Industrial lightning protection: LPS, SPDs and earthing

Short answer

An industrial plant is a structure with contents and internal systems whose lightning damage ranges from minor to unacceptable loss of production. IEC 62305 protects it with a system of air terminals, down-conductors and earthing that can use the reinforcing steel and the metal frame, a meshed earthing network between buildings, SPDs at the service entrance and in the panels, and shielded cables with careful routing.

Short answer. An industrial plant is a structure, or several, with equipment, processes and people. The IEC 62305 standard protects it on four fronts: a lightning protection system (LPS) with air terminals, down-conductors and earth electrodes that can use the reinforcing steel and the metal frame; a meshed earthing network linking buildings and equipment; surge protective devices (SPDs) at the service entrance and in the panels; and shielded cables routed to reduce induction loops. The risk assessment decides the protection level.

What has to be protected and why

Clause 5.1 of IEC 62305-1 lists what determines the effect of lightning on a structure: its construction (timber, brick, concrete, steel frame), its function, its occupants and contents (including combustible or explosive materials and electrical and electronic systems), the connected lines, the measures already installed and the extent of the hazard. In its Table 1, for an industrial facility, the effects depend on the contents of the factories and range from minor damage to unacceptable damage and loss of production.

Lightning can damage a plant in four ways (clause 5.1.2): a strike to the structure, near it, to a connected line or near a connected line. The damage is injury from electric shock, physical damage (fire, explosion, mechanical destruction) and failure of internal systems through the electromagnetic pulse.

The plant LPS

The LPS of IEC 62305-3 comprises air terminals (on the roof and, if needed, on the sides), down-conductors and earth electrodes. The class (I to IV) comes from the risk assessment and sets, among other things, the rolling sphere radius (20, 30, 45 and 60 m) and the air-terminal mesh size (5 × 5, 10 × 10, 15 × 15 and 20 × 20 m), per Table 2.

Structural steel as a down-conductor

In industrial halls and buildings, Annex E.4.3.7 permits using the reinforcing bars of walls and columns and steel frames as natural down-conductors. The points that count are:

There must be a termination joint at the roof to connect the air terminals and, unless the foundation serves as the only electrode, termination joints to connect the earthing system.
If a reinforcing bar is used as a down-conductor, the same bar must be followed all the way down, to maintain direct electrical continuity.
If vertical continuity cannot be guaranteed, dedicated conductors are added, tied or clamped to the reinforcement. Where there are doubts in existing buildings, an external down-conductor system is added.
Welds between bars inside concrete must be at least 50 mm long, and bars that cross are bent to run 70 mm in parallel before welding.
In steel structures, bolted joints are considered interconnected if the bolts have the necessary tightness. The connection is improved by exposing the bearing surfaces or with a weld of about 50 mm. Each steel column is connected to the foundation reinforcement.

In large, low halls, the roof is also supported on interior columns. If they are conductive, they are connected to the air terminals at the top and to the equipotential bonding at the floor, and become internal down-conductors (Figure E.10), which avoids dangerous sparking inside the hall. Close to them, electromagnetic interference increases, and this must be kept in mind when locating sensitive equipment. Internal down-conductors are connected to an electrode and to the floor reinforcement, to avoid step and touch voltages, and nearby expansion joints are bridged. The lower part of an exposed down-conductor is insulated with a PVC tube at least 3 mm thick or with equivalent insulation.

For equipotential bonding, Annex E.4.3.8 recommends, where many connections to the reinforcement are needed, conductive rings at each floor joined by vertical bars at intervals of no more than 10 m.

Meshed earthing network of the plant

A plant usually has several structures linked by many power and signal cables. Their earthing systems are decisive for electrical protection: a low-impedance earth reduces the potential difference between structures and, with it, the interference injected into the electrical links. Annex E.5.4.3.6 proposes providing each building with foundation electrodes and type B and type A arrangements, and interconnecting the electrodes, the foundation electrodes and the down-conductors at the test joints, connecting some of them to the equipotential bars of the internal LPS.

Figure E.42 of IEC 62305-3 shows the electrode mesh between buildings, towers, isolated equipment and cable trenches. Meshes can be of the order of 20 × 20 m next to buildings and other objects, and 40 × 40 m at more than 30 m. That mesh gives low impedance between buildings and important electromagnetic compatibility benefits. To reduce the probability of direct strikes on buried cable routes, an earth conductor is installed above them, and several if the route is wide. In the foundation, the reinforcing steel of the footing and buried walls forms an excellent electrode, and it is recommended to add an additional metal mesh tied to the reinforcement (Annex E.4.3.9).

