Systems and special applications · Complies to IEC 62305-3 §6.2 · IS 3043
Equipotential Bonding (Main and Supplementary)
Equipotential bonding connects every metal service, structural part and earthing system of a building to common bonding bars, so that no dangerous voltage can appear between them during a fault or a lightning strike. Main bonding is done where services enter; supplementary bonding is done locally.
When it applies
- Every building with an electrical installation
- Every building with a lightning protection system — lightning equipotential bonding is mandatory
- Medical locations, wet areas and plant rooms needing supplementary bonding
- Sites with several earthing systems that must be joined
When it does not
- Parts that must stay electrically separate, such as cathodically protected pipes — bond these through isolating spark gaps
- Live conductors, which are bonded through surge protective devices, not directly
- Hazardous-area bonding, which adds requirements from IEC 60079-14
How the system works
- DefineZones, bonding points and what must be evidenced are fixed on the drawings first.
- SelectEach component is chosen by its rating and the standard it is tested to.
- InstallConnections are kept short and every joint is made with a tested component.
- VerifyThe system is measured, labelled and recorded before handover.
Coordinated surge protection
Each device takes its share. The Class I device diverts lightning current at the service entry; Class II and III devices bring the remaining voltage below what the equipment can withstand.
Design parameters
The values a consultant or contractor needs to fix before the system is drawn.
| Parameter | Typical value or rule | Reference |
|---|---|---|
| Main earthing terminal | One bar near the service entry, connected to the earth electrode | IEC 60364-5-54 |
| Main protective bonding conductor | Minimum 6 mm² copper; sized from the largest protective conductor | IEC 60364-5-54 |
| Lightning bonding, bar to bar and to earth | Minimum 16 mm² copper, 25 mm² aluminium or 50 mm² steel | IEC 62305-3 Table 9 |
| Lightning bonding, internal metal to bar | Minimum 6 mm² copper, 10 mm² aluminium or 16 mm² steel | IEC 62305-3 Table 10 |
| Services to bond | Water, gas, heating, ducts, cable trays, lift rails, structural steel, cable screens | IEC 60364-4-41 · IEC 62305-3 |
| Isolating spark gaps | Where direct bonding is not permitted | IEC 62561-3 |
| Verification | Continuity between each bonded part and the bar | IEC 60364-6 |
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 |
|---|---|---|
| Earth Bars & Terminals | Main earthing terminal and local bonding bars | One main bar per service entry; one local bar per plant room and floor |
| Bonding Clamps & Leads | Clamps and leads for pipes, trays, ducts and steel | One per service per bonding point |
| Isolating Spark Gaps | Bond parts that must stay separate in normal operation | One per isolated service or separate earthing system |
| Earth Studs & Fixed Earthing Points | Fixed earthing points on structural steel and concrete | One per bonding level per riser |
From design to handover
- List every metal service and earthing system entering the building
- Position the main earthing terminal and the local bars
- Bond each service with a correctly sized conductor and a tested clamp
- Fit isolating spark gaps where direct bonding is not allowed
- Measure continuity and record every bonding point
Services for this solution
Industries that use it
What the standards require
Checked against the current editions held in the LPES standards library.
- IEC 62305-4:2024
- A coordinated SPD system protects against conducted surges only. Radiated fields need shielding and cable routing.
- IEC 62305-4:2024 Cl. 7
- Where an external LPS exists, a Class I (Type 1) SPD is required at the service entry.
- IEC 62305-4:2024
- Earthing conductor to the SPD: 16 mm² copper for Class I, 6 mm² for Class II and 1.5 mm² for Class III.
- IEC 62305-3:2024 Tables 9 and 10
- Bonding conductors: 16 mm² copper between bonding bars or to earth; 6 mm² copper from internal metal installations to a bar.
- IS 3043:2026 Cl. 6.3
- One interconnected, meshed earthing and bonding network, with all electrodes, the main earthing terminal and the LPS integrated.
