Lightning protection by structure · Complies to IEC 62305-3 §6.3 · IEC 62561-8
Isolated Lightning Protection
An isolated lightning protection system keeps the air termination and down conductors at a calculated separation distance from the protected structure and its equipment, so that no lightning current and no spark enters it. It uses free-standing masts, insulating supports or insulated conductors.
When it applies
- Structures with explosive or flammable contents
- Rooftop equipment, antennas and electronics that must not carry lightning current
- Telecom shelters, radar and satellite installations
- Roofs where bonding the equipment would bring partial lightning current into the building
When it does not
- Buildings whose own steel frame or reinforcement is used as the LPS — isolation is not achievable
- Sites where the separation distance cannot be kept in air and insulated conductors are not accepted
- Low-risk structures where a bonded, non-isolated LPS meets the requirement at lower cost
How the system works
- InterceptAir termination is placed by rolling sphere, mesh or protective angle so that the strike lands on the system, not the structure.
- ConductSeveral straight, short down conductors share the current and keep voltages low.
- DisperseThe earth termination spreads the current into the ground with a low impulse impedance.
- EqualiseBonding and surge protection stop sparks between the system and what is inside the building.
Rolling sphere method
A sphere of the radius set by the protection level is rolled over the structure. Wherever it touches, a strike can land, so that point needs air termination. Everything it cannot touch is protected.
Design parameters
The values a consultant or contractor needs to fix before the system is drawn.
| Parameter | Typical value or rule | Reference |
|---|---|---|
| Protection level | LPL I–IV from the risk assessment; sets rolling-sphere radius 20 / 30 / 45 / 60 m | IEC 62305-2 · IEC 62305-3 |
| Separation distance | s = ki × (kc ÷ km) × l | IEC 62305-3 Cl. 6.3 |
| ki | 0.08 / 0.06 / 0.04 for LPL I / II / III–IV | IEC 62305-3 Table 11 |
| kc | 1 for one down conductor · 0.66 for two · 0.44 for three or more | IEC 62305-3 Table 13 |
| km | 1 for air · 0.5 for concrete, brick or wood | IEC 62305-3 Table 12 |
| Supports | Insulating spacers in glass-reinforced plastic, or free-standing masts on tripods or foundations | IEC TS 62561-8 |
| Insulated conductor | Used where s cannot be kept in air; rated for an equivalent separation distance | IEC TS 62561-8 |
Solution pack: component BOQ
Each line opens the LPES product category. Part codes and final quantities are confirmed on your quote from the drawings.
| Component | Role in the system | Quantity rule |
|---|---|---|
| Isolated Air-Termination | Insulating spacers, isolated rods and supports | From the layout; spacers about every 1 m of isolated conductor |
| Step Touch Conductor® — Insulated Conductors | Insulated conductor where the separation distance cannot be kept in air | Route length from air terminal to earth connection |
| Self-supporting Masts | Free-standing masts that place the protected volume over the structure | From the rolling-sphere layout |
| Tripods & Mast Stands | Stands and ballast for masts on roofs without penetration | One per mast, with ballast for the site wind speed |
From design to handover
- Fix the protection level and model the protected volume
- Calculate the separation distance at every approach point
- Place masts or isolated rods and route conductors on insulating supports
- Use insulated conductor where the distance cannot be kept in air
- Check wind loading, inspect, test and record the as-built distances
Services for this solution
Industries that use it
What the standards require
Checked against the current editions held in the LPES standards library.
- IEC 62305-3:2024 Cl. 5.2
- The rolling sphere is the general method. The protection angle only supplements it for simple shapes, and the mesh method is for flat surfaces.
- IEC 62305-3:2024 Cl. 5.2
- An unprotected metal roof fixture needs its own protection if it stands more than 0.3 m above the roof, covers more than 1.0 m² or is longer than 2.0 m.
- IEC 62305-3:2024 Cl. 5.2.3
- Side flashes are negligible up to 60 m. Above that, protect the top 20% of the height.
- IEC 62305-3:2024 Cl. 5.3
- An exposed corner can share a down conductor if both neighbours are within half the Table 5 spacing.
- IEC 62305-3:2024 Cl. 5.3.5
- Natural down conductors are proven by a four-lead continuity test at about 10 A: 0.2 Ω or less from top to earth network.
- IEC 62305-3:2024 D.7.2
- Inspect visually every year and completely, with electrical tests, every three years; every year for public, sensitive and explosion-risk structures.
