IS 2309 : 1989Code of practice for protection of buildings and structures from lightning

IS 2309:1989 is the code of practice for the protection of buildings and structures against lightning. It is invoked through NBC 2016 Part 8 Section 2, which makes lightning protection a design requirement for risk-prone structures: tall buildings, chimneys and towers, structures with explosives/flammables, hospitals and public buildings, structures housing electronics/data, and isolated tall structures in high-isokeraunic regions.

It is read with:

• IS 3043 — earthing (the earth-termination network is shared/coordinated)
• NBC 2016 Part 8 Sec 2 — the trigger and broad requirements
• IS/IEC 62305 (Parts 1–4) — the modern risk-based alternative many consultants now design to

IS 2309 builds protection from three coordinated parts:

• Air termination — vertical finials, horizontal conductors or a mesh on the roof, positioned so the structure lies within the protected zone (IS 2309 uses the protective-angle / cone-of-protection concept; modern practice supplements this with the rolling-sphere method of IEC 62305)
• Down conductors — at least two for any sizeable structure, distributed around the perimeter at the prescribed spacing (closer for taller structures), routed as directly as possible, with test/disconnecting joints
• Earth termination — a low-impedance earth, with the whole installation's resistance generally kept ≤ 10 Ω, bonded to the building's other earths to prevent dangerous potential differences

Reinforcement and structural steel may be used as 'natural' down conductors if electrically continuous. Equipotential bonding of all metalwork is essential to prevent side-flash.

Step 1 — is protection required? IS 2309 gives a weighted index from the structure's *use and contents*, *type of construction*, *height/isolation*, and the *local thunderstorm-day level*. Score each factor, combine to an overall risk index, and compare against the threshold: e.g., a 40 m RCC office with electronics in a 40-thunderstorm-day region scores above the threshold ⇒ protection required.

Step 2 — air termination: roof grid/finials so every point lies within the protective zone for the chosen height of air terminal.

Step 3 — down conductors: for a building of perimeter ≈ 120 m with the IS 2309 spacing rule for a 40 m structure (closer spacing with height), provide on the order of 6 down conductors evenly distributed (never fewer than two), each with a test joint ~ 2 m above ground.

Step 4 — earthing: an earth electrode per down conductor, all interconnected, system resistance ≤ 10 Ω, bonded to the electrical earth.

1. A single down conductor. Minimum is two, distributed; one conductor gives high impedance and no redundancy and is a frequent audit failure.

2. Earth resistance > 10 Ω / un-bonded earths. A separate, un-bonded lightning earth at a different potential causes side-flash into the building services during a strike.

3. Protecting the structure but not the electronics. IS 2309's classical system protects the *fabric*; sensitive equipment also needs surge protection (SPDs) and bonding — design for both, ideally via IEC 62305.

4. Not using the rebar as a natural conductor (or assuming it without proving continuity). Either provide dedicated conductors or *prove* electrical continuity of the reinforcement — don't assume it.

5. No maintenance/test regime. Joints corrode and earths drift; without periodic resistance testing the system silently degrades.

• IS 3043 — earthing (earth-termination design and resistance)
• NBC 2016 Part 8 Section 2 — electrical and allied installations / lightning protection requirement
• IS/IEC 62305 Parts 1–4 — protection against lightning (risk, physical damage, electrical systems)
• IS 8437 — guide for protection of structures against lightning (effects)
• IS 732 — electrical installations (bonding/coordination)
• IS 3070 — surge/lightning arresters (system-side)

IS 2309:1989 is conceptually dated relative to IS/IEC 62305, which introduced lightning-protection levels (LPL I–IV), the rolling-sphere placement method, and an integrated approach to surge protection of internal systems. In current practice many consultants assess risk and design to IEC 62305 and use IS 2309 as the locally-referenced code for NBC compliance — documenting the basis in the design report. That is acceptable and usually superior.

The drivers today are data centres, hospitals, tall residential and renewable installations, where the real exposure is not the building burning down but equipment damage from the strike's electromagnetic pulse — which classical IS 2309 barely addresses. Treat IS 2309 as the minimum structural-protection baseline and layer IEC 62305-style surge/bonding design on top for any electronics-critical facility.