Electrical Isolation of External Lightning Systems (Separation Distance)
When the separation distance between the external lightning system or the landing conductor and the metal parts in the building and the electrical installation is not sufficient; There is an uncontrolled flash-over (arc) risk between the components of the lightning protection system and the metal components in the building and the electrical installation.
Depending on the magnetic field due to the rapidly changing lightning to metal installations such as water, air conditioning and power lines, the pulse voltage creates induction loops in buildings. These impact voltages must be prevented from causing uncontrolled flash-overs that could cause a fire. For example; Can damage flash-over wiring and connected loads in power lines
Figure 1: In practice, this formula is often difficult to use to calculate the separation distance. This formula shows the principle of ırma Separation Distance..
– s: Separation distance (meters)
– ki: Induction factor (Depends on selected LPS class.)
– kc: Division coefficient (Depends on geometrical design.)
– km: Material factor (Depends on material used at proximity point)
– l: We determine the separation distance is the length of the down conductor extending from one point to the potential equalization or grounding point with the external lightning system. (meter)
Two (Induction Factor)
The coefficient of two means the risk generated by the high current. Depending on the LPS class, it is specified in table 10 in IEC 62305-3 (EN 62305-3) standard (Table 1)
Table 1: Induction Factor (ki)
km (Material Factor)
The material factor km takes into account the insulation properties of the environment. The electrical insulation factor of the air is assumed to be 1. All other solid materials used in the construction industry (brick, wood, etc.) isolate only half of the air. This must also be taken into account for roof-mounted catch rods. As shown in Fig. 2, between the catch end base and the ceiling-mounted structure is km = 0.5 for solid material, and for the air gap between the upper edge of the ceiling-mounted structure and the catch rod is km = 1. Since no material factor values other than 0.5 and 1 for km are specified in the standard, the deviation values have to be verified or calculated by tests. Glass fiber reinforced plastic used for insulated catching systems (DEHNiso spacer, DEHNiso combi) in DEHN products
(GRP) for the material factor was determined as 0.7. This factor can be used in calculations like other material factors.
Fig. 2: Material Factor for Catch Rod on Roof
According to DIN EN 62305-3 standard APPENDIX 1, km factor can be calculated for multi-layered brick. The km factor consists of material thicknesses and the isolated properties of the materials. The following formula is used to calculate the km factor. (Figure 3)
– km total: Total material factor
– l1, l2,…, lm: Material thicknesses
– lg: Material thicknesses
– km1, km2,…, kmx: Insulation property of the respective material For a wall construction as shown in Figure 3, material factor km total is calculated as follows.
Figure 3: Mileage factor in case of air gap and different materials
However, in the construction of multi-layered bricks, fittings are often used between different materials.
Figure 4: km factor for different materials in cases where there is no air gap
As it can be seen in Figure 4, concrete, brick, thermal insulation composite material is a combination, so it cannot be assumed that there is air gap between the two materials. The km factor is lower for this group of materials.
In general, it is recommended to use the material factor km = 0.5, assuming the worst case.
As you can see in Figure 4, the length “l ir is the distance determined from the point at which the separation distance“ l belirlen is determined to the next lightning potential equalization point (zero potential level) or to the earth termination system along the pickup system or landing conductor. Each building with a lightning equipotential bonding system has the equipotential surface of the grounding electrode or the grounding electrode located near the ground surface. This surface is the reference plane to determine the length. If potential compensation for lightning protection is to be established in tall buildings, potential compensation for all electrical and electronic lines and all metal installations should be ensured. (For example, for buildings 20 meters and over.) Type 1 surge protection devices must also be used to establish equipotential bonding at this height. In tall buildings, the equipotential surface of the foundation earth electrode should also be used as a reference point that determines the length “l.. In high buildings, it is more difficult to maintain the required separation distance.