Regulation and standards cross-check
What Indian regulation and the wider standards add to this solution. Paraphrased; confirm against the current official text, amendments and state notifications before relying on a clause in a tender or inspection.
| Standard | Clause | What it requires |
|---|---|---|
| CEA Safety Regulations 2023 | Reg 38(3)(vi) | Lightning protection is required for multi-storeyed buildings above 15 m and for listed premises, which include hospitals, hotels, academic buildings, industrial installations and other public buildings. The regulation names no design standard, so IS/IEC 62305 applies through Reg 14. |
| CEA Safety Regulations 2023 | Reg 14(2), Reg 2(1)(zze) | Relevant standards, including the National Electrical Code and the National Building Code, shall be followed. Indian Standards come first, then IEC, IEEE and European Norms in that order. Where a standard and the regulations differ, the regulations prevail. |
| IEC 61936-1:2021 | 8.6, Annex E | For outdoor high-voltage installations the analysis method and protection level against direct strokes are agreed with the user. Lightning rods and shield wires shall be earthed. Buildings and similar structures follow IEC 62305. |
| IS 3043:2026 | Clause 12.1.2.1, 13.7 | For lightning protection, a Type A earth electrode targets a resistance below 10 Ω, and the continuity of the lightning protection path is checked against 0.2 Ω. |
| IS 3043:2026 | Clause 6.3.1 | Separate or dedicated earth electrodes for individual equipment, neutrals, electronics or the lightning protection system shall not be followed. Electrodes are interconnected into one earthing system. |
| CEA Safety Regulations 2023 | Reg 43(v) | Every building shall have protective equipotential bonding that interconnects exposed and extraneous conductive parts. |
| CEA Safety Regulations 2023 | Reg 32(2), Reg 33(3) | Periodic inspection or self-certification is set by the Appropriate Government at intervals not longer than five years. Test instruments are calibrated by a Government-authorised or NABL-accredited laboratory. |
What we ask for before design starts
- Roof plan, elevations and sections with heights
- Use of the structure and its contents, for the risk assessment
- Roof and facade materials with sheet thickness
- Metal installations on the roof and services that enter the structure
- Soil resistivity and the existing earthing layout
- Single-line diagram for the surge protection design
Engineering depth: from the LPES Knowledge Hub
Topic studies written against the current standards. Open each part for the method, the mistakes to avoid and the checklist.
IEC 62305-3:2024 · LPES Knowledge Hub H-45
Separation distance s simplified approach and practical evaluation
Separation distance, given the symbol s, is the minimum distance that must be kept between a conductor carrying partial lightning current (an air-termination conductor or a down conductor) and any other conductive part of the structure, so that dangerous sparking cannot occur between them during a lightning strike. IEC 62305-3:2024, 6.3 gives two calculation routes: a general approach (6.3.1) that adds up the contribution of every conductor run and its own current-partitioning coefficient, and a simplified approach (6.3.2) that collapses the whole down-conductor arrangement to a single coefficient read off a table. This document covers the simplified route, the coefficient tables and figures that support it (Table 13 and Annex B), and the practical, on-site rules for evaluating and applying s on a real structure (Annex D, D.6.3).
How it is applied
- Fix ki from the class of LPS the risk assessment has selected for the structure (Table 11).
- Fix km from the material in the gap between the LPS conductor and the nearby conductive part or internal system (Table 12); use the manufacturer's value for an insulating stand-off material not listed, and the lower of several values if more than one insulating material is in series.
- Count the total number of down conductors n (including internal ones) and read kc from Table 13, checking the Type A/Type B earthing-resistance proviso; where a more precise value is wanted or the arrangement matches a wire air-termination system, a simple multiple down-conductor system, a ring-interconnected system, or a meshed system, take kc instead from the matching figure (B.1 to B.4).
- Measure the reference length l along the actual conductor route, from the point being checked to the nearest equipotential bonding point or the earth-termination system, keeping the route as straight as practicable — a longer, winding route increases l and therefore increases the required s. For a structure whose horizontal footprint does not exceed three times its height, the vertical height of the structure may be used for l.
- Calculate s from Equation (6) and compare it with the actual distance d available on site.
- If d ≥ s, no further action is needed at that point.
- If d < s, choose a remedy: increase d by re-routing the LPS conductor or the electrical conductor; use an electrically insulated LPS with insulating down conductors or stand-offs (5.5.4, IEC TS 62561-8) so an equivalent separation distance is achieved without the physical clearance; or bond the metal installation or internal-system conductor to the LPS at the nearest and, where the distance cannot be kept along the whole run, also at the furthest point from the reference bonding point, in accordance with 6.2.
- For an isolated metallic body near both a lightning conductor and a grounded body, apply the D.6.3 check directly: bond the isolated body only if s > d1 + d2.
- At altitude, apply the correction to s given by IEC 60071-2 before finalising the design.