- IEC 62305-3:2024 D.7.2
- Inspect visually every year and completely every three years; every year for sensitive systems 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 43(v) | Every building shall have protective equipotential bonding that interconnects exposed and extraneous conductive parts. |
| CEA Safety Regulations 2023 | Reg 43(ix) | An earthing system must carry the prospective earth-fault current within temperature limits, keep earth-fault loop impedance low enough to operate the protective device in time, stay mechanically strong and corrosion resistant for the life of the installation, and be tested for effective bonding before energisation. |
| CEA Safety Regulations 2023 | Reg 43(vii) | Frames of generators and motors, transformer and control-gear metalwork and consuming apparatus above 250 V up to 650 V are earthed by two separate and distinct connections. Common guidance reads this as two connections to the earthing system, not two isolated electrodes in the soil. |
| IS 3043:2026 | Clause 11.1 | For industrial and commercial premises a global earthing system is recommended, and it is mandatory where a high-voltage substation stands on the premises. Low-voltage distribution is TN-S, with the source neutral earthed once. |
| 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. |
| IS 732:2019 | 5.4.2, 5.4.3, 5.4.4 | Earthing arrangements, protective conductor sizing by table or the adiabatic method, and main and supplementary protective bonding are designed to these clauses, with the main earthing terminal as the hub. |
| IS 3043:2026 | Clause 11.2 | Medical locations use TN-S, with a medical IT system in Group 2. Protective conductor impedance from the bonding bar to a socket is at most 0.2 Ω and from an extraneous part to the bonding bar at most 0.1 Ω. Dedicated clean earths are not permitted. |
| IEC 60364-6:2016 | 6.4.3 | Initial verification tests follow a set order: continuity of conductors, insulation resistance, polarity, automatic disconnection of supply, additional protection, phase sequence and functional tests, using IEC 61557 instruments. |
What we ask for before design starts
- Single-line diagram with sources, operating modes and earthing arrangement
- Equipment list with impulse withstand levels
- Cable routes and every point where services enter
- Fault levels and protection clearing times
- Existing earthing and lightning protection records
- Site layout with fences, pipes, rails and neighbouring installations
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 62305-4:2024 · LPES Knowledge Hub H-13
Bonding bars what must be bonded and installation rules
Every bonding scheme in Clause 5 - the meshed structural network (H-11), the star and mesh integration of internal systems (H-12), the boundary bonding of an LPZ (H-14) - comes down, physically, to a bonding bar somewhere. IEC 62305-4:2024, 5.4 says what has to be bonded to those bars (all conductive services entering an LPZ, the PE conductor, metal components of internal systems, and the magnetic shields of the LPZ) and gives five installation rules for making that bonding effective: a low-impedance bonding network as the basis, the shortest possible route from the bar to the earthing system, materials and dimensions per 5.6, the shortest possible SPD connections at the bar, and control of mutual induction on the protected (downstream) side of an SPD.
How it is applied
- For every LPZ, identify the four categories of item that need a bonding bar: conductive services entering the zone, the PE conductor, metal components of internal systems (per the S/M configuration chosen in H-12), and the LPZ's magnetic shields at the periphery and inside the structure.
- Place the bonding bar(s) so that the route from the bar to the earthing system is the shortest practicable one - this is an installation-rule requirement, not an afterthought, because route length adds impedance directly.
- Specify bar and connecting-conductor material and dimensions from H-15 (5.6, Table 1); do not select these independently of that table.
- Where SPDs are installed at or near the bar, route their connections - both to the bonding bar and to the live conductors - as short as possible, to minimise the inductive voltage drop the SPD itself will let through.
- On the protected (downstream) side of every SPD, control mutual induction either by minimising the loop area of the downstream wiring or by using shielded cables or cable ducts with the shield earthed at both ends.
- Where the bonding bar sits at an LPZ boundary rather than inside a zone, apply H-14's additional boundary-specific rules (closest possible position to the entrance point, common bonding bar or interconnected/ring bonding bar for multiple entries, equipotential bonding SPDs).