Kc (Bölme Katsayısı)
The factor Kc takes into account the current distribution of the outdoor lightning protection system in the down-conductor system. Different calculation formulas for kc are specified in the standard. In order to obtain separation distances in high buildings, it is recommended to make a ring with conductors. These interconnected ring conductors balance the current flow, thus reducing the required separation distance. The potential difference between the building and the downstream conductors is equal to zero near the earth and grows in relation to height. This potential slope can be considered as a cone. Thus, it appears that the separation distance to be maintained is greatest at the end of the building or on the roof surface and decreases towards the earth termination system. This shows that the distance from the down-conductors must be calculated several times with “l farklı of different lengths.Due to the different structures, kc proves that the partition coefficient is difficult to calculate. Kc for a single catch (Splitter Coefficient) For example, if a single catch pole is installed next to the building, the total lightning current flows through the catch and the down conductor. The factor kc is therefore equal to 1 and the lightning current cannot be divided here. Therefore, it is often difficult to maintain the separation distance. In Figure 5, the catch end post (telescopic lightning protection post) can be realized further away from the building.
Figure 5: Lightning Protection Pole (kc = 1)
Simplified Approach – Kc (Partition Coefficient) In
Order to evaluate kc easily and quickly, kc values can be determined based on the number of down conductors as shown in table 2. The simplified approach can only be used if the maximum horizontal extension (length or width) of the structure is no more than four times the height. The value of Kc applies to type B grounding electrodes. This value can also be used for type A electrodes if the ground resistance of the combined ground electrodes is not greater than 2. However, if the ground resistance of the ground electrodes is greater than 2, then kc = 1 should be assumed.
Table 2: Simplified Approach – kc (Partition Coefficient)
Kc (Partition Coefficient) for two interlocking catches
The lightning current can be divided between two current paths if the lightning current is spread over two catching rods or poles. (Fig. 6) However, lightning does not always come to the center of the plane (to the same impedances) and the current is not divided by 50% to 50% because of the different lengths (impedances) as it can hit any point along the air termination system. To calculate the factor kc, this formula takes into account the worst case.
– h: Length of down-conductor
– c: Distance between air termination rod or air termination poles This calculation assumes a type B earth termination system. If single type A ground electrodes are installed, they must be connected to each other.
Figure 6: Partition coefficient kc in case of two poles with cable and type B grounding electrode
The following example illustrates the calculation of the coefficient kc in a triangular roof with two down conductors. (Figure 7) Type B earth termination system (ring or basic earth electrode) is used.
Fig. 7: Calculation of the coefficient kc on a triangular roof with two down conductors
kc (Partition Coefficient) and s (Split Distance) for ≥4 down conductors in triangular or flat roofs The arrangement of the down conductors shown in Figure 7 should no longer be used even in a single family home. . The partition coefficient kc has been significantly improved by using a total of four down conductors using two further down conductors. (Figure 8) The following formula is used to calculate.
Figure 8: Roof and Four Down Conductors in Triangular Structure – h: Length of the downstream conductor up to the building troughs, which is the most negative point for the injection of lightning currents – c: Distance between down-conductors – n: Total number of down conductors The equation is an approach for three-dimensional structures with n≥ 4. The h and c values should be from 3 m to 20 m. If the down-conductors are installed internally, the number should be considered n. In buildings with flat roofs, the partition coefficient kc is calculated as follows. A ground electrode type B arrangement is a prerequisite in this case. (Figure 9) – h: Distance or height between ring conductors. – c: Distance between the down conductor and the next down conductor. – n: Total number of down conductors. Fig. 9: Grid values of grating kc coefficient values and type B grounding arrangement The distances of the down-conductors are based on the LPS class (Table 4 IEC 62305-3 (EN 62305-3)) and ± 20% deviation is acceptable. Describes the maximum distance between the down conductors.
s (Separation Distance) – detailed approach for determining the separation distance In addition to those described above, a more detailed calculation method can be used to determine the partition coefficient kc and the separation distance s. In the case of a lightning protection system in the form of mesh, the current is evenly divided due to the numerous existing paths formed by the flat roof and the down conductors. This has a positive effect on the separation distance. The roof-mounted structure as in Figure 10 is built on a building. The detailed calculation method allows you to calculate the separation distance “s olduğun as much as possible. The following general calculation formula is used.
Figure 10: Values of coefficient kc in a system consisting of several down conductors according to IEC 62305 Figure C.5
– l1, l2,…, ln: Length of conductor to the next point.
– kc1, kc2,…, kcn: The partition coefficient k according to the number of current paths depends on the number of current paths available. As a result, the following applies:
– kc = 0.5 in case of two conductors
– kc = 0.33 in case of three conductors
– kc = 0.25 in case of four conductors
At each subsequent node, the previous value of kc is halved. The minimum value of kc should not be less than “1 / number of down conductors..
