Lightning Rod / Air Terminal Symbol
Definition: The Lightning Rod / Air Terminal symbol represents a pointed metal conductor elevated above a structure to intercept lightning strikes and conduct the discharge current safely to earth via a down conductor, shown in lightning protection system (LPS) drawings as a vertical mast or spike with two pins — Air Terminal at the apex and Down Conductor at the base — per IEC 62305-3 (lightning protection systems, physical protection).
Also known as: air terminal, lightning conductor, Franklin rod, lightning finial, strike termination device, captive terminal.
What the Lightning Rod / Air Terminal symbol means
The lightning rod (air terminal) symbol identifies the point in a lightning protection system (LPS) where the lightning discharge is intended to be captured and directed away from a protected structure. The air terminal presents the point of lowest impedance to a descending stepped leader, attracting the stroke to itself rather than to the roof or building fabric. From the air terminal, a low-impedance down conductor routes the discharge current to the earth electrode system.
In LPS engineering drawings the lightning rod symbol sits at the apex of a mast or roof-mounted conductor network. The symbol's two pins — Air Terminal and Down Conductor — represent the physically separate upper and lower sections of the lightning rod assembly that must both be shown and connected in the drawing to verify the continuity of the discharge path from the interception point to the grounding electrode.
How to identify the Lightning Rod / Air Terminal symbol
In schematic and site-plan drawings the lightning rod (air terminal) symbol is drawn as a vertical line or thin mast with a pointed or rounded tip at the top (the Air Terminal pin) and a connection point at the base (the Down Conductor pin). Some representations add small horizontal lines at the base to indicate the mounting bracket and down-conductor bonding clamp. In architectural lightning protection drawings the symbol is overlaid on a building plan to show the protection zone radius (rolling sphere method per IEC 62305-3). The symbol is distinct from the lightning arrester symbol, which appears in single-line electrical diagrams for electronic surge protection.
Function in a circuit
A lightning rod (air terminal) functions as the interception element of an external lightning protection system. The sharp point concentrates the electric field during a thunderstorm, making upward leader formation more probable from the rod tip than from adjacent flat or curved surfaces of the structure. When the downward stepped leader from the cloud connects with an upward leader from the rod tip, the main discharge (return stroke, peak current 1–200 kA, typical 30 kA) flows through the air terminal, down the conductor, and into the earth electrode system. The low-impedance path provided by the LPS prevents side-flash damage to the building structure and reduces the hazardous step and touch voltages in the surrounding soil.
Standards: IEC vs ANSI
| IEC 60617 | IEC 62305-3:2010 (Protection against lightning — Part 3: Physical damage to structures and life hazard) governs the design, selection, and installation of air terminals (lightning rods) in external lightning protection systems. The standard defines four LPS protection levels (I–IV) with corresponding rolling sphere radii (20, 30, 45, 60 m) that determine the required air terminal placement. IEC 62305-1 provides general principles. |
|---|---|
| ANSI/IEEE 315 | NFPA 780 (Standard for the Installation of Lightning Protection Systems, published by the National Fire Protection Association) is the primary North American standard for lightning rod installation. IEEE 998 (Guide for Direct Lightning Stroke Shielding of Substations) covers air terminal placement in electrical substations. ANSI Y32.2 / IEEE 315 does not define a specific schematic symbol for the lightning rod in circuit diagrams. |
| Key difference | IEC 62305-3 and NFPA 780 both accept pointed solid metal conductors (copper or aluminium) as air terminals; the main difference is that NFPA 780 specifies minimum rod dimensions (solid copper ≥ 12.7 mm diameter, 254 mm above the highest point) while IEC 62305-3 defines protection levels by rolling sphere radius rather than prescriptive rod geometry. |
Terminals / pins
| Pin | Name |
|---|---|
| tip | Air Terminal |
| down | Down Conductor |
Typical values
Air terminal material: copper (minimum cross-section 50 mm²), aluminium (70 mm²), or galvanised steel (50 mm²) per IEC 62305-3; Minimum height above protected surface: 300 mm (NFPA 780), variable per IEC rolling sphere; Down conductor minimum cross-section: 50 mm² copper, 70 mm² aluminium; Earth electrode resistance: < 10 Ω (general practice), < 1 Ω (LPS class I); Peak lightning current (maximum design): 200 kA (LPS class I), 100 kA (LPS class IV).
