Reinforcing steel is considered continuous if most of the joints between vertical and horizontal bars are welded or securely connected, and the joints of vertical bars are welded, clamped or overlapped by at least 20 diameters. Continuity is checked with a total resistance of no more than 0.2 Ω between the highest part and ground level.
Short answer
The reinforcing steel of a reinforced concrete building can act as a natural down-conductor and as a foundation electrode of the LPS, and it is one of the most effective solutions because it provides many parallel paths. IEC 62305-3 sets one condition: the steel must be electrically continuous, which is achieved with welded joints, clamps or an overlap of at least 20 diameters, and is checked with a total resistance of no more than 0.2 Ω between the highest part and ground level. If that continuity is not achieved or cannot be measured, an external down-conductor system is installed.
Why it works so well
When the current enters the bars, it spreads over a large number of parallel paths. The impedance of the resulting mesh is low and, with it, the voltage drop caused by the lightning current. The magnetic field generated is weak because of the low current density and because parallel currents generate fields that oppose each other; interference in neighboring internal conductors is smaller.
In addition, well-used reinforcing steel forms the potential equalization cage of the internal system and can act as electromagnetic shielding. In a room fully surrounded by reinforced concrete walls with continuity in line with the standard, the magnetic field near the walls is lower than in a room with conventional down-conductors. That is an advantage for control rooms and for halls with GIS switchgear, which hold a lot of electronics.
When it is considered continuous
The standard sets three elements:
- Most of the joints between vertical and horizontal bars must be welded or securely connected.
- Joints of vertical bars are made welded, clamped or overlapped at least 20 times their diameter, and tied or securely joined.
- For new structures, the designer or installer specifies the connections between reinforcement elements in cooperation with the builder and the civil engineer.
Clamps for this purpose must meet IEC 62561-1. In precast structures, continuity must be established between adjacent concrete units. In prestressed concrete, care must be taken over the risk of unacceptable mechanical effects, both from the current and from the connection to the LPS.
How it is checked
The standard requires an electrical test between the highest part and ground level for structures with reinforced concrete (including precast and prestressed). The total resistance must not exceed 0.2 Ω. The informative annex describes measuring between the air-termination system and an earth plate at ground level with suitable four-terminal equipment (two for measurement and two for sensing), with an injection current of the order of 10 A. If access is difficult, a dedicated bar can be left running from top to bottom to measure at each point, and the total resistance is calculated from that of the joints and that of the down-conductor.
Which connections are used
- Welding and clamps are the preferred methods for connections that carry lightning current.
- Welds between bars inside the concrete: at least 50 mm long, with crossing bars bent to run at least 70 mm in parallel before welding. A weld of a few millimeters at the crossing is not enough: such joints often break when the concrete is poured. Conventional and exothermic welding are accepted.
- Wire ties: not suitable for conducting lightning current (the wire can explode and damage the concrete). The standard estimates that at least one in three ties conducts, so that practically all bars end up interconnected, which is enough for additional equipotential bonding and electromagnetic compatibility conductors.
- Where welding is not permitted, clamps or additional conductors are used, tied or clamped to a large number of bars.
Additional conductors inside the concrete can be of steel, mild steel, galvanized steel, stainless steel, copper or copper-clad steel. A smooth round bar of at least 8 mm diameter is recommended, to tell it apart from ribbed bars.
As a down-conductor
Bars in concrete walls or columns and the structural steel frame can be natural down-conductors. A termination joint is provided at the roof to connect the air-termination system and, unless the foundation is the only electrode, termination joints to connect the earth-termination. When using a specific bar, the route must guarantee continuity over the full height. If a straight path from roof to ground cannot be guaranteed, dedicated conductors are added. And if there is doubt about the most direct route (for example, in existing buildings), an external down-conductor system is added.
When the structure uses its interconnected steel as down-conductors, no ring conductor is needed. If many connections to different floors with low-inductance paths are wanted, the standard recommends ring conductors inside or outside the concrete at each level, joined by vertical bars at intervals of no more than 10 m.
As a foundation electrode
The reinforcing steel of the foundation, the slab and the buried walls forms an excellent foundation electrode, provided the earth-termination requirements are met. With good concrete cover (at least 50 mm for foundation conductors) corrosion is limited. Care must be taken with the joints to avoid cracking the concrete, and it should be considered that the resistance may increase over the years. After the works are finished, it may be impossible to measure the resistance; an alternative is to install one or more reference electrodes near the structure to monitor the evolution of the system.
You can estimate the resistance of an electrode or a grid with our earthing resistance tool.
Expansion joints and precast units
- Expansion joints. With a lot of electronic equipment, bonding conductors are installed between sections, at intervals of no more than half the distance between down-conductors in the class table. For effective shielding, the bond can be flexible or sliding, at intervals between 1 m and half that distance.
- Precast units. Connection points are provided from the design of the piece, with a continuous bar between one connection point and the next; if that is not possible, a conductor is added. In general, one connection point at each corner of a precast panel.
Corrosion
Where bonding conductors pass through a concrete wall, protection against chemical corrosion is needed: copper and copper-clad steel carry no risk with a solid conductor or a specific connection point; stainless steel needs no measures. Galvanized steel should be avoided in coastal areas and with salt in the groundwater, and not used without careful analysis.
Documenting concealed works
Once construction is finished, it is almost impossible to know the layout of the steel. The standard asks that its layout be very well documented for lightning protection purposes, with drawings, descriptions and photographs taken during construction.
Guides in the same series
The conditions for natural down-conductors are completed in LPS down-conductors. As an electrode, the foundation steel fits into the arrangement of LPS earth-termination, with the cross-sections of LPS materials and dimensions. The protection levels that set the system class are explained in protection levels; inspection of concealed works, in LPS inspection and maintenance.
What we do
We coordinate the connection points and continuity with the structural designer, and support the continuity measurement before and after the pour. We will quote the study.
Related guides
- IEC 62305: lightning protection, what each part covers
- LPS down-conductors: number, spacing and step voltage
- LPS earth-termination: type A and type B electrodes
- LPS materials: air-terminations, down-conductors and electrodes
- Industrial grounding
References
- IEC 62305-3:2010, Part 3: Physical damage to structures and life hazard: Cl. 4.3 (continuity of steel in reinforced concrete), 5.3.5 (natural down-conductors), 5.4.4 (natural electrodes) and Annex E.4.3 (E.4.3.1 to E.4.3.12).
- IEC 62561-1: requirements for connection components.
Frequently asked questions
What resistance should be measured on reinforcing steel used as a down-conductor?
IEC 62305-3 asks you to determine electrical continuity with a test between the highest part and ground level, with a total resistance of no more than 0.2 Ω. If it is not reached, or measuring it is not practical, the steel must not be used as a natural down-conductor and an external down-conductor system is recommended.
Is wire tying between bars good enough?
For connections that carry lightning current, the standard indicates that welding and clamps are preferable. Wire ties are not suitable for this because the wire could explode and damage the concrete; they do work for additional equipotential bonding and electromagnetic compatibility conductors.
Is reinforced concrete allowed as part of the LPS in every country?
No. A note in the standard says that in several countries reinforced concrete is not permitted as part of the lightning protection system; check the applicable regulations before designing.