Part 4 (clause 5) applies the same idea to the interior: a mesh of about 5 m under and around the building and a three-dimensional bonding network. The lightning earth does not replace the plant earth grid for power-frequency faults: both are integrated in the design. For these, see industrial earthing and earth grid.

You can estimate the resistance of an electrode or a grid with our ground resistance tool.

SPDs at the service entrance and in the panels

Clause 7 of Part 4 calls for SPDs at the entrance of each zone: with a single inner zone (LPZ 1), at least at the service entrance; with more zones, at the boundary of each, and additional SPDs if the equipment is far away. In a plant, that means SPDs at the power service entrance, in the sub-distribution boards and close to sensitive equipment, for power and for signals. If the plant transformer is inside a building and there are no SPDs on the high-voltage side, Part 4 (Figure 4) extends zone LPZ 0 inward and the SPDs are installed only at low voltage. Selection and class coordination are covered in transient overvoltages in plants, which explains the causes of overvoltages and zone-based protection.

Shielded cables and routing

The control, instrumentation and communication cables between buildings and equipment form loops where the lightning field induces voltage. They are reduced with shielded cables with the screen bonded to earth at both ends, closed and interconnected metal ducts, power and signal routed together, and sensitive equipment kept away from shield walls and internal down-conductors. The immunity of drives and field equipment is covered in EMC and shielding of variable frequency drives. External equipment (sensors, cameras, antennas, instruments on process vessels) is brought into zone LPZ 0B with a local air terminal where possible, and its cables leave at a single point and use the natural shielding of pipes and ladders (Annex B.12 of Part 4).

Work sequence to protect an industrial plant

Assess the risk. Contents, processes, loss of production and consequences for the surroundings
Define the LPS of each building. Class, air terminals, natural down-conductors and foundation electrodes
Mesh the plant earthing. Interconnect buildings, towers and equipment; cable trenches included
Define the LPZ zones. Boundaries with SPDs, bonds and shields
Shield and route the cables. Screens at both ends, ducts and small loops
Inspect and maintain. Periodically and after alterations or a lightning strike

What not to do

Do not assume the continuity of the reinforcing steel is complete: confirm it by measurement before using it as a down-conductor.
Do not locate sensitive equipment next to columns that act as internal down-conductors.
Do not assume electronics are protected by an LPS that only has equipotential bonding SPDs.
Do not run power and signal cables on separate routes that form large loops.

Guides in the same series

Tanks and pipework with an explosion hazard are covered in lightning protection for structures with an explosion hazard, and inspection of the system in LPS inspection and maintenance. The interior protection of the plant is in SPM against LEMP, equipotential bonding network and 3D earthing system, magnetic shielding and cable routing and coordinated SPDs; substations are in substation lightning protection.

When to call us

Lightning protection of a plant is decided together with the earthing, the protection, the cables and the equipment of the same project. If you are going to build, extend or review a plant, our engineering design and studies team can support you on the electrical side, and our testing and commissioning services cover earthing measurements, in Colombia or elsewhere on request. Write to us through the contact page.

References

IEC 62305-1:2010, Protection against lightning, Part 1: General principles: clause 5.1 and Table 1.
IEC 62305-3:2010, Part 3: Physical damage to structures and life hazard: Table 2 and Annex E (E.4.3.7 to E.4.3.9, E.5.4.3.6, Figures E.10 and E.42).
IEC 62305-4:2010, Part 4: Electrical and electronic systems within structures: clauses 4 to 7 and Annex B.12.

Frequently asked questions

Can the structural steel of an industrial hall be used as a lightning down-conductor?

Yes. IEC 62305-3 accepts the reinforcing bars of walls and columns and steel frames as natural down-conductors, provided there is electrical continuity: bolted joints with the necessary mechanical tightening are considered interconnected, and they are improved by cleaning the contact surfaces or with a weld of about 50 mm. If vertical continuity cannot be guaranteed, dedicated conductors are added.

What advantage does a meshed earthing network between the buildings of a plant have?

It reduces the potential difference between buildings and, with it, the interference injected into the power and signal cables linking them. Figure E.42 of IEC 62305-3 shows meshes of the order of 20 × 20 m next to the buildings and 40 × 40 m at more than 30 m.

How does lightning affect a plant with continuous processes?

According to Table 1 of IEC 62305-1, in an industrial facility the effects depend on the contents of the factories, from minor damage to unacceptable damage and loss of production. For that reason the protection level and the LEMP protection measures (SPM) are decided with the plant's risk assessment, not with a single recipe.