Common mistakes
- Bonding only the PE conductor and the cabinet metalwork to the bar, and treating the conductive service itself as "already earthed elsewhere": the clause lists conductive services entering the LPZ as their own category, bonded directly or via a suitable SPD, regardless of any other earthing the service may have.
- Running a long earthing conductor from the bonding bar to the earthing system because the physically nearest earthing point is inconvenient: the rule is the shortest possible route, and route length should drive the bar's placement, not the other way round.
- Selecting bonding-bar or connecting-conductor material and cross-section by convention or by what is in stock, instead of by 5.6/Table 1 (H-15).
- Installing an SPD with long leads to the bonding bar "for tidiness" or ease of access: this directly increases the inductive voltage drop the installation rule is written to avoid, and can make the SPD ineffective even though it is correctly rated.
- Ignoring the protected (downstream) side of an SPD: the clause explicitly asks for loop-area minimisation or shielded/earthed-at-both-ends cabling there too, not only on the bonding-bar side of the SPD.
Design checklist
- Has every conductive service entering the LPZ been bonded to a bonding bar, directly or via a suitable SPD?
- Is the PE conductor connected to the same bonding-bar discipline?
- Are the internal system's metal components (per the S/M configuration from H-12) bonded to the bar?
- Are the LPZ's magnetic shields, at the periphery and inside the structure, bonded to the bar?
- Is the bonding bar connected to the earthing system by the shortest possible route?
- Do the bar's material and dimensions, and those of its connecting conductors, comply with 5.6/Table 1 (H-15)?
- Are SPD connections to the bonding bar and to live conductors as short as possible?
- On the protected side of each SPD, has mutual induction been controlled by loop-area minimisation or by shielded/earthed-at-both-ends cabling?
IEC 62305-4:2024 · LPES Knowledge Hub H-14
Bonding at the boundary of an LPZ entry point, ring bonding bar and equipotential bonding SPDs
Clause 5.5 of IEC 62305-4:2024 requires that wherever a lightning protection zone (LPZ) boundary is defined, every metal part and every service (metal pipes, power lines, signal lines) that penetrates that boundary must be bonded, and it fixes exactly how: bonding is performed via bonding bars installed as close as possible to the entrance point; where possible all incoming services enter at one location and connect to one bonding bar; where services enter at different locations, each gets its own bonding bar and those bars are interconnected, with a ring bonding bar (ring conductor) recommended as the way to realise this; and equipotential bonding SPDs are always required at the LPZ entrance to bond incoming lines connected to internal systems within the LPZ back to the bonding bar, with an interconnected or extended LPZ offered as a way to reduce how many SPDs that requirement demands.
How it is applied
- The designer applies Clause 5.5 at the point where an LPZ's boundary is drawn on the design: every metal part and every service crossing that boundary line is identified, and for each one it is confirmed that it is bonded via a bonding bar installed as close as possible to its entrance point.
- If the design allows it, all such services are routed to enter the structure or the LPZ at a single, common location, so a single bonding bar can serve them all.
- If site conditions force services to enter at multiple, separated locations, each entry gets its own bonding bar, and those bars are then interconnected — the standard's recommended method for this is a ring bonding bar running around the boundary, to which each individual entry-point bar connects.
- Independently of how many entry points and bars there are, every incoming line that connects to an internal system inside the LPZ must have an equipotential bonding SPD installed at the entrance, bonding that line to the local bonding bar; the designer can reduce the total SPD count needed across the facility by choosing to interconnect several same-order LPZs into one joint LPZ, or by extending one LPZ outward to the next boundary using shielded cables or interconnected, boundary-bonded metal cable ducts, rather than treating every LPZ crossing as a fresh boundary requiring its own SPD set.
- Where any of the entering services belong to an external network provider (power or telecom utility) and the boundary in question is LPZ 1's own boundary, the standard requires that provider to be consulted, because the utility's own bonding/earthing rules can conflict with the structure's SPM design.
