ABSTRACT
The purpose of this assignment is to study several identified buildings taken around 2006 in Klang Valley appeared to have physical damage which is believe was struck by lightning. Preliminary found that some of the buildings were installed by lightning protection system but still being struck by lightning especially at the sharp edge while some were not protected and exposed to the strike. A grounding measurement was taken using one rod method and the result was analyzed. It found that not installing lightning protection, LPS installed not adhere to the standard and wrongly place the air terminal had increase a risk of the building being struck by lightning.
INTRODUCTION
Lightning is a natural phenomenon that happen due to electrical discharge between cloud to cloud (inter cloud and intra cloud) and cloud to ground (CG) generated by Cumulonimbus cloud. However in most cases, we are much interested with cloud to ground discharges because it can cause a physical damage to a structure, electrical damage to equipment as well as a threat to human safety. The discharge begins with stepped leader and it’s completed with return stroke (streamer). For multiple strokes it leads by dart leader as shown in Figure 1. The discharge was identified as negative flash (CG-) if a negative charges flow from the cloud to the ground and positive flash (CG+) if a positive charges flow from cloud to the ground.
Figure 1
There are several parameters related to lightning, for instance lightning ground flash density, thunderstorm day, isokeraunic, lightning stokes current and probability. Lightning ground flash density, Ng can be defined as the number of cloud-to-ground flashes in km-2 per year-1. Thunderstorm day, TD is also called keraunic level and defined as a local calendar day during which thunder is heard at least once at a given station while isokeraunic level map is a map of thunderstorm days in a year. The lightning ground flash density Ng is an important meteorological data that is used in calculating the risk of lightning strikes to a structure or system. The Ng used to be calculated from the annual thunderstorm days TD. A more accurate method of determining the Ng is by the use of a lightning detection system (LDS).
Malaysia is situated in a very active region of thunderstorm activity. Figure 2 shows a keraunic level map for Peninsular Malaysia. For further understanding about lightning behavior at that location, normally a study was done in term of temporal analysis and spatial analysis.
Figure 2
In 1995, Peninsular Malaysia was estimated to have average ground flash density (Ng) 4.40 flashes/sq.km/year[1]. 4% of the flashes were positive flash as shown in Table 1
Table 1
METHODS OF LIGHTNING PROTECTION
Lightning protection is essential for the protection of humans, structures, contents within structures, transmission lines and electrical equipment from thermal, mechanical and electrical effects caused by lightning discharges. Lightning cannot be prevented, but it can with some success be intercepted, and its current can be conducted to a grounding system without side flashes where it is harmlessly dissipated.
The methods of lightning protection are well described using the zone of protection concepts for a structure. The zone of protection is the space within the volume to be protected and where the lightning protection air termination is located. Air termination can have different forms such as:
• Rods;
• Stretched wires; and
• Meshed conductors.
In order to place the air termination in a proper position on the building to be protected, the following methods can be used:
• The protective angle method.
• The rolling sphere method.
• The mesh method.
• Faraday cage method.
Protective Angle Method
The structure to be protected lies within an imaginary cone ABC with the highest point of the structure providing protection as shown in Figure 3. Table 2 provides the recommended values for the cone angle, α (in degrees).
Table 2
The protection angle method is suitable for simple buildings. If the height is larger than the rolling sphere radius, R, the protection angle method cannot be used.
The zone of protection offered by an air termination system is considered to be 45º for heights up to 20m. Above this height, the zone of protection is determined by the rolling sphere method.
Figure 3
Rolling Sphere Method
The rolling sphere method is used when we are dealing with complex structures. When the use of the protective angle method is excluded, the rolling sphere method is used to identify the protected volume of the structure.
This involves rolling an imaginary sphere of radius given in Table 3 over a structure. The areas touched by the sphere are deemed to require protection as shown in Figure 4. On tall structures, this can obviously include the sides of the building.
Table 3
Figure 4
If two parallel horizontal LPS air terminations are placed above the horizontal reference plane as shown in Figure 5, the penetration distance p of the rolling sphere below the level of the air terminations is calculated as follows:
Equation 1
p is the penetration distance of rolling sphere, R is the radius of rolling sphere and d is the distance separating two air terminations.
Figure 5
1 – Two air termination, 2- horizontal reference plane, 3- whole protected area, ht- physical height of the air termination above the reference plane. This height is given in Table 1.
The Mesh Method
The mesh method is suitable for protecting flat surfaces. BS 6651 recommends that on high risk structures such as explosive factories and thatched roofs, no part of the roof should be more than 2.5m from an air termination conductor. This is generally achieved by applying a 5m x 10m mesh to the roof.
