Tall steel structures attract lightning. In Kenya, where thunderstorm activity is intense around Lake Victoria, the Western highlands, and parts of the Rift Valley, a tower may be struck several times a year. A direct strike can deliver tens of thousands of amperes within microseconds. Without a proper lightning protection and earthing system, that energy can destroy radio equipment, injure technicians, and damage foundations. This article explains how lightning interacts with towers and how protection is designed.
How Lightning Affects Towers
Lightning damage occurs through several mechanisms.
- Direct strike: current enters the tower top or an antenna, then flows down the structure to earth.
- Ground potential rise: current dispersing into the soil raises the potential around the base, creating dangerous voltage gradients.
- Induced surges: magnetic fields from the lightning channel induce voltages in nearby cables and equipment.
- Surge conduction: current travels along power, telecom, and feeder cables into the equipment shelter.
- Side flash: current jumps from the tower to nearby conductive objects.
Good protection aims to capture the strike safely, conduct it to ground, dissipate it, and limit surges entering sensitive equipment.
The Standards Framework
The international reference is the IEC 62305 series (EN 62305), covering general principles, risk management, physical damage to structures and electronic systems. IEC 62305-2 describes risk assessment, which determines whether protection is needed and what level to apply. Lightning protection levels (LPL I to IV) correspond to different peak current assumptions, with level I being the most onerous. Telecom-specific guidance includes ITU-T K.27, K.56, and K.66 and ANSI/TIA-222-H, and national regulations may add further requirements.
Components of a Lightning Protection System (LPS)
1. Air termination
Air terminals (lightning rods) at the tower top intercept strikes. They should rise above antennas and lights so that equipment lies within the protected zone. The rolling sphere method or protection angle method determines coverage. On lattice towers, the steel structure itself often acts as the air terminal and down conductor.
2. Down conductors
Lightning current must flow from the air terminal to earth by a low-impedance path.
- Lattice towers: the steel legs provide multiple parallel paths if joints are electrically continuous, which is ideal because current is shared and inductance is low.
- Monopoles: the shaft acts as the conductor, but flange joints and internal cables must be bonded.
- Concrete or insulating structures: need dedicated copper or galvanized steel conductors.
Bolted joints should be checked for electrical continuity, and painted or corroded surfaces can interrupt the path. Bonding jumpers across flanges and joints are often added.
3. Earth termination system
This is the buried part that disperses current into the soil. It may include:
- Ring electrodes around the tower base, typically 0.5 to 1 m from footings.
- Radial electrodes extending outward from the ring.
- Vertical rods driven to reach lower-resistivity soil.
- Foundation (Ufer) earthing, using reinforcement steel in concrete footings.
- Chemical or enhanced earth rods in high-resistivity ground.
The aim is a low resistance and, more importantly, a low impedance to high-frequency lightning current. A common target for telecom sites is 10 ohms or less, with many operators demanding 5 ohms or less. A mesh or ring with several radials performs better than a single deep rod under lightning conditions.
4. Equipotential bonding
All metallic parts, including the tower, cable ladders, feeder shields, shelter frame, fence, generator, fuel tank, mains neutral earth and incoming metallic services, are bonded to a common earth bar or ring. Equipotential bonding prevents dangerous voltage differences between items during a strike. Feeder cable shields should be grounded at the tower top, mid-height (on tall towers) and bottom, with grounding kits that bond the shield to the tower.
5. Surge protection devices (SPDs)
SPDs protect electrical and electronic equipment from transient overvoltages.
- Power SPDs at the main distribution board and at sensitive equipment (coordinated Type 1, Type 2 and Type 3 devices).
- Coaxial SPDs at the entry to the equipment shelter.
- Data and signal line SPDs on Ethernet, alarm and control lines.
- DC SPDs on 48 V power systems and remote radio units on the tower.
SPDs only work if they have a short, low-inductance connection to the earth bar, so lead lengths should be kept as short as possible.
Designing the Earthing System
Soil resistivity measurement. The Wenner four-pin method measures the soil resistivity at several depths. Resistivity in Kenya varies from a few tens of ohm-metres in wet clay to several thousand in dry volcanic rock, so each site should be tested.
Electrode layout. Based on resistivity and available area, the designer chooses ring, radial and rod combinations. In rocky ground, long horizontal radials or trenches filled with conductive backfill are often more effective than vertical rods.
Material. Copper, copper-clad steel, and hot-dip galvanized steel are common. Copper gives longer life but is attractive to thieves, so many sites use copper-bonded steel with anti-theft measures.
Connections. Exothermic (Cadweld) welds or compression connectors give permanent low-resistance joints. Bolted joints in the ground should be avoided.
Step and touch voltage. The layout should limit voltage gradients near the base, so people and animals are not endangered during a strike. Gravel surfacing and fencing help.
Protecting the Shelter and Equipment
- Run power and signal cables in bonded metal trays or conduits.
- Route cables away from down conductors.
- Use a single-point earth or a well-designed mesh bonding network.
- Install a bonded copper bar at the shelter entry.
- Use isolation transformers or fibre optic links where practical.
Testing and Maintenance
Earthing performance can degrade because of corrosion, theft, dry soil, and mechanical damage. Regular testing is critical.
- Earth resistance testing using fall-of-potential or clamp-on testers, at least annually and after any strike or repair.
- Continuity testing of bonds and joints.
- Visual inspection of air terminals, connectors, SPDs, and clamps.
- Record keeping of test results to identify trends.
- Replacement of consumed or stolen conductors promptly.
Safety Considerations
Technicians should not climb during thunderstorms. Most operators enforce a standstill when lightning is within a certain radius, commonly 10 km or when thunder is audible. Warning signs, restricted access, and personal lightning detectors add protection.
Common Problems
- High earth resistance due to dry or rocky soil.
- Stolen copper conductors.
- Corroded clamps and joints.
- Missing bonding of feeder cables.
- SPDs that are damaged by previous strikes but not replaced.
- Poor separation of power and signal cables.
Conclusion
Effective lightning protection is a system, not a single device. Air terminals, down conductors, a low-impedance earth system, equipotential bonding, and coordinated surge protection work together to keep people safe and networks running. Design based on site-specific risk and soil resistivity, build with quality connections, and test regularly to keep protection effective.
Key references: IEC 62305-1 to -4; ITU-T K.27; ANSI/TIA-222-H; IEEE 80.
