SPDA: What it is, how it works and why it's mandatory

1. What is SPDA

The SPDA (Lightning Protection System), popularly known as a "lightning rod", is a set of devices and measures designed to safely intercept, conduct and dissipate the energy of an atmospheric electrical discharge before it causes damage to the structure, facilities and, above all, people.

Unlike what many imagine, a modern lightning rod does not "attract" lightning — it offers a low impedance path for the lightning current to flow safely to the ground, preventing it from taking unpredictable paths such as metallic structures, pipes, electrical wiring or people.

2. How an atmospheric discharge works

An atmospheric discharge is the result of the accumulation of opposite electrical charges between the base of storm clouds and the earth's surface. When the potential difference exceeds the dielectric strength of the air (approximately 3 MV/m), air ionization occurs and a conductive channel is formed — the lightning.

A typical discharge carries from 10 to 200 kA of peak current, lasting microseconds. This energy, if not conducted in a controlled manner, causes:

• Fires due to thermal effect on the materials it passes through
• Explosion of high resistance materials (such as wet masonry)
• Overvoltages that destroy electronic equipment
• Step and touch voltages, fatal to people near the impact point

3. SPDA Components

A lightning protection system is composed of four main subsystems, according to NBR 5419:

3.1 Air termination system

It is the exposed part that intercepts the lightning. It can be composed of:

• Vertical rods (Franklin): metallic rods installed at the highest points of the building. Simple and effective for smaller buildings.
• Protective conductors (Faraday cage): mesh of horizontal conductors over the roof, indicated for large surfaces such as warehouses and shopping malls.
• Shield wire: horizontal conductor stretched over linear structures such as chimneys or bridges.

3.2 Down conductors

They conduct the current from the air termination to the grounding system. They must be installed on the exterior of the building, with the straightest possible path (tight bends increase impedance and the risk of sparks). The most common material is bare copper 50 mm² or galvanized steel 70 mm².

3.3 Grounding system

It is the element that safely dissipates the lightning current into the ground. It can be composed of vertical electrodes (copper-bonded steel rods), horizontal electrodes (cables buried in a mesh), or a combination of both. The grounding resistance must be measured periodically and ideally kept below 10 Ω.

3.4 Equipotentialization and SPDs

All metallic structures, pipes, rails and signal cables that enter the building must be connected to the equipotentialization busbar to avoid dangerous potential differences. SPDs (Surge Protective Devices) complement the system by protecting electronic equipment against overvoltages induced by lightning.

4. NBR 5419: the standard that governs SPDA

The ABNT NBR 5419 is the Brazilian standard that establishes the requirements for the protection of structures against atmospheric discharges. The current version, from 2015, was updated based on the international standard IEC 62305 and is divided into 4 parts:

5. Protection levels (PL)

The sizing of the SPDA begins with the risk analysis defined in NBR 5419-2. Based on this analysis, the project is classified into one of the four Protection Levels (PL):

The higher the protection level, the greater the design requirements — smaller spacing between interceptors, more down conductors, more elaborate grounding.

6. When SPDA is mandatory

NBR 5419 defines the mandatory nature of the SPDA based on a risk analysis that considers factors such as:

• Density of atmospheric discharges in the region (Ng — number of lightning strikes/km²/year)
• Dimensions and construction characteristics of the building
• Type of occupation (residential, commercial, industrial, hospital, etc.)
• Consequences of a discharge (risk of fire, explosion, loss of lives)
• Contents and activities carried out inside the structure

In addition, state and municipal legislation and sectoral standards frequently require the SPDA regardless of the risk analysis. Examples:

• Amusement parks and circuses (ABNT NBR 15486)
• Fuel stations
• Buildings more than 30 meters high (in many municipalities)
• Hospitals and health facilities
• Facilities that store flammable or explosive materials

7. The technical report and the ART

The design and execution of the SPDA must be carried out by an electrical engineer registered with CREA, with the mandatory issuance of an ART (Technical Responsibility Annotation) with the state CREA.

The technical report of the SPDA must contain:

• Descriptive memorandum with detailed risk analysis
• Plans and technical drawings of the installed system
• Specification of materials and equipment used
• Results of grounding resistance measurements
• ART signed by the responsible engineer
• Material compliance certificates

8. Maintenance and periodicity

The SPDA is not an install-and-forget system. NBR 5419 establishes mandatory periodic inspections:

9. Conclusion

The SPDA is an investment in safety that protects assets, equipment and, above all, lives. A system properly sized, installed and maintained by a qualified professional with an ART is the only way to guarantee real protection against atmospheric discharges.

If you do not know whether your building needs an SPDA, or if the installed system complies with the current NBR 5419, the correct path is to hire a risk analysis with an electrical engineer registered with CREA.