However, for most structures, a mesh of 10m x 20m is considered sufficient, giving a maximum distance from any part of the roof to the nearest conductor of 5m. All metal structures on the roof would be interconnected using this mesh.
The recommended sizes for the mesh according to their protection level are listed in Table 4.
Table 4
Faraday Cage Method
Electromagnetic radiation is associated with lightning and can cause damages to the sophisticated electronics in a building. Prevention of such intrusion of undesired electromagnetic impulses through both radiation and conduction is termed Electro Magnetic Compatibility (EMC). “Faraday Cage” is the usual method of radiation prevention, in which building will be screened with the conductive materials so that the structure behaves like cage as shown in Figure 6. The interior of a completely enclosed metal shell is free from the effects of any external changes of electric field. This method of protection is provided to the part of the building where sophisticated electronics are installed (such as computer rooms, medical theatres and scanning rooms, control chambers of power plants, airports, military bases, and communication bases etc.).
Figure 6
LIGHTNING PROTECTION - ROD
The standard LPS which are comply with the national or international technical standard that has been scientifically proven to provide safety to users. The standard LPS component consists of conventional air terminal, down conductor and earth terminal [2].
The main component of standard LPS is conventional air terminal (Franklin rod) which has been invented by Benjamin Franklin in 18th century. After more than 350 years, the Franklin rod is still in use throughout the whole world and has been scientifically validated by two major studies group which was AGU and Federal Interagency Lightning Protection User Group. It is a passive device and serves as a sacrificial device when the lightning strikes it rather than building structure. It can be installed at various locations on the rooftop that are likely strike by lightning. Therefore, the building is protected by direct lightning strike [2]. Figure 7 shown Conventional lightning rod installed on the building rooftop.
Figure 7
The ESE Air Terminal is a device which creates an upward propagating streamer faster than conventional lightning rod (Franklin rod). When downward leader from the thunderstorm is approaching to ground surface, ESE Air Terminal has short initiation time in microseconds of creates an upward streamer compare to conventional lightning rod. There are different types of ESE Air Terminal on the market today. Each type of ESE Air Terminal has a different protective radius stated by its manufacture [3] [4]. Figure 8 below shown ESE Air Terminal lightning rod installed on the building rooftop.
Figure 8
FACTORS AFFECTING SOIL RESISTIVITY
Factors that affecting soil resistivity may be summarized as below:
a) Type of earth (eg, clay, loam, sandstone, granite).
b) Stratification; layers of different types of soil (eg, loam backfill on a clay base).
c) Moisture content; resistivity may fall rapidly as the moisture content is increased, however after a value of about 20% the rate of decrease is much less. Soil with content greater than 40% does not occur very often.
d) Temperature; above freezing point, the effect on earth resistivity is practically negligible
e) Chemical composition and concentration of dissolved salt.
f) Presence of metal and concrete pipes, tanks, large slabs, cable ducts, rail tracks, metal pipes and fences
g) Topography; rugged topography has a similar effect on resistivity measurement as local surface resistivity variation caused by weathering and moisture
SOIL RESISTIVITY TEST METHOD
Wenner Method
Figure 9
In the Wenner method, all four electrodes are moved for each test with the spacing between each adjacent pair remaining the same. The Wenner array is the least efficient from an operational perspective. It requires the longest cable layout, largest electrode spreads and for large spacings one person per electrode is necessary to complete the survey in a reasonable time. Also, because all four electrodes are moved after each reading the Wenner Array is most susceptible to lateral variation effects.
However the Wenner array is the most efficient in terms of the ratio of received voltage per unit of transmitted current. Where unfavorable conditions such as very dry or frozen soil exist, considerable time may be spent trying to improve the contact resistance between the electrode and the soil.
Equation 2
Where ρaw = apparent resistivity (Ω) I = injected current (Amps)
a = probe spacing (m) R = measured resistance (Ω)
Δv = voltage measured (volts)
Schlumberger Method
Most widely used in electrical prospecting. Two current electrodes may be placed a large distance apart and the potential electrodes moved along the middle third of the line.
Economy of manpower is gained with the Schlumberger array since the outer electrodes are moved four or five times for each move of the inner electrodes. The reduction in the number of electrode moves also reduces the effect of lateral variation on test results.
Considerable time saving can be achieved by using the reciprocity theorem with the Schlumberger array when contact resistance is a problem. Since contact resistance normally affects the current electrodes more than the potential electrodes, the inner fixed pair may be used as the current electrodes, a configuration called the ‘Inverse Schlumberger Array’. Use of the inverse Schlumberger array increases personal safety when a large current is injected. Heavier current cables may be needed if the current is of large magnitude. The inverse Schlumberger reduces the heavier cable lengths and time spent moving electrodes. The minimum spacing accessible is in the order of 10 m (for a 0.5m inner spacing), thereby, necessitating the use of the Wenner configuration for smaller spacings.