- Remember the shielding exemption: for structures with a metal enclosure (metal facades, metal roofs) or an electrically continuous reinforced-concrete framework with a mesh width under 20 cm in the outer walls, floor and roof, a separation distance inside the structure is not required — small apertures such as doors and windows covering a few per cent of the facade do not materially affect this shielding.
Common mistakes
- Applying Table 13's kc = 1 value to an attached (non-isolated) LPS; kc = 1 is stated to apply only in the case of an isolated LPS with a single down conductor.
- Forgetting the Type A earthing proviso on Table 13: if individual earth-electrode resistances differ by more than a factor of 2, kc = 1 must be assumed, not the tabulated 0,66 or 0,44.
- Measuring l as a straight-line distance rather than the actual (and often longer) length along the conductor's real route; a winding roof-to-earth route increases l and is itself a design fault worth fixing at source by routing conductors as straight as possible.
- Treating the separation-distance check as unnecessary for the whole building once one metal-clad section has been assessed — the shielding exemption applies only where the metal enclosure or continuous reinforced-concrete mesh (under 20 cm mesh width) is continuous around the section in question, not by default across a mixed-construction structure.
- On larger structures, discovering late that the required s cannot physically be achieved between the LPS and nearby metal installations, and having to add bonding (with its associated partial lightning current and EMC consequences inside the structure) as an afterthought rather than a planned design choice.
- Bonding an installation only at the nearest reference point when the separation distance cannot be maintained along the whole run; the standard requires bonding at the furthest point of proximity as well, or a full re-route, or a continuous conductive shield bonded to the LPS at that furthest point.
- Omitting the altitude correction (IEC 60071-2) for LPS installations at higher elevations, which understates the required separation distance.
- Using the general-approach multi-term equation's per-conductor kc values (Figures B.3, B.4) together with the simplified Table 13 kc value in the same calculation — the two approaches are alternatives, not to be mixed on the same run.
- Applying the Figure B.2 approximation formula outside its stated validity range (n ≥ 4 down conductors, h and c between 3 m and 20 m).
Design checklist
- LPS class confirmed and ki taken from Table 11.
- Insulation material in every gap identified and km taken from Table 12 (or manufacturer's value, lowest of any series of materials).
- Number of down conductors n counted (including internal ones) and kc taken from Table 13, with the Type A/Type B earthing proviso checked.
- Where precision matters, kc re-derived from the matching Annex B figure (wire, multiple, ring-interconnected, or meshed system) instead of the flat Table 13 value.
- Reference length l measured along the actual conductor route to the nearest bonding/earth-termination point, with routes kept as straight as practicable; vertical-height shortcut applied only where the footprint rule is met.
- s calculated and compared with the actual distance d at every point of proximity along the run, not just at one location.
- Wherever d < s: a remedy chosen and recorded — re-route, electrically insulated LPS section, or bonding at nearest and furthest points.
- Isolated conductive bodies near a lightning conductor checked against the s > d1 + d2 bonding trigger.
- Altitude correction applied for elevated installations.
- Metal-enclosure/continuous-reinforced-concrete shielding exemption applied only where genuinely continuous, not assumed.
- Design communicated to the building owner wherever internal constructions prevent full assessment or connection.
IEC 62305-4:2024 · LPES Knowledge Hub H-43
Maintaining electrical insulation to the LPS separation distance and SPD test class
Where an external LPS shares a structure with metallic services that enter thebuilding — ducts, pipes, cables — the electromagnetic interference those servicescarry inside, and the amount of lightning current that flows into the structurealong them, can be reduced by keeping the LPS (air termination and/or downconductors) electrically insulated from those conductive paths. IEC 62305-4:2024Clause B.11.3, read together with Figure B.6, gives the retrofit-orientedstatement of this principle for existing structures with externally installedequipment (aerials, sensors, cameras — the subject of the immediately precedingClause B.11.2): insulation is achieved either by an isolated LPS, or by locallymaintaining the IEC 62305-3 separation distance s between the LPS and theconductive paths, and the outcome of that choice directly sets which SPD testclass is required — test class I where the separation distance cannot bemaintained, test class II or III at the equipment (with test class I still keptat the entry point) where it can.
How it is applied
- In practice, a designer working on an existing structure with externallyinstalled equipment checks, for eachpiece of external equipment and each conductive service feeding it, whether thephysical distance to the LPS's air termination and down conductors meets orexceeds the IEC 62305-3 separation distance s.
- If it does not, and an isolatedLPS is not practical to install (common in retrofit situations), the designeraccepts that the electrical line must be treated like a down conductor for SPDselection purposes and specifies test class I SPDs at both the equipment and thestructure entry point (Figure B.6 a).
