Lightning equipotential bonding connects metal installations, internal systems and incoming lines to the LPS, directly or through SPDs, close to where they enter the structure and at an accessible bar connected to earth. The minimum cross-section of an incoming cable screen is calculated with SCMIN = (IF · ρC · LC · 10⁶) / UW.
Short answer
Lightning equipotential bonding prevents dangerous sparking by connecting to the LPS everything that could end up at a different potential: metal installations, internal systems and incoming lines. Each connection is direct where possible, through an SPD when direct bonding is not viable and through an isolating spark gap (ISG) when direct bonding is not permitted. It is made as close as possible to the point of entry, normally at an accessible bonding bar connected to the earth-termination. For screened cables, the standard gives the minimum screen cross-section with SCMIN = (IF · ρC · LC · 10⁶) / UW.
What is bonded, and how
The standard lists three forms of interconnection:
- Bonding conductors, where electrical continuity is not provided by natural elements.
- SPDs (surge protective devices), where direct bonding is not possible.
- Isolating spark gaps (ISG), where direct bonding is not permitted.
The way the bonding is made must be agreed with the operators of the telecommunications, power, gas and other networks involved, because conflicting requirements may arise. When bonding to internal systems, part of the lightning current may enter them, and that effect must be taken into account. SPDs are installed so that they can be inspected.
The bonding bar
In a non-isolated system, equipotential bonding is installed in the basement or roughly at ground level and wherever the insulation requirements are not met. The bonding conductors go to a bonding bar built and installed with easy access for inspection and connected to the earth-termination. In large structures (typically more than 20 m long) a ring bar or several interconnected bars can be used. In an isolated system, bonding is made at ground level only. Connections must be as direct and straight as possible.
Minimum cross-sections
The cross-sections hold for all four classes:
| Use of the conductor | Copper | Aluminum | Steel |
|---|---|---|---|
| Between bonding bars, or from the bar to the earth-termination | 16 mm² | 25 mm² | 50 mm² |
| From internal metal installations to the bar | 6 mm² | 10 mm² | 16 mm² |
Where bonding is needed but an LPS is not, the standard indicates that the earthing of the low-voltage electrical installation can be used.
Metal installations and external conductive parts
- Metal installations (pipes, ladders, elevator rails, ducts): bonded at the bar. If there are insulating sections in gas or water pipes inside the structure, they are bridged with ISGs by agreement with the provider.
- External conductive parts (pipes, metal cable screens, ducts): bonding is made as close as possible to the point of entry. The conductors must withstand the part IF of the lightning current that flows through them, evaluated with Annex E of Part 1. If direct bonding is not acceptable, ISGs are used with an impulse current rating not less than IF and a nominal sparkover voltage below the withstand level of the insulation between parts.
Internal systems and incoming lines
- If the cables of internal systems are screened or run in metal ducts, bonding the screens and ducts may be enough. This bonding alone does not prevent equipment failures from overvoltage; for that, see Part 4.
- If they are neither screened nor in metal ducts, they are bonded with SPDs. In TN systems, the PE and PEN conductors are bonded to the LPS directly or through SPDs.
- On incoming power and telecommunications lines, all conductors are bonded directly or through SPDs; live conductors only through SPDs. The SPDs are tested with an impulse current Iimp not less than the lightning current IF flowing in the line, and their protection level UP must be below the impulse withstand level of the insulation between parts.
- If protection of internal systems against overvoltage is required, a coordinated SPD system according to Part 4 is used.
The guide on transient overvoltages in the plant covers SPDs by zone and their coordination.
Minimum cross-section of the cable screen
If lines are screened or run in metal ducts, the standard allows bonding only the screens and ducts, and not the conductors, provided the cross-section SC of the screen or duct is not less than a minimum SCMIN evaluated according to Annex B (normative). Screen bonding is made close to the point of entry to the structure.
The overvoltage between the live conductors and the screen can cause dangerous sparking because of the lightning current carried by the screen. It depends on the material and dimensions of the screen and on the length and position of the cable. Annex B gives:
SCMIN = (IF · ρC · LC · 10⁶) / UW (mm²)
where IF is the current flowing in the screen (kA), ρC the resistivity of the screen (Ω·m), LC the cable length (m) and UW the impulse withstand voltage of the electrical or electronic system the cable feeds (kV).
- If the screen is in contact with soil of resistivity ρ (Ω·m), LC is at most 8 × √ρ.
- If the screen is insulated from the soil or in air, LC is the distance between the structure and the nearest earthing point of the screen.
- The current is limited as follows: in cables with a copper screen, IF = 8 × SC (SC in mm², IF in kA); in unscreened cables, IF = 8 × n′ × S′C, with n′ the number of conductors and S′C the cross-section of each.
The standard reminds you to check that there is no unacceptable temperature rise in the line insulation when current flows in the screen or conductors. The values of UW and cable length come from the project; the calculation with your data is part of the study.
Common mistakes
- Bonding live conductors directly to the bar. Only with SPDs.
- Bonding far from the point of entry. The bond must be close to the entry of each service.
- Undersized screen. Sparking occurs between screen and conductors.
- Inaccessible bar. The standard requires easy access for inspection.
- Forgetting the service operator. How gas pipes and telecommunications lines are bonded is agreed with their operator.
Guides in the same series
The cross-sections of bonding conductors are in LPS materials and dimensions. The distance the LPS must keep from metal installations is calculated in separation distance s, and the complete network inside the building is described in bonding network and earthing in 3D.
What we do
We define the bonding bars, the SPDs per service and the cross-section of your cable screens with your project data. We will quote the study.
Related guides
- IEC 62305: lightning protection, what each part covers
- LPS materials: air-terminations, down-conductors and electrodes
- Separation distance s against dangerous sparking
- Bonding network and earthing system in 3D
- Transient overvoltages in the plant
References
- IEC 62305-3:2010, Part 3: Physical damage to structures and life hazard: Cl. 6.1 and 6.2 (lightning equipotential bonding), Tables 8 and 9, Annex B (minimum cable screen cross-section, Eq. B.1 and Table B.1) and Annex E.6.2.
- IEC 62305-4:2010, Cl. 7 (coordinated SPD system) and IEC 62305-1:2010, Annex E (part IF of the current), cited by Part 3.
Frequently asked questions
Can a live conductor be connected directly to the bonding bar?
No. The standard states that the live conductors of a line are bonded to the bar only through an SPD. The PE or PEN earthing conductors of TN systems can be bonded directly or through an SPD.
Must metal pipes entering the building be bonded?
Yes, by direct bonding if acceptable or, if not, with isolating spark gaps (ISG) designed for that function. On gas or water pipes with insulating sections, bridging is agreed with the service provider.
What data does the cable screen formula need?
The current flowing in the screen IF (in kA), the screen resistivity ρC (in Ω·m), the cable length LC (in m) and the impulse withstand voltage UW of the system the cable feeds (in kV). With them you get the minimum screen cross-section in mm².