Where the Lightning Rod / Air Terminal symbol is used
- Rooftop lightning protection systems on residential, commercial, and industrial buildings to intercept direct strikes
- Electrical substation shielding: tall masts with Franklin rods protecting open-air switchgear and transformer yards from direct strokes
- Communications towers and antenna masts: air terminals at the apex of broadcast towers to protect structural steel and transmission lines
- Petro-chemical storage tanks and refineries: catenary wire and mast air terminal systems protecting flammable-liquid storage areas
- Historic buildings and heritage structures where an aesthetically minimal interception system is required
- Wind turbine nacelles: air terminals integrated into blade and nacelle tips routed through the tower to the foundation earth electrode
Example
In an LPS design drawing for a two-storey residential building, two copper air terminal masts (labelled AT1 and AT2, each 500 mm above the ridge line) are shown at both ends of the roof ridge; the Down Conductor pin of each rod connects to a 50 mm² bare copper down conductor routed down the exterior wall to a ring earth electrode buried 0.5 m deep around the building perimeter, with an equipotential bonding bar at foundation level linking the LPS to the main earthing terminal of the electrical installation per IEC 62305-4.
Key facts
- A lightning rod (air terminal) is the interception component of an external lightning protection system (LPS); it intercepts the lightning discharge and routes the current through a down conductor to the earth electrode, protecting the building structure from direct stroke damage.
- The lightning rod symbol has two pins in schematic drawings: Air Terminal (the tip, connected to the lightning interception point) and Down Conductor (the base, connected to the down-conductor network leading to earth).
- IEC 62305-3:2010 is the international standard governing air terminal design, placement, and installation; NFPA 780 is the equivalent North American standard.
- The protection zone of a single air terminal is determined by the rolling sphere method: a sphere of radius 20 m (LPS class I) to 60 m (LPS class IV) is notionally rolled over the structure; any surface touched by the sphere requires an air terminal at that point.
- Air terminals must be fabricated from corrosion-resistant materials: copper (50 mm² minimum cross-section), aluminium (70 mm²), or hot-dip galvanised steel (50 mm²) per IEC 62305-3.
- A lightning rod does not prevent a lightning strike — it ensures the strike is directed to a safe, controlled discharge path, preventing uncontrolled side-flash, fire ignition, and structural damage.
- The lightning rod is distinct from a lightning arrester (surge arrester): the rod is a physical conductor intercepting direct strokes; the arrester is an electronic protective device in a distribution circuit clamping transient overvoltages.
Diagrams that use this symbol
Frequently asked questions
What does the lightning rod symbol look like in a drawing?
The lightning rod (air terminal) symbol is drawn as a vertical line or mast with a pointed tip at the top (the Air Terminal pin) and a connection stub at the base (the Down Conductor pin). In site-plan LPS drawings it is overlaid on building outlines to show interception coverage; in elevation drawings it appears as a vertical spike above the roof line connected downward to the down conductor.
What is the difference between a lightning rod and a lightning arrester?
A lightning rod (air terminal) is a physical metal conductor mounted above a structure to intercept direct lightning strikes and route the current to earth via a down conductor; it is part of the external lightning protection system. A lightning arrester (surge arrester) is an electronic device installed in an electrical distribution circuit to clamp voltage transients on conductors; it protects electrical equipment from indirect lightning effects and switching surges, not from direct physical strikes.
What does the lightning rod symbol mean in an LPS drawing?
The lightning rod symbol marks the intended point of lightning strike interception on a building or structure. It indicates that this elevated conductor will attract and capture the lightning discharge, preventing it from striking the building fabric, and that the discharge current will be safely conducted to earth through the connected down-conductor and earth-electrode system.
What standard governs lightning rod installation?
IEC 62305-3:2010 (Protection against lightning — Part 3: Physical damage to structures and life hazard) is the international standard for air terminal selection, placement, and installation. NFPA 780 is the equivalent North American standard. Both use the rolling sphere method to determine required air terminal positions.
How does a lightning rod protect a building?
A lightning rod works by presenting a pointed, elevated metal conductor that concentrates the local electric field during a storm, making upward leader formation more probable from the rod tip than from the surrounding building structure. When the downward stepped leader connects with the rod's upward leader, the main lightning return stroke (up to 200 kA peak) flows through the rod, down conductor, and earth electrode rather than through the building. This controlled path prevents fire, structural damage, and dangerous voltages inside the building.
What material is a lightning rod made from?
IEC 62305-3 requires lightning rod air terminals to be made from copper (minimum 50 mm² cross-section), aluminium (70 mm²), or hot-dip galvanised steel (50 mm²). Copper is the preferred material for its high conductivity and corrosion resistance; aluminium is used where weight or cost is a factor. Stainless steel rods are also used in corrosive environments such as coastal installations.
Does a lightning rod attract more lightning to a building?
A lightning rod does not increase the number of lightning strikes a building receives — the strike probability is determined by the building's height and location. The rod ensures that when a strike does occur in its vicinity, the discharge is directed to the rod rather than to an unprotected part of the structure, providing a safe, low-impedance path to earth instead of an uncontrolled path through the building.
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