- If the separation distance can bemaintained — either because the equipment/line is genuinely far enough from theLPS, or because an isolated LPS is installed — the designer can specify a lowertest class II or III SPD at the equipment itself, while still keeping a testclass I SPD at the point where the line enters the structure, because a strikeelsewhere on the LPS can still drive partial lightning current onto the line atthat entry point (Figure B.6 b).
- This decision then feeds directly into theAnnex C selection procedure for the actual SPD componentchoice, and into Annex E.4.2/Annex F for current-sharing estimates whereseparation is not maintained.
Common mistakes
- Assuming that maintaining separation distance at the equipment end automatically removes the need for a test class I SPD at the structure entry point — Figure B.6 b) keeps test class I at the entry regardless, because the LPS can still drive partial current onto the line there.
- Treating "isolated LPS" and "maintaining separation distance locally" as the same thing; an isolated LPS is a whole-system design choice made from the outset, while locally maintaining s is a point-by-point check that can be applied selectively to specific pieces of equipment in a retrofit.
- Specifying a lower test class SPD (II or III) at the equipment without first confirming, against IEC 62305-3, that the actual distance d genuinely meets or exceeds the calculated separation distance s at every relevant point, not just at the point where it looked closest by eye.
- Forgetting that this clause is specifically framed for existing structures (Annex B) with externally installed equipment; the general new-build separation-distance requirement between the LPS and internal/external conductive paths is set by IEC 62305-3 itself and is not restated here in full.
Design checklist
- Separation distance s calculated per IEC 62305-3 for every point where the LPS and a conductive service/equipment path could come close.
- Actual distance d measured or confirmed on site (or on drawings, for a new design) at each of those points.
- d ≥ s or d < s determination made and documented for each service/equipment, not assumed globally for the whole structure.
- Where d < s: line treated as a down conductor; test class I SPD specified at both the equipment and the structure entry point; B.11.4 cable-routing measures considered as a complementary reduction.
- Where d ≥ s: test class II or III SPD specified at the equipment; test class I SPD retained at the structure entry point regardless.
- Isolated-LPS option considered and costed as an alternative to point-by-point separation-distance maintenance, where retrofit constraints make the latter impractical.
- SPD test class decisions here cross-checked against the Annex C selection procedure for the actual Iimp/In component ratings.
Try the numbers
Protection level explorer
- Rolling-sphere radius
- 45 m
- Mesh size
- 15 × 15 m
- Down-conductor spacing
- 15 m
- Peak current covered
- 100 kA
- Coefficient ki
- 0.04
- Interception efficiency
- 90%
Values from IEC 62305-1 and IEC 62305-3. The level for your structure comes from the risk assessment.
What we find on audits
- Rooftop equipment standing outside the protected volume
- A single down conductor, or long loops around parapets
- Separation distance ignored, so current jumps to ducts and cable trays
- No test joints and no record of earth resistance
- External lightning protection installed with no surge protection on incoming lines
What you receive: the LPES Protection Passport
- As-built drawings of the installed system
- Risk assessment and design calculations
- Test results: earth resistance, continuity, SPD checks
- Component list with part codes and type-test references
- Inspection schedule for the protection level
Lightning to Load: one accountable partner
- Assess
- Design
- Specify
- Supply
- Implement
- Verify
- Assure
Standards basis
IEC 62305-1, -2 and -3:2024 (Ed. 3) · IS/IEC 62305 · IEC 62561 series · CEA Safety Regulations 2023 · IS 732:2019 · IEC 60364-6:2016 · IEC 61936-1:2021
Reviewed by LPES engineering, November 2026. Clause and table numbers follow IEC 62305:2024 (Edition 3) and IS 3043:2026.
Questions engineers ask
What is separation distance?
The minimum gap between the LPS and conductive parts of the structure that prevents a dangerous spark. It is calculated from the protection level, current sharing and the insulating material. Lightning Protection System Design
When is an isolated LPS required?
Where lightning current must be kept out of the structure: explosive areas, sensitive rooftop electronics and telecom installations. Hazardous Area Lightning and Static Protection
What if there is no room for the separation distance?
An insulated conductor with a tested equivalent separation distance replaces the air gap. Step Touch Conductor® — Insulated Conductors
How tall can a free-standing mast be?
Masts are available in a range of heights; height and base are chosen from the protected volume and the site wind speed. Self-supporting Masts
Can masts stand on a roof without drilling?
Yes, on tripods with concrete ballast sized for wind load. Tripods & Mast Stands
Get this solution quoted for your site
Send the drawings or the basic dimensions. LPES returns the design parameters, the BOQ with part codes and the price in one offer.