Lower voltage readings are obtained when using Schlumberger arrays. This may be a critical problem where the depth required to be tested is beyond the capability of the test equipment or the voltage readings are too small to be considered.
Figure 10
With the Schlumberger array the potential electrodes remain stationary while the current electrodes are moved for a series of measurements. In each method the depth penetration of the electrodes is less than 5% of the separation to ensure that the approximation of point sources, required by the simplified formulae, remains valid.
Schlumberger array
Equation 3
Where ρas = apparent resistivity (Ωm)
l = distance from centre line to inner probes (m)
L = distance from centre line to outer probes (m)
R = measured resistance (Ω)
One rod method
This is the simplest method compared to Wenner and Schlumberger method above. The test is based on measuring the resistance of a single rod that is driven into the ground for a known depth. The resistance measurement and rod dimensions are then used to calculate the average soil resistivity required to produce the measured resistance.
The earth resistance of a rod will usually reduce as its driven depth is increased. The resistance of a rod should never increase with driven depth. It is the rate at which the resistance decreases with depth that allows the soil structure and layer resistivity to be determined. Soil structure where the deeper layer has a lower resistivity than the upper, will produce sudden changes in the gradient of the rod’s resistance curve. Where the top layer has a lower resistivity than the lower layers then the structure is more difficult to determine, as the test current will tend to continue to flow in the top, lower resistivity layer. The resulting low current density in the higher resistivity layer has little influence on the measured of the rod. Where a very high resistivity stratum is penetrated, the rod resistance may remain virtually constant with increasing depth. If a further lower resistivity layer is penetrated beneath this, then the rod resistance will again begin to decrease with increasing depth.
CASE STUDY 1: RISDA building, Jalan Ampang
The building was struck by lightning at the corner roof structure, where the ESE air terminal installed about 30metres away at the highest location from the corner roof structure. This was primarily due to air terminal is not effective in providing sufficient protection on all the roof structure.
Figure 11
From our observation recently, the defect at the building had been rectified by RISDA management. To improve the lightning protection, the management had increased the number of lightning conductor at strategic point on top of the building to enlarge the protection radius in case if the lighting strikes to the building. A lightning conductor mounted on top of a building and electrically connected to the ground through a wire, to protect the building from damage due to lightning strikes. In case if the lightning strikes to the building it will preferentially strike the rod and be conducted to ground through the wire, instead of passing through the building.
Figure 12
Figure 13
To measure soil resistivity of the RISDA building, we had used driven rod method or also called the three probe method or three pin method. The purpose of resistivity testing is to obtain a set of measurements which may be interpreted to yield an equivalent model for the electrical performance of the earth, as seen by the particular earthing system.
From the investigation of soil resistivity of the RISDA area it shows that in many locations, the types of soil are sand and it can be quite homogenous. A set of readings taken with various probe spacing gives a set of resistivity (see Appendix 1). The range of soil resistivity is between 371 Ωm – 445 Ωm. The factors chiefly affecting soil resistivity is type of soil, climate, seasonal conditions and others factors. From all of these factors, there is a large variation of soil resistivity between different soil types and moisture contents.
In addition, the weather was around 32 ºC measurement reading taken and it was contributed to a low resistance value since the condition of the soil was dry with low percentage of moisture content.
CASE STUDY 2: The Mall, Kuala Lumpur
The Mall is located at Jalan Putra, Kuala Lumpur. The Legend Hotel is located beside The Mall. Currently, the lightning struck below the edge of the roof still visible from the distance can be shown in Figure 12 and haven’t repaired it. So, our group member will use KYORITSU (Model 4102) to measure the grounding resistivity on 25-9-2010 Saturday. After measurement has been done, we record our results on the earth resistivity measurement report.
Figure14
Figure15
From the report that we recorded (see Appendix 2), we found that the grounding resistivity is relativity small. It means that The Mall of the grounding resistivity is in good condition. Since The Mall did not install with any lightning rod at their rooftop, The Legend Hotel was installed with ESE Air Terminal on their rooftop which was higher height than The Mall can be shown in Figure 13. Although The Legend Hotel was installed ESE Air Terminal, but the lightning struck below the edge of the roof at The Mall and The Legend Hotel. The reason is because The Legend Hotel installed with non-standard LPS. The non-standard LPS which are did not comply with the national or international technical standard that has been scientifically proven to provide safety to users. Besides that, the non-standard LPS are usually easier and cheaper to install compared to standard LPS [2]. So, the consultant of the building installed the ESE Air terminal at The Legend Hotel did not follow according to the MS IEC 61024-1-1:2001 Malaysia Standard.
