lighting protection of an office building

Date: Lightning protection of an office building page:1

Electrical power engineering, EEP course

Lightning protection of an office building

Esta obra está bajo una licencia de Creative Commons. Participants:

Instructor name:


Index Name Page 1. Specification 3 1.1. A risk assessment for the outer lightning of the office building is to be done to verify the protection level 3 1.2 Work out a description of how the outer lightning protection should be created. Include a system for ”catching”, conducting and earthing. Additional information is found in appendix 1. 4 1.2.1. The Air-termination system 6 1.2.2. Down-conducting system 11 1.2.3. The earth-termination system 14 1.3 For the inner lightning protection there should be selected equipment for protection of the high voltage side of the supply transformer. 17 1.4 Over-voltage protection for the electrical installations in the building. 18 2. Conclusion / Evaluation 23 3. Literature 23 4. Appendix 23


Specifications

Our group had read and understand the normes before look for the specifique case. After, with new knowledge to prepare a proposal for lightning protection of an office building.

1. 1 A risk assessment for the outer lightning of the office building is to be done to verify the protection level

gN is the average annual ground flash density, in lightning flashes per square kilometer, during the year, in the region where the building is located.

236,29,5504,004,0 25,1 dg TN ;

dN - average of annual frequency of direct lightning flashes to a structure can be accessed from.

EA - is the equivalent collection area of structure ( 2m ) 0261612,01011700236,210 66

Egg ANN ; Risk management contains:

CBANC ;

.;;

321

4321

4321

CCCCBBBBBAAAAA

5,01 A The construction of a building is made from concrete units, where the armament iron is not connected between the units;

5,02 A roof construction is made from ferro concrete with segment parts; 5,03 A because the roof is covered with roofing felt; 5,04 A on the roof there is only cooling and ventilation system which do not have

electronical material; 5,01 B Building usage comparing by persons inside is between low and medium, because it

is an office building; 2,02 B flammable material inside; 2,03 B valuable materials inside;

104 B automatical fire protection; 11 C no hazard to the nature – no trees or forest near, there is no material to poison the water

also; 1,02 C serious breakdown if the power is off. It is an office building, so probably a lot of

people will be using computers there, and in case of power loss all the work will be stopped; 13 C there is no other consequences to evaluate;

0125,011,01102,02,05,05,05,05,05,0 CBANC ;


Cd NN so we need protection:

952219,01 d

C

NNE ;

while E is equal between 0,98 and 0,95 we need at least second level protection. 1.2 Work out a description of how the outer lightning protection should be created. Include a system for ”catching”, conducting and earthing. Additional information is found in appendix 1. In the following exercise we will create a outer lightning protection system, based on the given data from the appendix 1 and with the protection level 2. The Outer protection system consists of 3 parts.

The first part is the Air-termination system, which is intended to intercept the lightning flashes.(catching)

The second part is the Down-conductor which is intended to conduct lightning current from the Air-termination system to the Earth-termination system. ( conducting)

The third part is the Earth-termination system which is intended to conduct and disperse lightning current to the earth. (earthing)

For the construction and planning of an effective and secured outer lightning protection system, we need information about the size and buildup of our building. So at first we made a construction plan of the building. This is necessary to see how we can get a high security ( according to protection level 2) without using to much protection devices and money as required.


Fig.1: top view of the building with size information ( the plan based on the picture in the appendix, it is not

exact but for our use adequate) The height of the building is 20m.


1.2.1. the Air-termination system For planning an Air-termination system, different methods are possible. The method we will mainly use for our system is the meshed size method, because we have a plan surface on our roof. We will also use the protective angle method, for example to protect the air conditioning system on the roof with small rods. The rolling sphere method we will also use. Table.1: positioning of air termination system for the protection level 2 (taken from IEC 1024-1)

Protection radius [m] h=20m h=30m h=45m h=60m Mesh level Sphere a° a° a° a° width [m]

2 30 35 25 * * 10 For our building it is enough, if we put the air-termination system on the roof, because the radius of the sphere is bigger than the height of our building (r=30m>h=20m).

