannot accept even a moderate risk, so the protection level must be higher.
Based on these factors, you determine the required protection level: Level I, II, III, or IV. Level I gives you the highest degree of protection, while Level IV provides a lower but still controlled level. This decision is critical. If you underestimate the risk, the system may fail during an actual lightning event. If you overestimate it, you may spend more than necessary. A proper risk assessment is mandatory before you move to system design.
Selecting the ESE Lightning Arrester
Once the risk assessment is complete and the protection level is decided, you select the ESE lightning arrester based on calculation. According to NFC 17-102, the protection radius of an ESE arrester depends on three factors:
Height of installation: The protection radius is the horizontal distance around the arrester that remains protected. If you install the arrester higher, the radius increases.
Protection level: If you choose a higher protection level, the radius becomes more conservative, but it still provides umbrella protection.
Delta T: This represents how much earlier the device emits an upward leader compared to a conventional rod, measured in microseconds. As per NFC, Delta T can be between 10 and 60 microseconds.
You calculate the protection radius using this data, then check if the radius fully covers the structure. If the calculated radius does not cover the entire building, you cannot assume protection. You must either increase the height or add another arrester. Selection is not about choosing a model at random. You calculate, verify coverage, and only then finalise the device.
Placement of the ESE Arrester
Place the arrester at the highest point of the structure. This gives you the largest protection radius for the given height and protection level. Next, match the placement to the shape of the structure. A long roof, multiple levels, or separate blocks can leave corners and edges outside the covered zone, so place the arrester where the calculated protection radius covers the full structure.
Then check the surrounding area. If a nearby object sits higher, such as a tower, mast, or another building, lightning will prefer that point instead. After placing the ESE arrester, keep a clear direct path for the down conductors to reach the grounding system, since the lightning current must travel from the ESE arrester to ground through a safe path.
Down Conductor Design
In an ESE system, connect each air terminal to at least two down conductors. Using two conductors divides the lightning current and reduces stress on a single path. Install them on opposite sides of the structure, spreading the current flow and reducing the chance of side flashing on one side.
Keep the down conductors at a safe distance from other metal parts and internal cables, as lightning can jump to nearby conductive parts if they sit too close. The aim is simple: give the lightning current a direct path to earth, so it does not travel through the structure.
Grounding the System
Lightning current must enter the soil in a controlled way. If the current cannot disperse into the ground, it becomes a threat to the structure and its equipment. Connect each down conductor to the earth electrode system, giving the lightning current a direct path into the soil.
Ensure the earth electrodes are properly interconnected. This keeps the system at the same potential during a strike and reduces the risk of side flashing. The required earth pit resistance shall be less than or equal to 10 ohms.
Conclusion
You now have a brief idea of how an ESE lightning protection system should be designed. Please note that these calculations and designs should only be carried out by a competent or authorised engineer.
At Axis, our team of 50+ engineers is ready to assist you in designing, installing, and testing your Lightning Protection Systems. Our products are used in substations, data centres, factories, and in everyday residential and commercial buildings.
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