Figure E.35 shows an example of conductive bridging between metal facade plates acceptable in those applications where the plates are to be used as natural down-conductors. Two methods are presented: bridging by flexible metal strapping and bridging by means of self-threading screws.
Figure Е.35а - Flexible metal strapping bridging Figure E.35b - Self-tapping screw bridging
NOTE Electrically conducting bridging improves, in particular, the protection against LEMP. More information concerning protection against LEMP can be found in IEC 62305-4.
Figure E.35 - Construction of the bridging between the segments
of the metallic facade plates
E.5.2.6 Isolated air-termination
Air-termination masts adjacent to structures or equipment to be protected are intended to minimize the possibility of lightning strikes to structures within their zone of protection when an isolated LPS is installed.
When more than one mast is installed, they may be interconnected by means of overhead conductors and the proximity of the installations to the LPS should be in accordance with 6.3.
Overhead conductor connections between the masts extend the protected volume and also distribute the lightning current between several down-conductor paths. The voltage drop along the LPS and the electromagnetic interference in the space to be protected are therefore lower than in the case when the overhead conductors are not present.
The strength of the electromagnetic field in the structure is reduced because of the greater distance between the installations within the structure and the LPS. An isolated LPS may also be applied to a structure of reinforced concrete, which will improve the electromagnetic shielding even more. However, for tall structures the construction of an isolated LPS is not practical.
Isolating air-termination systems made of stretched wires on isolating supports could be suitable where a large number of extensive protruding fixtures on the roof surface are to be protected. The isolation of the supports should be adequate for a voltage calculated from the separation distance in accordance with 6.3.
NOTE Environmental conditions (pollution) can lower the voltage breakdown of the air; this should be taken into account when determining the required separation between the isolated air-termination system and the structure.
Е.5.3 Down-conductor systems
General
The choice of number and position of down-conductors should take into account the fact that, if the lightning current is shared in several down-conductors, the risk of side flash and electromagnetic disturbances inside the structure is reduced. It follows that, as far as possible, the down-conductors should be uniformly placed along the perimeter of the structure and with a symmetrical configuration.
The current sharing is improved not only by increasing the number of down-conductors but also by equipotential interconnecting rings.
Down-conductors should be placed as far as possible away from internal circuits and metallic parts in order to avoid the need for equipotential bonding with the LPS.
It should be remembered that
the down-conductors should be as short as possible (to keep inductance as small as possible),
the typical distance between down-conductors is shown in Table 4,
the geometry of down-conductors and equipotential interconnecting rings has an influence on the value of the separation distance (see 6.3),
in cantilevered structures the separation distance should also be evaluated with reference to the risk of side-flashing to persons (see E.4.2.4.2).
If it is not possible to place down-conductors at a side, or part of a side, of the building because of practical or architectural constraints, the down-conductors that ought to be on that side should be placed as extra compensating down-conductors at the other sides. The distances between these down-conductors should not be less than one-third of the distances in Table 4.
A variation in spacing of the down-conductors of ±20 % is acceptable as long as the mean spacing conforms to Table 4.
In closed courtyards with more than 30 m perimeter, down-conductors have to be installed. Typical values of the distance between down-conductors are given in Table 4.
E.5.3.2 Number of down-conductors for isolated LPS
No additional information.
E.5.3.3 Number of down-conductors for non-isolated LPS
As stated in 5.3.3, a down-conductor should be installed at each exposed corner of the structure, where this is possible. However an exposed corner does not need a downconductor if the distance between this exposed corner to the nearest down-conductors complies with the following conditions:
the distance to both adjacent down-conductors is half the distance according to Tables 4 or smaller; or
the distance to one adjacent down-conductor is one-quarter of the distance according to Tables 4 or smaller.
Inside corners can be disregarded.Е.5.3.4 Construction
General information
External down-conductors should be installed between the air-termination system and the earth-termination system. Wherever natural components are available they can be used as down-conductor.
If the separation distance between down-conductors and the internal installations, calculated on the basis of the down-conductor spacing according to Table 4, is too large, the number of down-conductors should be increased to meet the required separation distance.
Air-termination systems, down-conductor systems and earth-termination systems should be harmonized to produce the shortest possible path for the lightning current.
Down-conductors should preferably be connected to junctions of the air-termination system network and routed vertically to the junctions of the earth-termination system network.