Fig.2: rolling sphere model

Figure 2 demonstrate very good the mode of operation of the sphere method, the header of the lightning is in the center of the sphere, and the lightning will strike into points which have a contact with the sphere. Now we move the sphere over the building and look for all the contact points at the building, these points should be protected very well. Also we see wherever we put the sphere, the side of our building is always protected by the roof. By complex buildings it is sometimes usual to build a small model, and move the sphere over the model, but our building has a flat roof so this is not necessary. For the Air-termination system we can use these elements. - rods; - stretched wires; - meshed conductors. Also it is possible to use metal pipes, metal plates or other conducting material which is normally installed on the roof. Our building has a flat roof and on the most flat roofs it is common that metal plates are installed on the corners of the roof. So we will use these metal plates for the Air-termination system, because this is a cheap and effective way. If metal plates


are not installed or if they are not confirm to the requirements, it is possible to replace them with conducive lines in the corners of the building. According to the material and thickness of the metal plates we can use them for the air-termination system (table 2). The thickness of the metal plates is important to prevent a melt threw in case of a lightning strike. Also it is important that these metal plates have no conductive connection into the building, because then it is possible that they conduct the lightning power into the building. And that can cause a lot of damage (see Fig.5.). The metal plates on the roof should have a conductive connection between each other, so you have to take care of the expansion and shrinking of the metal plates according to the temperature. The best way is to connect them with flexible conductors. Table 2: minimum thickness of metal plates or pipes (taken from IEC 1024-1)

Protection level Material Thickness [mm]Fe 4

1,2,3&4 Cu 5Al 7

Table 3: minimum dimension of LPS materials (taken from IEC 1024-1)

Protection Material Airtermin- Downcond- Earthermin- level ation [mm²] uctor[mm²] ation[mm²]

Cu 35 16 50 1,2,3,4 Al 70 25 -

Fe 50 50 80 We will protect the flat roof with a system of meshed conductors and the constructional systems on the roof we will protect with small rods. This is the most effective way for our building. The mesh width is according to protection level 2, 10 meters (table 1). It is important to say that this is just a protection for the building, so it is not allowed for people to enter the roof in case of a thunderstorm. If we want to protect people on the roof, we need to install big rods. But for our building this is not necessary.


Fig.3: meshed conductor system for a flat roof (taken from www.dehn.de)

In Fig.3 is shown a meshed conductor system, similar to the system we want to install. The meshed conductors are connected with the metal plates by a flexible connector (conductive), this is because of mechanical moving, shrinking and expansion of the conductors and the plates. There are also conductor holders installed (in distance of about 1m), to fix the conductors and to prevent damage from the roof (melt threw the roofing felt). So we install a meshed conductor system with the maximum mesh width of 10m and with a thickness of the conductors according to table 3. The Air-termination system has to be connected with the down-conducting system. In the next step we want to protect constructional systems on our roof (e.g. cooling and ventilation system).

Fig.5: example of connecting the ventilation system


In Fig.5 we have a very bad example of connecting a ventilation system. The ventilation system is connected with the Air-termination circuit through a contact with the meshed conductors. So if a lightning strikes into our protection system on the roof, current will run through the conductive parts of the ventilation system into the building (The yellow arrows symbolize the current flow in case of a lightning strike). Inside the building it is possible that electronic components are placed close to the ventilation system, which can be destroyed by an induced voltage. Also the ventilation system is not protected to a direct lightning strike.

Fig.6.example of protecting the ventilation system Fig.6. is a much better example how to protect a constructional system. The size of the rods can be calculated with the protective angle method. The Air-termination system is not connected to the ventilation system (pipe), no lightning power will run into the building. It is not necessary to protect these small pipe with two rods, one rod is enough if it is high enough to protect the hole pipe. Table 4: shows the relationship between size of the rod and the angle alpha for the protection level 2 (This cutout of a table is taken from www.dehn.de)

It is important that no component of the air termination system is close to electronic or other sensitive installations (automatic fire detection, alarm system and sprinkler system), because threw the components of the lightning protection system runs a very high current in case of a lightning strike. So these sensitive installations can be damaged because of sparks, heat or induced voltage. The protective rods have to be placed in a distance to the installations we have to protect, by strong wind influences the rods can move with the wind, and therefore it is important to stabilize the rods with isolated distance pieces.


