Lightning Protection System (LPS): Classes I to IV

Short answer

A lightning protection system (LPS) is the set of measures that reduces the physical damage a lightning strike causes to a structure. IEC 62305-3 divides it into an external system (air-termination, down-conductors and earth-termination) and an internal system (lightning equipotential bonding and, where applicable, electrical isolation of the external system). The standard defines four classes, I to IV, which correspond to the four protection levels of Part 1: class I is the most demanding. Which one is installed comes from the risk assessment of Part 2, not from preference.

What it covers and what it does not

Part 3 applies to the design, installation, inspection and maintenance of an LPS for structures of any height, and to measures against injury from step and touch voltages near the down-conductors. It does not aim to protect against failures of electrical and electronic systems caused by overvoltage: that is Part 4. Wind turbines have their own standard, IEC 61400-24.

It is a standard for structures. An outdoor substation yard is protected with overhead shield wires and masts, a subject covered in the guide on lightning shielding of substations. The LPS in this guide is the one for the building: control room, switchgear hall, industrial building.

The parts of the LPS

Air-terminations can be rods (including free-standing masts), catenary wires or meshed conductors. Every air-termination must comply with the standard in full, and only the real physical dimensions of the metal count when working out the protected volume.

Isolated or non-isolated

The standard defines an isolated external system as one where air-termination and down-conductors are placed so that the current path does not touch the structure. In a non-isolated system that path may touch it.

In most cases the external system can be fixed to the structure. An isolated system should be considered when the thermal and explosive effects at the point of strike, or in the conductors carrying the current, can damage the structure or its contents. The standard gives as examples structures with combustible roofs or walls and areas with an explosion or fire hazard. It can also make sense when changes to the structure or its use are expected, or when the contents are so sensitive that reducing the electromagnetic field radiated by the down-conductor current is justified.

What depends on the class and what does not

The class sets the data that tighten as the protection level rises:

• the lightning current parameters of Part 1;
• the rolling sphere radius, the mesh size and the protection angle;
• the typical distance between down-conductors;
• the separation distance against dangerous sparking;
• the minimum length of the earth electrodes.

Other requirements are the same for all four classes: lightning equipotential bonding, the minimum thickness of the metal sheets that act as air-termination, the materials and their conditions of use, and the minimum cross-sections of air-terminations, down-conductors, electrodes and bonding conductors. The performance of each class is summarized in Annex B of Part 2.

How the class is chosen

The class is chosen with the risk assessment of IEC 62305-2. If the authority having jurisdiction, the insurer or the purchaser has already specified the LPS, you start from that specification; if not, the designer decides whether the structure needs an LPS by following the risk procedure.

For an existing structure the standard also asks for an economic criterion: building an LPS must be compared with other protection measures compliant with the standard that give the same level of protection at lower cost. That comparison is the cost-benefit analysis of Part 2.

Design together with the structure

The standard recommends coordinating the LPS design with the design and construction of the structure. In particular, the structure should make use of its metal parts as part of the LPS. For existing structures, the design of class and layout is adapted to the constraints that already exist.

This coordination saves conductors and shortens lengths, and forces early decisions on matters such as the location of the down-conductors, the connection points to the reinforcing steel and the routing of lines. The standard also asks that the LPS be designed and installed by trained and experienced personnel.

Design documentation

The documentation must contain all the information needed for a correct and complete installation. In practice it includes drawings with the position of the conductors and main components, the location of connections to the reinforcing steel, and the access conditions for parts that will later be concealed. That documentation is useful again at inspections: it is the baseline against which you check that the LPS still conforms to the design.

Common mistakes

• Thinking the LPS protects the equipment. It prevents physical damage; equipment is protected with SPDs and shielding (Part 4).
• Choosing the class on cost. The class comes from the calculated risk.
• Designing the LPS once the building is already built. Connections to the reinforcing steel and primary fixings are resolved before the concrete is poured.
• Forgetting inspection. The standard asks for inspections during construction, after installation, periodically and after any alteration or strike.

Guides in the same series

Each element of the system has its own guide: LPS air-termination, air-termination on tall structures and metal roofs, LPS down-conductors, LPS earth-termination, LPS materials and dimensions, reinforcing steel of the concrete, lightning equipotential bonding and separation distance s. The class is chosen with the risk (lightning risk management, protection levels and cost-benefit analysis) and the system is reviewed as set out in LPS inspection and maintenance.

What we do

We define the class with the risk assessment, design the LPS in coordination with the civil works and document the design. Tell us about your structure and we will quote the study.

Related guides

• IEC 62305: lightning protection, what each part covers
• Lightning protection levels: LPL I to IV
• LPS air-termination: rolling sphere, mesh and protection angle
• LPS down-conductors: number, spacing and step voltage
• Lightning shielding of substations

References

• IEC 62305-3:2010, Protection against lightning, Part 3: Physical damage to structures and life hazard: Cl. 1 (scope), 3.1 to 3.5 (definitions), 4.1 and 4.2 (LPS class and design), 5.1 (application and choice of the external system), Table 1 and Annex E.4 (LPS design).
• IEC 62305-2:2010, Annex B (performance of each class).

Frequently asked questions