If it is not possible to make a straight connection because of large roof overhangs, etc. the connection of the air-termination system and the down-conductor should be a dedicated one and not through natural components like rain gutters, etc.
It is permitted, where aesthetic consideration need to be taken into account, to use a thin coating of protective paint or PVC covering over the external down-conductors.
Figure E.36 is an example of an external LPS for a structure with different levels of roof construction and Figure E.25 is an example of the external LPS design for a 60 m high structure with a flat roof with roof fixtures
.
Key
horizontal air-termination conductor
down-conductor
T-type joint - corrosion resistant
test joint
type В earthing arrangement, ring earth electrode
T-type joint on the ridge of the roof
mesh size
NOTE The distance between the down-conductors should comply with 5.2, 5.3 and Table 4.
Figure E.36 - installation of external LPS on a structure of insulating material
with different roof levels
In structures without extensive continuous conductive parts, the lightning current only flows through the ordinary down-conductor system of the LPS. For this reason the geometry of down-conductors determines the electromagnetic fields within the interior of the structure (see Figure E.37).
Figure Е. 37е
ІЕС 2719/10
Key
natural components of the LPS, e.g. gutters
LPS conductors
test joint
joint
NOTE The distance between the down-conductors and the mesh size should comply with the selected lightning protection level according to Tables 2 and 4.
Figure E.37 - Five examples of geometry of LPS conductors
When the number of down-conductors is increased, the separation distance can be reduced according to the coefficient kc(see 6.3).
According to 5.3.3, at least two down-conductors should be used on a structure.
Key
electric equipment
electric conductors
LPS conductors
main electric power distribution box with SPD
test joint
earth-termination system
electric power cable
foundation earth electrode
separation distance according to 6.3
/ length for the evaluation of the separation distance s
NOTE The example illustrates the problems introduced by electric power or other conductive installations in the roof space of a building.
Figure E.38 - Construction of an LPS using only two down-conductors
and foundation earth electrodes
For large structures, such as high-rise apartment buildings and, in particular, industrial and administrative structures, which are often designed as steel skeletons or steel and concrete skeleton structures, or which use steel-reinforced concrete, the conductive structure components may be used as natural down-conductors.
The total impedance of the LPS for such structures is fairly low and they afford a very efficient lightning protection for inner installations. It is particularly advantageous to use conductive wall surfaces as down-conductors. Such conductive wall surfaces might be: reinforced concrete walls, metallic sheet facade surfaces and facades of prefabricated concrete elements, provided they are connected and interlinked according to 5.3.5.
Figure E.4 provides a detailed description of the proper construction of an LPS using natural LPS components such as interconnected steel.
Use of natural components containing structural steel reduces the voltage drop between the air-termination system and the earth-termination system and the electromagnetic interference caused by lightning current within the structure.
If the air-termination system is connected to the conductive parts of the columns within the structure complex and to the equipotential bonding at ground level, a portion of the lightning current flows through these internal down-conductors. The magnetic field of this partial lightning current influences neighbouring equipment and has to be considered in the design of the internal LPS and electrical and electronic installations. The magnitude of these partial currents depends on the dimensions of the structure and on the number of columns, assuming the current waveform follows the waveform of the lightning current.
If the air-termination system is isolated from the internal columns no current flows through the columns within the structure complex, provided the isolation does not break down. If the isolation breaks down at an unpredicted point, a larger partial current may flow through a particular column or group of columns. The current steepness may increase due to the reduced virtual duration of the wave front caused by the breakdown and the neighbouring equipment is affected to a greater extent than it would be in the case of controlled bonding of columns to the LPS of the structure.
Figure E.10 is an example of the construction of internal down-conductors in a large steel- reinforced concrete structure used for industrial purposes. The electromagnetic environment near to the inner columns shall be considered when planning the internal LPS.
E.5.3.4.2 Non-isolated down-conductors
In structures with extensive conductive parts in the outer walls, the air-termination conductors and the earth-termination system should be connected to the conductive parts of the structure at a number of points. This will reduce the separation distance according to 6.3.
As a result of these connections the conductive parts of a structure are used as downconductors and also as equipotential bonding bars.
In large, flat structures (typically industrial structures, exhibition halls, etc.) with dimensions over four times the spacing of the down-conductors, extra internal down-conductors should be provided, wherever possible, approximately every 40 m to minimize the separation distance when the lightning current is flowing long distances over a flat roof.