Final plan of the Air-termination system:

Fig. 7: Air-termination system for the bulding


1.2.2. Down-conducting system The Down-conducting system conducts the lightning power from the Air-terminations system to the Earth-termination system. To prevent damage it is important that the Down-conducting system consists of:

- A satisfactory number of parallel down-conducting current paths (as larger the number of the current paths as smaller is the current threw each path)

- The length of the current-paths should be as short as possible (vertical and no ties, loops)

In the Appendix it is written that an electrical continuity of the armament iron should be established. If this is possible we can use the armament iron construction for our Down-conducting system. If we use the armament iron, we have to take care about different points:

- the thickness of the metal-constructions should be not smaller then 0,5mm - the different parts of the metal constructions have to be connected in the vertical

direction - a ring conducting system around the building is not necessary - in case of a lightning strike current is running threw the armament iron

If we connect the different parts of the armament iron, we have to take care of a secured connection. The distance between the connectors should be not bigger then 15m, table 5 (if an electrical continuity of the iron is not possible). If we are not aware about this we get a high current in some current paths, electrical installation can be damaged. But in our case we have continuity connection, so that the current in each path should not become to high. Table 5: Average distance between down-conductors according to the protection level (taken from IEC 1024-1)

Protection Average

level distance [m] 1 10 2 15 3 20 4 25

Calculation of the safety distance d: d=ki*(kc / km)*l ki= depends on the selected protection level of LPS kc= depends on dimensional configuration km= depends on separation material l= length along the down-conductor from the point where the proximity is to be considered to the nearest equipotential bonding point Table 6: Values of coefficient km & ki (taken from IEC 1024-1)

Protection level ki km 2 0,075 Air 1 Solid 0,5


In case of a lightning strike, current will run threw the armament iron. Inside the building are also electrical devices and other conductive parts installed, which can be damaged by sparks and induced voltage. Therefore it is important to take care about the safety distance; the installations should not get closer then the distance d to the Down-conducting parts. If this is not possible we have to make a special isolation for these components.

Fig.8: connection between down-conducting and air-termination system

Fig.8 shows a schematic picture of our roof construction, several down conductors are connected between the metal plates on the roof and the armament iron. By this connection we have to take care about loops, loops should be avoided. If this is not possible the proximity distance should be larger then the safety distance d, else we can get unintentional sparks. Roof break-threw are not necessary for our building, because this is just necessary for areas bigger then 40x40m. Block B is longer then 40m but it is also smaller then 40m, so the current will not have a to long way to earth. The down-conductors from the roof to the armament iron should be placed in a distance not bigger then 15m and close to corners of the building. If the down conducting system threw the armament iron is not confirm to the requirements, we have to install down conductors on the surface of the building.


Fig.8c: Down-conducting system threw armament iron (taken from 61024-2)

1=horizontal air-termination conductors 2=test joint 3=equipotential bar of the internal LPS

Fig.9: places of down-conductor connections

The connection of the Down-conducting system to the earth-termination system is already installed (“the foundation electrode placed in the border foundation of the building and connected to the armament iron” Appendix). We should install test joints on the connections


between the down-conducting 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 possible to check the continuous connection between air-termination system and earth. Also it is possible to measure the earth resistance. If the down conducting system threw the armament iron is not confirm to the requirements or is not possible, we have to install down conductors on the surface of the building. The biggest distance between the down-conductors we can take from table 5, is 15m. So we will place the down-conductors on the same position like we place the down-conductors to connect with armament iron (Fig 9., red spots). The minimum thickness of the down-conductors is shown in table 3.

Fig.10: down-conducting system on the surface of the building

In Fig.10 is shown a down-conducting system on the surface of the building. The lightning current is running outside the building to the earth-termination system, but you also have to take care of the safety distance d. We will place distance holders each meter. Also you have to prevent that the down-conductors get to close to windows and doors. By down-conductors on the surface it is sometimes necessary to install ring-conductors around the building, and connect them to the down-conductors. This installation helps to distribute the lightning current on each path, also it realize a better equipotential between the down-conductors. On buildings of the height of 20m and less this is not necessary; the foundation electrode is enough to put the down-conductors on the same potential. The down-conductors have to be connected with the earth-termination system threw test joints. 1.2.3. The earth-termination system The task of an earth-termination system is:

Conducting of the lightning current into the earth without causing dangerous over voltages

Equipotential bonding between the down-conductors Potential control in the vicinity of conductive building walls Interception of the lightning current when propagation on earth’s surface


The installed foundation electrode meets all these requirements. Earthing and equalization connections: The earth electrode for installations (lightning protection, protection against indirect contact, EMC,etc.) is established as a foundation electrode placed in the border foundation of the building and connected to the armament iron. The electrodes are “connected” to the room with the main electrical panel. To assure and to make the protection systems effective, electrical continuity of the armament iron should be established. Taken from Appendix1 The earth-termination system already exists and it is already connected to the down-conductor (armament iron). So our work is to check, if everything is confirm to the requirements. The necessary length of the earth-electrode is shown in Fig.11, p is the specific resistance for a cubic meter of earth.

