NOTE In the case of pre-stressed concrete, consideration should be given to the consequences of the passage of lightning discharge currents, which may produce unacceptable mechanical stresses.
E.5.4.2 Types of earth electrode arrangements
E.5.4.2.1 Type A arrangement
The type A earth-termination system is suitable for low structures (for example family houses), existing structures or an LPS with rods or stretched wires or for an isolated LPS.
This type of arrangement comprises horizontal or vertical earth electrodes connected to each down-conductor.
Where there is a ring conductor, which interconnects the down-conductors, in contact with the soil the earth electrode arrangement is still classified as type A if the ring conductor is in contact with the soil for less than 80 % of its length.
In a type A arrangement the minimum number of earth electrodes should be one for each down-conductor and at least two for the whole LPS.
E.5.4.2.2 Type В arrangement
The type В earth-termination system is preferred for meshed air-termination systems and for LPS with several down-conductors.
This type of arrangement comprises either a ring earth electrode external to the structure, in contact with the soil for at least 80 % of its total length, or a foundation earth electrode.
For bare solid rock, only the type В earthing arrangement is recommended.
E.5.4.3 Construction
E.5.4.3.1 General
Earth-termination systems should perform the following tasks:
conduction of the lightning current into the earth;
equipotential bonding between the down-conductors;
potential control in the vicinity of conductive building walls.
The foundation earth electrodes and the type В ring-type earth electrodes meet all these requirements. Type A radial earth electrodes or deep-driven vertical earth electrodes do not meet these requirements with respect to equipotential bonding and potential control.
The structure foundations of interconnected steel-reinforced concrete should be used as foundation earth electrodes. They exhibit very low earthing resistance and perform an excellent equipotentialization reference. When this is not possible, an earth-termination system, preferably a type В ring earth electrode, should be installed around the structure.
E.5.4.3.2 Foundation earth electrodes
A foundation earth electrode, which conforms to 5.4.4, comprises conductors, which are installed in the foundation of the structure below ground. The length of additional earth electrodes should be determined using the diagram in Figure 3.
Foundation earth electrodes are installed in concrete. They have the advantage that, if the concrete is of adequate construction and covers the foundation earth electrode by at least 50 mm, they are reasonably protected against corrosion. It should also be remembered that reinforcing steel rods in concrete generate the same magnitude of galvanic potential as copper conductors in soil. This offers a good engineering solution to the design of earthtermination systems for reinforced concrete structures (see E.4.3).
Metals used for earth electrodes should conform to the materials listed in Table 7, and the behaviour of the metal with respect to corrosion in the soil should always be taken into account. Some guidance is given in 5.6. When guidance for particular soils is not available, the experience with earth-termination systems in neighbouring plants, with soil exhibiting similar chemical properties and consistency, should be ascertained. When the trenches for earth electrodes are refilled, care should be taken that no fly ash, lumps of coal or building rubble is in direct contact with the earth electrode.
A further problem arises from electrochemical corrosion due to galvanic currents. Steel in concrete has approximately the same galvanic potential in the electrochemical series as copper in soil. Therefore, when steel in concrete is connected to steel in soil, a driving galvanic voltage of approximately 1 V causes a corrosion current to flow through the soil and the wet concrete and dissolve steel in soil.Earth electrodes in soil should use copper, copper coated steel or stainless steel conductors where these are connected to steel in concrete.
At the perimeter of a structure, a metal conductor, in accordance with Table 7, or a galvanized steel strip, should be installed in the strip foundation and be taken upwards with connection leads to the designated terminal points of the lightning down-conductor test joints.
Upward routing of the conductors connected to the down-conductors can be performed on the brickwork, within the plaster or within the wall. The steel connection leads installed within the wall may penetrate the asphalt-saturated paper normally used between the foundation and the brick wall. Piercing of the humidity barrier at this point generally presents no problem.
The water-insulating layer often inserted below the structure foundation to reduce the humidity in basement floors provides consistent electrical isolation. The earth electrode should be installed under the foundation in the sub-concrete. An agreement should be obtained with the builder for the design of the earth-termination system.
Where the groundwater level is high, the foundation of the structure should be isolated from subsoil water. A sealing waterproof layer should be applied to the outer surface of the foundation, which also provides electrical isolation. The usual practice in establishing such a waterproof foundation is to pour a clean layer of concrete approximately 10 cm to 15 cm in depth on the bottom of the foundation pit, onto which the isolation, and later the concrete foundation, is laid.
A foundation earth electrode consisting of a network of mesh size not exceeding 10 m shall be installed in the clean concrete layer at the bottom of the foundation pit.