Common mistakes
- Treating "bonding exists somewhere in the building" as satisfying Clause 5.5, instead of confirming bonding specifically at every point where a metal part or service actually penetrates the LPZ boundary in question.
- Allowing services to enter an LPZ at multiple scattered locations without interconnecting the resulting bonding bars — Clause 5.5 requires interconnection (ring bonding bar recommended) whenever there is more than one entry point, not just a bonding bar at each.
- Omitting the equipotential bonding SPD on a line that "only" carries signal or telecom traffic rather than power, on the assumption SPDs are a power-only requirement — Clause 5.5 requires them on any incoming line connected to internal systems within the LPZ, regardless of service type.
- Forgetting to consult the external service-network provider for services entering LPZ 1, and then discovering a conflict between the structure's bonding design and the utility's own requirements after installation rather than at the design stage.
- Sizing the bonding bars and their conductors from general practice rather than from Clause 5.6's Table 1, or skipping Clause 5.4's general installation rules (shortest route to earth, minimising inductive voltage drop, minimising downstream mutual induction) when installing the boundary-specific bonding bars Clause 5.5 requires.
- Assuming an interconnected or extended LPZ automatically eliminates the need for entrance SPDs entirely, rather than reading the clause correctly: it "can reduce the number of SPDs required," not eliminate the requirement outright.
Design checklist
- Identify every LPZ boundary in the design and list every metal part and service that penetrates each one.
- Confirm a bonding bar exists as close as possible to each entrance point where penetrations occur.
- Where multiple services enter at one location, confirm they connect to a single shared bonding bar.
- Where services enter at multiple locations, confirm each has its own bonding bar and that all such bars are interconnected — a ring bonding bar around the boundary is the recommended method.
- Confirm every incoming line connected to an internal system inside the LPZ has an equipotential bonding SPD at the entrance, bonding it to the local bar.
- Where an interconnected or extended LPZ design is used (shielded cables or interconnected, boundary-bonded metal cable ducts), confirm it is documented as the basis for any reduction in entrance SPD count.
- For services entering LPZ 1, confirm the relevant service-network provider (power or telecom authority) has been consulted for conflicting requirements.
- Confirm the resulting bonding bars and conductors follow Clause 5.4's general installation rules and Clause 5.6's material/cross-section requirements.
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.
Maximum earth-fault loop impedance
- Trip current Ia
- –
- Design limit for Zs
- –
- Without the 2/3 margin
- –
Zs ≤ (2 ÷ 3) × U0 ÷ Ia for miniature circuit breakers to IS/IEC 60898, instantaneous trip. Every circuit and every source is checked, as CEA Reg 43(xii) requires.
What we find on audits
- Bonding done at the board but not where services enter the building
- Type 2 devices used where a Type 1 is required
- Isolating joints and insulated flanges left without spark gaps
- Monitoring fitted with nobody assigned to act on alarms
- Records kept by the contractor and never handed to the owner
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 62305-3 and -4:2024 (Ed. 3) · IEC 61643 series · IEC 60364-6:2016 · IS 3043:2026 · IS 732:2019 · IEC 62561 series · 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 the difference between earthing and bonding?
Earthing connects the installation to the ground. Bonding connects metal parts to each other so that they stay at the same potential. Earthing Audit
What size is a bonding conductor?
At least 6 mm² copper for protective bonding, and at least 16 mm² copper between bonding bars that may carry lightning current. Bonding Clamps & Leads
Where is the main earthing terminal placed?
Close to the point where services enter the building, with short connections to the earth electrode. Earth Bars & Terminals
When is an isolating spark gap used?
Where two parts must stay separate in normal operation but be connected during a lightning strike, such as cathodically protected pipes. Isolating Spark Gaps
How is bonding tested?
By a low-resistance continuity test between each bonded part and its bonding bar. Continuity & Bonding Test
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.