Fig.11: minimum length l1 of earth electrodes according to the protection level (IEC 1024-1)

In the building a foundation electrode is installed, the foundation electrode is a type B arrangement of an earth electrode. For this type the radius of the area enclosed by earth electrode shall be not less than the length l1.

r ≥ l1; For the protection level 2 the length l1 is a constant of 5 meters, which is not depending on the specific resistance. So it is not necessary to measure the specific resistance of the earth. To calculate the radius we can use the formula:

r= √(A/π); A= area enclosed by earth electrode. The electrode is close to the border of the building, so we will calculate approximately the area for the bloc A.


A= 13,44m*25,2m*2+13,44m*(25,2m-12,8m)= 844m² r=16,39m ;

The length / radius of the earth electrode is long enough in our building. The minimum thickness of the earth-electrode is shown in table 3. Especially in the earth termination system we have to take care about the corrosion of the material, because the material is exposed to water, soil drying and freezing. Table7: LPS materials and condition of use (taken from IEC 1024-1)

Table 7 shows the condition of use of the material, e.g. for our earth-termination system stainless steel is a good solution. The electrode is surrounded by concrete so it is protected very well against water and corrosion. Also copper we can use because in the appendix is written, “There may not be any material combinations that can cause galvanic corrosion in the construction.” If it is possible a galvanized steel strip, should be installed in the foundation and be taken upwards with a connection to the down-conducting test joints, else it is possible to perform the connection to the down conducting system on the brick wall. A foundation electrode is normally a good solution for an earth-termination system, because the concrete has a low specific resistance (150-500Ωm) and protects the metal from corrosion.


1.3 For the inner lightning protection there should be selected equipment for protection of the high voltage side of the supply transformer. Electrical installation: The building is connected to the public grid at voltage level 10.5 kV. The grid voltage is transformed down by a 10.5/0.42 kV Dyn 5 transformer. Rated power is 800 kVA. The distance from the transformer to the main electrical panel located in the basement of the building is 12 m. Taken from appendix

Fig.1.3.1: electrical connection of the building

Used Cable information: Cable type Nkt-cable 3x95mm² PEX-Cu 12kV Length =10000m Vcable =200*106m/s C´ =0,34µF/km X´ =0,097Ω/km Calculation of wave impedance and wave time constant: F =50Hz L =X / (2*π*f)= 3,08*10-4 Vs / A*km Z =√ (L / C)= √ ((3,08*10-4 Vs / A*km) / (0,34μF/km)) = 30,13Ω Ψ = length / Vcable = 10000m / (200*106m/s) = 50µs Surge arrester information: more information on appendix 2 Type = Esra 12 Ur = 12 kV guaranteed protective characteristics Uc =Max. conti. operating voltage= (UmaxL*1,05) / √3 =7,27kV <9,6kV UmaxL =max. system voltage= 12kV Ur0 =voltage marking at component= Uc / 0,8= 9,09kV Ue =temporary over voltage= ke*Uc= √3 * 7,27kV=12,6kV <14,2kV


ke =earth fault factor; kt = 3; UL=(kt*Ur*ke*√2) / √3 = 44,55kV; Calculation of the consuming energy: Urest = switching surge voltage taken from the surge arrester information = 28,6kV; W=( (UL- Urest) / Z) * Urest*2* Ψ= 1509,25 VAs= 0,419Wh= 4,19*10-4kWh; Transforming kWhJ: 4,19*10-4kWh * 3,6 · 106 = 1509,25J; W / UmaxL= 0,125 kJ / kV; The surge arrester ESRA 12 can withstand a Energy of 2,1 kJ / kV Line discharge, our calculated value is much lower. So we can use the ESRA 12 for the protection of the transformer on the high voltage side. 1.4 Work out over-voltage protection for the electrical installations in the building. In the following exercise:

Describe the necessary initiative to achieve potential equalization (same voltage). The proposal should include the main and group electrical panels (switch boards) and

possible instrument/device protection. The proposal should be based on appendix , which only contains information about

block B of the building. Hence the proposal for the inner lightning protection should be limited to block B.

equipotential bonding: We need equipotential bonding to set all the metal installations to one voltage level. So the metal installation inside the building should be connected to an equipotential bonding. If there is no equipotential, dangerous touch voltage can occur for example between PE-conductor of the electrical equipment and metal pipes of the radiator. Threw the PE-conductor can run voltage which can damage electrical devices. In the building two types of equipotential bonding can be installed, the main equipotential bonding (EB) and additional equipotential bondings. To the main equipotential bonding should be connected:

foundation earthing electrodes; central heating system; metal drain pipe; internal gas pipe; earthing conuctor for antennas;


PEN-conductor (main electrical panel) / electrical equipment; Conductive parts of the construction of the building.

The main equipotential bonding should be placed in the basement, near to the earth-level and not to far from the main electrical panel (service entrance box).

Fig.12:Internal lightning protection with a common entry of all supply lines (taken from www.dehn.de)

Fig.12 shows how we can connect the different metal installations in our building, the connection to the power supply is protected with a surge arrester. The equipotential cables can be marked like the PE-conductor, yellow and green. The thickness of these cables should be not smaller then the half of the main equipotential cable ( normally the PE-conductor of the power supply), but at least not smaller then 6mm². The equipotential bounding is connected to the foundation electrode.


Fig.13: equipotential bonding

Protection of electrical devices:


Fig.14:single-line diagram for the power supply system of the building (taken from appendix)

There are three different classes of over voltage protection. We will use all three classes of protection. - Class B Lightning Current Arrester

Single-pole arrester in the specification class B for the protection of electrical consumer installations and equipment in case of over voltage or direct lightning strikes. will protect the input side of the main electrical panel (F1)

- Class C Surge Arrester Single-pole arrester in the specification class C for the protection of electrical consumer installations and equipment against over voltage caused by distant lightning strikes or switching operations. will protect the input side of the group electrical panel (F2)

- Class D Surge Arrester for Device Protection Double-pole arrester in the specification class D for the protection of electrical installations against over voltage caused by distant lightning strikes or switching operations. will protect the electrical equipment (F3-F28)

The voltage and the current of the lightning will be reduced from the class B arrester to the class C arrester, from the class C arrester to the class D arrester and at the output side of the class D arrester the voltage and the current surge should cause no damage in the electronic devices. Protection plan of electric devices:

Fig.15: protection of the electric devices in the building

Cable length 2 should be not longer then 5 meters, so we can put it also inside the group panels on each floor. Cable length 1 should be not longer then 15m, but in our building the


cable length is between 30 and 50m. So it will be a good idea to install also a limiter link between the main electrical panel and the group panel. - Limitor Link Decoupling Inductor

Supplies the inductance for energy-control coordination between the lightning current arrester and the surge arrester when the impedance of the lines connecting these components does not provide sufficient damping.

Principle of a surge arrester :

Fig.16: principle of a surge arrester (switch) A – Isolator B – Metal C – Transfer connectors D – Break down plates E – Choke absorber A ignition tension appears in point number 1 and the spark rise until the 4th point were it’s break and spread out in two different ways. At the 6th point the small plates transform the half of spark into less powerful sparks. The small sparks that goes back, to the side of the stroke, the remove power to the stroke, and the another sparks go to the earth using PE connection.


Example for used equipment:

Fig.17: example of equipment (taken from www.abb.de)

2. Conclusion / Evaluation While we were doing this work, we got a lot of knowledge about Lightning protection. However this kind installation and the tools is very specific and so this kind of installations only can be done by a specialize company. After this work we are able to understand the principles of lightning protection and work in the area. 3. Literature:

1. IEC 61024-1 (-1), (-2) Protection of structures against lightning. 2. www.abb.de. 3. IEC 611312-1 Protection against lightning electromagnetic impulse. 4. Dehn+Sohne „Lighting Protection Guide“ www.dehn.de

4. Appendix: 1. PROB-P, appendix for report no.2. 2. Surge arrester.

lighting protection of an office building

Education

outer protection system

inner lightning protection

level protection

protection c1

protection level31

protection devices

lightning current

protection leveln g