A conductor in accordance with Table 7 shall connect the meshed earth-termination with the reinforcement in the foundation, the ring earth electrodes, and the down-conductors external to the damp-proof membrane. Where permitted, pressure-waterproof bushings may be used to penetrate the insulation.
When penetration of the conductor through the isolation layer is not permitted by the building contractor, connections should be made to the earth-termination outside the structure.
Figure E.40 shows three different examples of how to install foundation earth electrodes on a structure with waterproofed foundations
.
Figure Е.40а - Isolated foundation with foundation earth Figure E.40b - Isolated foundation with earthelectrode in the non-reinforced concrete layer below the termination conductor partly passing through the soil bitumen insulation
Figure E.40c - Connection from the foundation earth electrode
to the steel reinforcement passing through the damp proof membrane
Key
down-conductor
test joint
bonding conductor to the internal LPS
non-reinforced layer of concrete
connecting conductor of the LPS
foundation earth electrode
damp proof membrane, watertight insulating layer
connecting conductor between steel reinforcement and the test joint
steel reinforcement in concrete
watertight bushing through the damp proof membrane
NOTE Permission from the structure constructor is necessary.
Figure E.40 - Construction of foundation earth ring for structures
of different foundation design
Several solutions of an adequate connection of the earth-termination on structures with isolated foundation are also illustrated.
Figures E.40a and E.40b show connections external to the insulation, so that the insulation is not damaged; Figure E.40c shows a watertight bushing through the insulation to avoid compromising the integrity of the damp-proof membrane.
E.5.4.3.3 Type A - Radial and vertical earth electrodes
Radial earth electrodes should be connected to the lower ends of the down-conductors by using test joints. Radial earth electrodes may be terminated by vertical earth electrodes if appropriate.
Each down-conductor should be provided with an earth electrode.
Figure E.41 shows examples of type A earth electrodes where Figure E.41a shows how a lightning conductor in accordance with Table 7 is pushed into the soil using special driving rods. This earthing technique has several practical advantages and avoids the use of clamps and joints in the soil. Sloped or vertical earth electrodes are generally hammered in.
/EC 2728/10
Key
short upper-most driving rod
earthing conductor
soil
short driving rods
driving steel dart
NOTE 1 A continuous wire conductor is driven into the soil by means of short driving rods. The electrical continuity of the earth electrode conductor is of great advantage; using this technique, no joints are introduced into the earth electrode conductor. Short driving rod segments are also easy to handle.
NOTE 2 The short upper-most driving rod may be removed.
NOTE 3 The uppermost part of the earthing conductor may have an insulating jacket.
Figure E.41a - Example of a type A earthing arrangement with a vertical conductor type electrode
ІЕС 2729/10
Key
extensible earth rod
rod coupling
soil
conductor to rod clamp
earthing conductor
Figure E.41b - Example of a type A earthing arrangement with a vertical rod type electrode
Figure E.41 - Two examples of vertical electrodes in type A earthing arrangement
There are also other types of vertical electrodes. It is essential to ensure a permanent conducting connection along the whole length of the electrode during the service life of the LPS.
During installation it is advantageous to measure the earthing resistance regularly. The driving may be interrupted as soon as the earthing resistance stops decreasing. Additional electrodes can then be installed in more suitable locations.
The earth electrode should have sufficient separation from existing cables and metal pipes in the earth, and due allowance should be made for the earth electrode departing from its intended position during driving. The separation distance depends on the electrical impulse strength and resistivity of the soil and the current in the electrode.
In the type A arrangement, vertical earth electrodes are more cost-effective and give more stable earthing resistances in most soils than horizontal electrodes.
In some cases it may be necessary to install the earth electrodes inside the structure, for example in a basement or cellar.
NOTE Special care should be taken to control step voltages by taking equipotentialization measures according to Clause 8.
If there is a danger of an increase in resistance near to the surface (e.g. through drying out), it is often necessary to employ deep-driven earth electrodes of greater length.
Radial earth electrodes should be installed at a depth of 0,5 m or deeper. A deeper electrode ensures that in countries in which low temperatures occur during the winter, the earth electrode is not situated in frozen soil (which exhibits extremely low conductivity). An additional benefit is that deeper earth electrodes give a reduction of the potential differences at the earth surface and thus lower step voltages reducing the danger to living creatures on the earth surface. Vertical electrodes are preferred to achieve a seasonally-stable earthing resistance.
When the type A earthing arrangement is provided, the necessary potential equalization for all electrodes is achieved by means of equipotential bonding conductors and bonding bars .