EXAMPLE: To illustrate this, let us calculate the separation distance s for a flat roof mounted structure. A class air conditioning system was installed on the roof of a building (Figure 11 & Figure 12). Lightning protection level 2 is taken.
Figure 11: Current distribution in case of several conductors
Figure 12: Roof-mounted structure and several down-conductors
– Lightning protection level: LPS II – Induction factor: 0.06
– Length: 60m
– Width: 60m
– Height: 7m
– Number of descent conductors: 24
– Minimum kc value (1 / number of down conductors): 0.042
– Grounding system, type B basic ground electrode: -1.0m
It is assumed that the air conditioning system is protected from lightning by means of two transverse catch rods (LPZ 0B). The separation distances must be determined at the base of the catch tip bar. The current paths with different conductor lengths are formed by the grid on the roof surface. Moreover, the lightning current is divided according to the points as follows.
– The base of the air-termination rod (two conductors) o kc1 = 0.5, with a conductor length l1 of 0.8 m
– Node 1 (two conductors) o kc1 = 0.25, a conductor length of 0.40 m at l2
– Node 2 (two conductors) o kc1 = 0.125, a conductor length of 10.0 m at l3
– Node 3 (three conductors) o kc1 = 0.063, with a conductor length l4 of 10.0 m
– Node 4 (three conductors) o kc1 = 0.042, with a conductor length l3 of 10.0 m
Let’s calculate the separation distance according to the following formula:
A separation distance of 0.87m (solid metal) must be applied to the base of the air conditioning system.
Determination of the zero potential level To calculate the separation distance, it is important to determine the zero potential level. The zero potential of buildings is at the same height as the ground or ring ground electrode. Thus, the definition of the zero potential level is decisive for the separation distance s. Buildings with a wall and ceiling reinforcement connected to each other to carry lightning currents can be used as a down-conductor system. Therefore, separation distances must be maintained due to constant potential. However, the roof surfaces are typically covered with insulation and ceiling membranes, and the gripping system is fitted with a mesh (grid) method. These mesh trapping systems are also connected around the roof parapet. In the event of a lightning strike, the separation distances from the grid and conductors must be maintained. It is therefore recommended to use insulated conductors that allow the separation distances to be maintained. In buildings with interconnected steel frames and metal roofs, it can be assumed that the zero potential level is equal to the height of the building and the separation distances must be maintained
Consequently, the requirements of the standard IEC 62305-3 (EN 62305-3) must be observed. DEHNSupport software can be found in the DEHN Distance Tool tab 126.96.36.199. As can be seen in the section, based on node analysis allows easy calculation of separation distance.
SOME ISSUES THAT CAN BE AWARE IN THE CHOICE OF THE AR Surge
As the awareness of lightning protection increases, the use of surge arrester has become widespread.
Although there is a perception that it is sufficient to know the definitions of old terms B, C, D or new terms Type 1, Type 2, Type 3, according to the reference IEC 62305, IEC 60364 and IEC 61643, the situation is not so simplistic. Deficiencies are observed in many subjects from the specifications to the project implementation.
In the following, we have compiled some of the most common unaware issues.
1. The performance of the Type1 + Type2 surge arrester is not only understood by its protection level Vp and the value I imp. What matters is how little residual energy is left in the device to be protected after the incoming lightning energy is discharged. The purpose of the surge arrester is to protect the installation and devices against lightning, mains or electromagnetic impacts. The lower the residual energy left after the discharge of the surge arrester, the less your device will wear out.
2. I max 100kA ≠ I imp 100kA
In surge arresters, I max value and I imp value do not mean the same. If 100kA is mentioned in type1 surge arresters, this value should be in the 10 / 350us curve. The energy it carries is approximately 19 times less than I imp 100kA and is meaningful for Type2 Surge Arresters.
3. If you are using a Type1 + Type2 (B + C) LV surge arrester and this value is 100kA, the I imp value must be 100kA (10 / 350us) and according to the relevant IEC 61643-11 standard, I imp 100kA must be written on the surge arrester label. If there is no T1 mark on the surge arrester, it should definitely indicate I imp 100kA (10 / 350us) in the technical document. If these statements do not exist, this product is either not really Type1 (B) or is not of the specified value.