All internal columns and all internal partition walls with conductive parts should be connected with the air-termination system and with the earth-termination system at suitable points.
Figure E.10 shows the LPS of a large structure with internal columns made of steel-reinforced concrete. To avoid dangerous sparking between different conductive parts of the structure, the reinforcement of the columns is connected to the air-termination system and to the earthtermination system. As a result, a portion of the lightning current will flow through these internal down-conductors. However, the current is divided among numerous down-conductors and has approximately the same waveform as the current in the lightning stroke. The steepness of the wavefront, however, is reduced. If these connections are not made and a flashover occurs, only one or a few of these internal down-conductors may carry the current.
The waveform of the flashover current will be considerably steeper than the lightning current, so the voltage induced in neighbouring circuit loops will be considerably increased.
For such structures, it is particularly important that before commencing the design of the structure, the structure’s design as well as the design of the LPS should be harmonized so that conductive parts of the structure can be utilized for lightning protection. By means of well- coordinated design, a highly effective LPS is achieved at minimum cost.
Lightning protection of space and persons below an overhanging upper storey, such as a cantilevered upper floor, should be designed according to 4.2.4.2 and Figure E.3.
Direct installation of down-conductors within the external plaster is not recommended since the plaster may be damaged as a result of thermal expansion. Moreover, the plaster may be discoloured as a result of chemical reaction. Damage to the plaster is particularly likely as a result of temperature rise and mechanical forces exerted by lightning current; PVC-covered conductors prevent staining.
E.5.3.5 Natural components
The use of natural down-conductors to maximize the total number of parallel current conductors is recommended as this minimizes the voltage drop in down-conductor systems and reduces the electromagnetic interference within the structure. However, it should be ensured that such down-conductors are electrically continuous along the entire path between the air-termination system and the earth-termination system.
The steel reinforcement in concrete walls should be used as a natural component of the LPS as illustrated in Figure E.27.
Steel reinforcement of newly erected structures should be specified in accordance with E.4.3. If electrical continuity of the natural down-conductors cannot be guaranteed, conventional down-conductors should be installed.
A metallic rain-pipe which satisfies the conditions for natural down-conductors according to 5.3.5 may be used as a down-conductor.
Figures E.22a, E.22b and E.22c show examples of fixing the conductors on the roof and the down-conductors including appropriate geometrical dimensions, and Figures E.22c and E.22d show the connections of the down-conductor to the metallic rain-pipe, the conductive gutters and the earth-termination conductor.
The reinforcing rods of walls or concrete columns and steel structural frames may be used as natural down-conductors.
A metal facade or a facade covering on a structure may be used as a natural down-conductor conforming to 5.3.5.
Figure E.8 shows construction of a natural down-conductor system using metal facade elements and steel reinforcing in the concrete walls as the equipotentialization reference plane to which the equipotentialization bars of the internal LPS are connected.
Connections should be provided at the top of the wallcovering, to the air-termination system and at the bottom of the earth-termination system and to the reinforcing rods of the concrete walls, if applicable.
The distribution of current in such metal facades is more consistent than in reinforced concrete walls. Sheet metal facades comprise individual panels generally of trapezoidal cross-section with a width between 0,6 m and 1,0 m and a length corresponding to the height of the structure. In the case of high-rise structures, the panel length does not correspond to the structure height due to transport problems. The whole facade then comprises a number of sections mounted one above the other.
For a metal facade, the maximum thermal expansion should be calculated as the difference in length produced by a maximum temperature of the metal facade in sunlight of approximately +80 °С and a minimum temperature of -20 °С.
The temperature difference of 100 °С corresponds to a thermal expansion of 0,24 % for aluminium and 0,11 % for steel.
Thermal expansion of the panels results in movement of the panels with respect to the next section or to the fixtures.
Metal connections, such as those depicted in Figure E.35, encourage uniform current distribution in metal facades and thus reduce the influence of the electromagnetic field inside the structure.
A metal facade produces maximum electromagnetic shielding when it is electrically interconnected over its whole area.
High electromagnetic shielding efficiency of a structure is obtained when permanent bonding of adjacent metal facades is carried out at sufficiently small intervals.
Symmetry of current distribution relates directly to the number of connections.