E.5.4.3.4 Type В - Ring earth electrodes
For structures using isolating material such as brickwork or wood with no steel-reinforced foundation, a type В earth-termination should be installed conforming to 5.4.2.2. Alternatively a type A arrangement incorporating equipotential bonding conductors may be used. In order to reduce the equivalent earth resistance, the type В earthing arrangement may be improved, if necessary, by adding vertical earth electrodes, or radial earth electrodes conforming to 5.4.2.2. Figure 3 gives the requirements regarding the minimum length of earth electrodes.
The clearance and depth for a type В earth electrode, as mentioned in 5.4.3, are optimal in normal soil conditions for the protection of persons in the vicinity of the structure. In countries with low winter temperatures, the appropriate depth of earth electrodes should be considered.
Type В earth electrodes also perform the function of potential equalization between the downconductors at ground level, since the various down-conductors give different potentials due to the unequal distribution of lightning currents due to variations in the earth resistance and different lengths in the above ground conductor current paths. The different potentials result in a flow of equalizing currents through the ring earth electrode, so that the maximum rise in potential is reduced and the equipotential bonding systems connected to it within the structure are brought to approximately the same potential.
Where structures belonging to different owners are built closely to each other, it is often not possible to install a ring earth electrode that will fully surround the structure. In this case the efficiency of the earth-termination system is somewhat reduced, since the conductor ring acts partly as a type В electrode, partly as a foundation earth and partly as an equipotential bonding conductor.
Where large numbers of people frequently assemble in an area adjacent to the structure to be protected, further potential control for such areas should be provided. More ring earth electrodes should be installed at distances of approximately 3 m from the first and subsequent ring conductors. Ring electrodes further from the structure should be installed more deeply below the surface i.e. those at 4 m from the structure at a depth of 1 m, those at 7 m from the structure at a depth of 1,5 m and those at 10 m from the structure at a depth of 2 m. These ring earth electrodes should be connected to the first ring conductor by means of radial conductors.
When the area adjacent to the structure is covered with a 50 mm thick slab of asphalt of low conductivity, sufficient protection is provided for people making use of the area.
E.5.4.3.5 Earth electrodes in rocky soil
During construction, a foundation earth electrode should be built into the concrete foundation. Even where a foundation earth electrode has a reduced earthing effect in rocky soil, it still acts as an equipotential bonding conductor.
At the test joints, additional earth electrodes should be connected to the down-conductors and foundation earth electrodes.
Where a foundation earth electrode is not provided, a type В arrangement (a ring earth electrode) should be used instead. If the earth electrode cannot be installed in the soil and has to be mounted on the surface, it should be protected against mechanical damage.Radial earth electrodes lying on or near the earth surface should be covered by stones or embedded in concrete for mechanical protection.
When the structure is situated close to a road, if possible, a ring earth electrode should be laid beneath the road. However, where this is not possible over the whole length of the exposed road segment, such equipotential control (typically a type A arrangement) should be provided at least in the vicinity of the down-conductors.
For potential control in certain special cases, a decision should be made as to whether to install a further partial ring in the vicinity of the structure entrance, or to artificially increase the resistivity of the surface layer of the soil.
E.5.4.3.6 Earth-termination systems in large areas
An industrial plant typically comprises a number of associated structures, between which a large number of power and signal cables are installed.
The earth-termination systems of such structures are very important for the protection of the electrical system. A low impedance earth system reduces the potential difference between the structures and so reduces the interference injected into the electrical links.
A low earth impedance can be achieved by providing the structure with foundation earth electrodes and additional type В and type A earth arrangements conforming to 5.4.
Interconnections between the earth electrodes, the foundation earth electrodes and the downconductors should be installed at the test joints. Some of the test joints should also be connected to the equipotential bars of the internal LPS.
Internal down-conductors, or internal structural parts used as down-conductors, should be connected to an earth electrode and the reinforcement steel of the floor to avoid step and touch voltages. If internal down-conductors are near expansion joints in the concrete, these joints should be bridged as near to the internal down-conductor as possible.
The lower part of an exposed down-conductor should be insulated by PVC tubing with a thickness of at least 3 mm or with equivalent insulation.
In order to reduce the probability of direct lightning flashes to cable routes in the ground, an earthing conductor and, in the case of wider cable routes, a number of earthing conductors should be installed above the cable routes.
