To protect a building's electronics against lightning, IEC 62305-4 calls for a complete earthing system: the earth-termination system, which disperses the current into the ground, and a three-dimensional bonding network with a typical 5 m mesh that reduces potential differences and the magnetic field. Services crossing the boundary of each zone are bonded to bonding bars with the minimum cross-sections of Table 1.
Short answer. A building with electronics needs more than an earth electrode. IEC 62305-4 (clause 5) calls for a complete earthing system in two parts: the earth-termination system, which disperses the lightning current into the ground, and the bonding network, a three-dimensional web of conductors and metal parts with a typical 5 m mesh that prevents potential differences between items of equipment and reduces the magnetic field. Every conductive service entering a protection zone is bonded to that network at the boundary, directly or through an SPD.
The two parts of the system
Clause 5.1 defines the earthing system as the combination of:
- The earth-termination system, which disperses the lightning current into the ground.
- The bonding network, which minimises potential differences and can reduce the magnetic field.
Figure 5 of the standard shows a three-dimensional example: all the conductors drawn are bonded structural metal elements or bonding conductors, and some also serve to intercept, conduct and disperse the lightning current.
Earth-termination system
The earth electrode of the structure must comply with Part 3 of IEC 62305. Type A arrangements are horizontal or vertical electrodes outside the structure, connected to each down-conductor. Type B arrangements are a buried external ring or a foundation electrode in a closed loop, which can also be a mesh. Part 4 states:
- In structures with only power systems, type A may be used, although type B is preferable.
- In structures with electronic systems, type B is recommended.
- The earth ring around the structure, or the ring in the concrete of the foundation perimeter, should be integrated with a mesh under and around the structure with a typical mesh width of 5 m. This improves the performance of the system markedly. The reinforced-concrete floor of the basement can also serve, if it forms a well-defined mesh and is connected to the electrode roughly every 5 m.
To reduce potential differences between two internal systems that, in special cases, refer to separate earthing systems, the standard proposes parallel bonding cables on the same routes as the electrical cables, or cables in reinforced-concrete cable ducts forming a grid or in continuous metal conduits connected to both earthing systems, or shielded cables with a screen of adequate cross-section bonded to both systems at both ends.
The bonding network
Clause 5.3 calls for a low-impedance bonding network to avoid dangerous potential differences between all items of equipment in the inner zones. It is achieved with a meshed structure that integrates the conductive parts of the building or of the internal systems, and by bonding the metal parts and conductive services at the boundary of each zone.
It is conceived as a three-dimensional mesh with a typical width of 5 m, which requires multiple interconnections between the metal components inside and on the structure:
- Reinforcing steel in concrete.
- Rails of lifts and cranes.
- Metal roofs and facades.
- Metal window and door frames.
- Metal floor structures.
- Service pipes and cable trays.
Bonding bars (for example, ring bars or several bars at different levels) and the magnetic shields of the zones are integrated in the same way.
Figure 7 of the standard illustrates the use of reinforcing steel: a mesh of conductors at typical spacings of 5 m is laid over the reinforcement, connected to the reinforcement roughly every 1 m, and that mesh is bonded to the internal bonding bars, to the foundation electrode and, if there is one, to the earth ring. Figure 8 shows a structure with steel reinforcement in which the electrical and electronic equipment is connected to bonding bars that are in turn connected to the reinforcement, and the services enter at a common point.
How equipment is connected to the network
Cabinets, enclosures and racks of the internal systems, and the protective conductor (PE), are integrated into the network in one of two configurations (Figure 9):
- Configuration S (star). All metal components of the system are insulated from the earthing system and integrated into it only through a single bar that acts as the earthing reference point (ERP). All lines between items of equipment run in parallel and close to the bonding conductors, to avoid induction loops. It is used when the systems are in a relatively small zone and all lines enter at a single point.
- Configuration M (mesh). The metal components are not insulated: they are integrated into the network at multiple connection points. It is preferred when the system extends over wide areas or the whole structure, there are many lines between items of equipment and the lines enter at several points.
In complex systems the two can be combined (Figure 10): a star integrated into a mesh, or a mesh integrated through a star, together with a general mesh.
Bonding bars and bonding at each zone boundary
Bonding bars (clause 5.4) must be installed to bond the conductive services entering a zone, the PE conductor, the metal components of the internal systems and the magnetic shields of the zone. The installation rules are:
- The basis of every bond is a low-impedance network.