If stringent requirements with respect to shield attenuation exist and continuous strip windows are incorporated in such a facade, the continuous strip windows should be bridged by means of conductors at small intervals. This may be done by means of metal window frames. The metal facade should be connected to the window frame at short intervals. Generally, each ridge is connected to the horizontal tie-beam of the window frame at intervals not exceeding the spacing of the vertical members of the window construction. Bends and detours should always be avoided (see Figure E.9).
Metal facades comprised of relatively small elements which are not interconnected cannot be used as a natural down-conductor system or for electromagnetic shielding.
More information on the protection of electrical installations and electronics in structures is available in IEC 62305-4.
E.5.3.6 Test joint
Test joints facilitate measurements of the earth resistance of earth-termination system.
Test joints conforming to 5.3.6, should be installed in the connection of the down-conductors to the earth-termination system. These joints facilitate the determination by measurement that an adequate number of connections to the earth-termination system still exist. It is thus possible to validate the existence of continuous connections between the test joint and the air-termination system or the next bonding bar. On tall structures, ring-conductors are connected to the down-conductors, which may be installed in the wall and invisible to the eye; their existence may only be confirmed by electric measurement.
Figure E.39a through Figure E.39d show examples of test joint designs, which may be installed on the inner or outer wall of a structure or in a test box in the earth outside the structure (see Figure E.39b). To make the continuity measurements possible, some conductors may have to have isolating sheaths on critical sections.
Figure Е.ЗЭа
Figure Е.ЗЭЬ
ІЕС 2722/10
Figure Е.39с ІЕС 2723/10
ІЕС 2721/10
Alternative 1 - Test joint on wall
down-conductor
type В earth electrode, if applicable
type A earth electrode, if applicable
foundation earth electrode
bonding to the internal LPS
test joint on the wall
corrosion-resistant T-joint in soil
corrosion-resistant joint in soil
joint between lightning conductor and a steel girder
Alternative 2 - Test joint in the floor
down-conductor
type A earth electrode, if applicable
bonding bar of the internal LPS
type В - Ring earth electrode
type В - Ring earth electrode
test joint in the floor
corrosion-resistant, T-joint in soil
corrosion-resistant joint in soil
joint between lightning conductor and a steel girder
NOTE 1 The test joint detailed in Figure E.39d should be installed on the inner or outer wall of a structure or in a test box in the earth outside the structure.
NOTE 2 To make the loop resistance measurements possible, some of the connecting conductors should have isolating sheaths along critical sections.
Figure E.39 - Four examples of connection of earth-termination to the LPS of structures using natural down-conductors (girders) and detail of a test joint lf it makes sense (e.g. in the case of earthing connections to steel columns via connecting conductors), connections from natural down-conductors to earth-termination electrodes may be provided with isolated conductor segments and testing joints. Special reference earth electrodes should be installed to facilitate monitoring of the earth-termination system of an LPS.
E.5.4 Earth-termination system
E.5.4.1 General
The LPS designer and the LPS installer should select suitable types of earth electrodes and should locate them at safe distances from entrances and exits of a structure and from the external conductive parts in the soil, such as cables, metal ducts, etc. Hence the LPS designer and the LPS installer should make special provisions for protection against dangerous step voltages in the vicinity of the earth-termination networks if they are installed in areas accessible to the public (see Clause 8).
The recommended value of the overall earth resistance of 10 Q is fairly conservative in the case of structures in which direct equipotential bonding is applied. The resistance value should be as low as possible in every case but especially in the case of structures endangered by explosive material. Still the most important measure is equipotential bonding.
The embedded depth and the type of the earth electrodes should be such as to minimize the effects of corrosion, soil drying and freezing and thereby stabilize the equivalent earth resistance.
It is recommended that the first half metre of a vertical earth electrode should not be regarded as being effective under frost conditions.
Deep-driven earth electrodes can be effective in special cases where soil resistivity decreases with depth and where substrata of low resistivity occur at depths greater than those to which rod electrodes are normally driven.
When the metallic reinforcement of concrete is used as an earth electrode, special care should be exercised at the interconnections to prevent mechanical splitting of the concrete.
If the metal reinforcement is also used for the protective earth, the most severe measure in respect of thickness of the rods and the connection should be chosen. In this case, larger sizes of reinforcement bars could be considered. The need for short and straight connections for the lightning protection earthing should be recognized at all times.