By interconnecting the earths of a number of structures, a meshed earthing system is obtained as shown in Figure E.42
.
b
Key
1 2
3
4
NOTE
uilding with meshed network of the reinforcement tower inside the plantstand-alone equipment
cable trenches
This system gives a low impedance between buildings and has significant EMC advantages. The size of the meshes next to buildings and other objects may be in the order of 20 m * 20 m. Beyond a 30 m distance they may be enlarged to the order of 40 m * 40 m.
Figure E.42 - Meshed earth-termination system of a plant
Figure E.42 shows the design of a meshed earth electrode network, including cable trenches, between associated structures of lightning-protected buildings. This will give a low impedance between buildings and has significant LEMP protection advantages.
E.5.5 Components
No additional information.
NOTE Distances between fixings are given in Table E.1.
Е.5.6 Materials and dimensions
E.5.6.1 Mechanical design
The lightning protection designer shall consult with the persons responsible for the structure, on mechanical design matters following the completion of the electrical design.
Aesthetic considerations are particularly important as well as the correct selection of materials to limit the risk of corrosion.
The minimum size of lightning protection components for the various parts of the LPS are listed in Tables 3, 6, 7, 8 and 9.
The materials used for the LPS components are listed in Table 5.
NOTE Components such as clamps and rods selected in accordance with the future IEC 62561 series are adequate.
The LPS designer and the LPS installer should verify the fitness of purpose of the materials used. This can be achieved, for example, by requiring test certificates and reports from the manufacturer, demonstrating that materials have successfully passed quality tests.
The LPS designer and the LPS installer should specify conductor fasteners and fixtures which will withstand the electrodynamic forces of lightning current in the conductors and also allow for the expansion and contraction of conductors due to the relevant temperature rise.
Connections between sheet metal panels should be compatible with the panel material, represent a minimum contact surface area of 50 mm2 and be capable of withstanding the electrodynamic forces of a lightning current and the corrosion threats of the environment.
When excessive temperature rise is a concern for the surface to which the components are to be attached because it is flammable or has a low melting point, either larger conductor crosssections should be specified, or other safety precautions should be considered, such as the use of stand-off fittings and the insertion of fire-resistant layers.
The LPS designer should identify all corrosion problem areas and specify appropriate measures to be taken.
The corrosion effects on the LPS may be reduced either by increases in material size, using corrosion-resistant components or by taking other corrosion protection measures.
E.5.6.2 Selection of materials
E.5.6.2.1 Materials
LPS materials and conditions of use are listed in Table 5.
Dimensions of LPS conductors, including air-termination conductors, down-conductors and earth-termination conductors, for different materials such as copper, aluminium and steel are given in Tables 6 and 7. The recommended values for copper and aluminium of 50 mm2 round are based on the mechanical requirements (e.g. keep the wires straight between supports, so they don’t sag to the roof). If mechanical constraints are of no concern the values from footnote b) of Table 6 (copper 28 mm2) may be used as minimum values.
Minimum thickness of metal sheets, metal pipes and containers used as natural airtermination components are listed in Table 3, and minimum dimensions for bonding conductors are given in Tables 8 and 9.Е.5.6.2.2 Protection against corrosion
The LPS should be constructed of corrosion-resistant materials such as copper, aluminium, stainless steel and galvanized steel. The material of the air-termination rods and airtermination wires should be electrochemically compatible with the material of the connection elements and the mounting elements, and it should have a good corrosion resistance to a corrosive atmosphere or moisture.
Connections between different materials should be avoided; otherwise they are to be protected.
Copper parts should never be installed above galvanized or aluminium parts unless those parts are provided with protection against corrosion.
Extremely fine particles are shed by copper parts which result in severe corrosive damage to galvanized parts even where the copper and galvanized parts are not in direct contact.
Aluminium conductors should not be directly attached to calcareous building surfaces such as concrete limestone and plaster, and should never be used in soil.
E.5.6.2.2.1 Metals in soil and air
Corrosion of metal will occur at a rate depending on the type of metal and the nature of its environment. Environmental factors such as moisture, dissolved salts (thus forming an electrolyte), degree of aeration, temperature and extent of movement of electrolyte combine to make this condition a very complex one.
In addition, local conditions, with different natural or industrial contaminants, can cause significant variations to be observed in different parts of the world. To resolve particular corrosion problems, consultation with corrosion specialists is strongly recommended.
The effect of contact between dissimilar metals, in association with a surrounding, or partially surrounding, electrolyte, will lead to increased corrosion of the more anodic metal, and to decreased corrosion of the more cathodic metal.
The corrosion of the more cathodic metal will not necessarily be fully prevented. The electrolyte for this reaction may be groundwater, soil with some moisture content or even moisture condensate in above-ground structures where it is retained by crevices.