- Bars are connected to the earthing system by the shortest path.
- SPDs are installed with connections as short as possible to the bar and to the live conductors, to reduce the inductive voltage drop.
- Downstream of an SPD, mutual induction is minimised by reducing the loop area or by using shielded cables or conduits.
At the boundary of each zone (clause 5.5), all metal parts and services passing through it are bonded, with bars as close as possible to the point of entry. If possible, the services enter at the same place and at the same bar. If they enter at different places, each goes to a bar and the bars are bonded to one another, preferably with a ring bar. At the entrance to the zone, equipotential bonding SPDs are always required for the lines feeding the internal systems. Bonding of services entering LPZ 1 should be agreed with the operators of those services, because conflicting requirements may exist.
Minimum cross-sections from Table 1
| Bonding component | Material | Minimum cross-section |
|---|---|---|
| Bonding bars | Copper, copper-clad or galvanised steel | 50 mm² |
| Conductors from bars to the earthing system or to other bars (all or a significant part of the lightning current) | Cu / Al / Fe | 16 / 25 / 50 mm² |
| Conductors from internal metal installations to bars (partial current) | Cu / Al / Fe | 6 / 10 / 16 mm² |
| Earth conductors of class I SPDs | Cu | 16 mm² |
| Earth conductors of class II SPDs | Cu | 6 mm² |
| Earth conductors of class III SPDs and other SPDs (telecommunications and signal) | Cu | 1 mm² |
With other materials, the cross-section must give an equivalent resistance. In some countries smaller cross-sections may be used if they meet the thermal and mechanical requirements of Annex D of IEC 62305-1. Clamps are sized with the lightning current of the protection level and with the current sharing of Part 3.
Relationship to fault earthing
The bonding network is the building’s lightning earth, at frequencies up to several MHz. It does not replace the design of the 50/60 Hz fault earthing. For the substation earth grid and step and touch voltages, see earth grid and step and touch voltage. Measurement of the earthing system is explained in earth resistance, and to estimate rods and grids you can use our ground resistance tool.
Guides in the same series
The network is part of the set of measures in SPM against LEMP. Type A and type B earthing is covered in LPS earthing, the use of foundation steel in reinforcing steel in concrete and cable routing in magnetic shielding and cable routing.
When to call us
The bonding network is defined together with the earthing, the cables and the location of equipment. If you are designing a control room, a plant or an extension, or need to review the earthing of an installation with surge-related failures, our engineering design and studies team can support you on the electrical side. We carry out field measurements, in Colombia or elsewhere on request, as part of our testing and commissioning.
Related guides
- IEC 62305: lightning protection, what each part covers
- LPS earthing: type A and type B electrodes
- Reinforcing steel in concrete as down-conductor and electrode
- SPM against LEMP: what they are and how they are managed
- Earth grid: what it is, design and testing
References
- IEC 62305-4:2010, Protection against lightning, Part 4: Electrical and electronic systems within structures: clause 5 (earthing and bonding), Figures 5 to 10 and Table 1.
- IEC 62305-3:2010, clause 5.4 (earth-termination system, types A and B).
- IEC 62305-1:2010, Annex D.
Frequently asked questions
What is the equipotential bonding network and how does it differ from the earth electrode?
The earth electrode disperses the lightning current into the ground. The bonding network is the set of metal parts of the structure and of the internal systems connected to one another so that dangerous potential differences do not appear and to reduce the magnetic field. Together they form the three-dimensional earthing system described in the standard.
Can the reinforcing steel in concrete be used as part of the bonding network?
Yes. IEC 62305-4 includes reinforcing steel, lift rails, cranes, metal roofs and facades, door and window frames, metal floors, pipes and cable trays among the elements that are interconnected, with multiple connections. When a mesh of conductors is laid over the reinforcement, it is connected to the reinforcement at typical distances of 1 m.
What minimum cross-section must bonding bars and conductors have?
Table 1 of IEC 62305-4 sets, among others, 50 mm² for copper or steel bonding bars, and for conductors carrying all or a significant part of the current to earth or to another bar, 16 mm² in copper, 25 mm² in aluminium and 50 mm² in steel. Those carrying partial current from internal metal installations to a bar: 6, 10 and 16 mm².