CASE STUDY 3: General Hospital, Kuala Lumpur
In set picture taken in 2006 shown that the edge of the building was struck by lightning that cause a physical damage to the building as shown in Figure 14. From our observation in a visit recently to the building found that the building was left unoccupied for quite some time, this could justify the damage remains unrepaired. It was understand that the building will undergo major renovation to improve services.
Figure 16
Figure 17
Figure 18
Figure 18, shown that no lightning protection was installed on top of that old building. Alternatively, water piping was used to measure soil resistivity at that area and the result shows it has a good soil resistivity (see appendix 3).
CASE STUDY 4: SuCasa Service Apartment, Kuala Lumpur
SuCasa Service Apartment is located at Jalan Ampang, Kuala Lumpur. The soil resistivity test was carried out on October 9, 2010 using one-rod method. The lightning protection system installed at the building is a conventional franklin rod type. We found that this building is exposed to the lightning strike because of the earth resistivity is quite high (see Appendix 4). This may be regarding to the soil type at that area (sand/gravel).
Figure 19
CONCLUSION
As a summary to the case study:
1) Case Study 1: Risda.
• Lightning protection was installed but not according to the standard, hence insufficient protection to the building.
• An improvement was done by increasing the number of air terminals at a strategic location.
2) Case Study 2: The Mall.
• Grounding measurement shows a relatively small resistivity and good for conductivity.
• No lightning protection system installed has exposed the building to lightning strike.
3) Case Study 3: General Hospital.
• Grounding measurement shows a relatively small resistivity and good for conductivity.
• No lightning protection system installed has exposed the building to lightning strike.
4) Case Study 4: Sucasa Service Apartment.
• Lightning protection was installed but not according to the standard, hence insufficient protection to the building.
• High grounding resistivity at that area also contribute to the failure of lightning protection.
Based on the case study, it can be concluded adhering to the standard is important when installing lightning protection to ensure the building is fully protected. All figures above shown that the lightning will strike at the edge due to charge density is high at a sharp edge. Hence by placing the air terminal at a strategic location also can reduce a risk of being struck by lightning.
REFERENCES
[1] The Study of Lightning Ground Flash Phenomena in Peninsular Malaysia, MD Ahsanul Alam, 1996.
[2] Conventional and Un-conventional Lightning Air Terminal: An Overview, Hartono Zainal Abidin & Robiah Ibrahim, 8th January 2004.
[3] Evaluation of Early Streamer Emission Air Terminal, Scott D. Mclvor, Roy B. Carpenter, Jr., Mark M. Drabkin.
[4] Eritech System 1000: ESE Lightning Protection Products.
APPENDIX
Appendix 1: measurement taken at RISDA
Appendix 2: measurement taken at The Mall
Appendix 3: measurement taken at HKL
Appendix 4: measurement taken at Sucasa Service Apartment.
GREAT EFFORT: MMSC Consultancy Team Member
MMSC member
The purpose of this assignment is to study several identified buildings taken around 2006 in Klang Valley appeared to have physical damage which is believe was struck by lightning. Preliminary found that some of the buildings were installed by lightning protection system but still being struck by lightning especially at the sharp edge while some were not protected and exposed to the strike. A grounding measurement was taken using one rod method and the result was analyzed. It found that not installing lightning protection, LPS installed not adhere to the standard and wrongly place the air terminal had increase a risk of the building being struck by lightning.
INTRODUCTION
Lightning is a natural phenomenon that happen due to electrical discharge between cloud to cloud (inter cloud and intra cloud) and cloud to ground (CG) generated by Cumulonimbus cloud. However in most cases, we are much interested with cloud to ground discharges because it can cause a physical damage to a structure, electrical damage to equipment as well as a threat to human safety. The discharge begins with stepped leader and it’s completed with return stroke (streamer). For multiple strokes it leads by dart leader as shown in Figure 1. The discharge was identified as negative flash (CG-) if a negative charges flow from the cloud to the ground and positive flash (CG+) if a positive charges flow from cloud to the ground.
Figure 1
There are several parameters related to lightning, for instance lightning ground flash density, thunderstorm day, isokeraunic, lightning stokes current and probability. Lightning ground flash density, Ng can be defined as the number of cloud-to-ground flashes in km-2 per year-1. Thunderstorm day, TD is also called keraunic level and defined as a local calendar day during which thunder is heard at least once at a given station while isokeraunic level map is a map of thunderstorm days in a year. The lightning ground flash density Ng is an important meteorological data that is used in calculating the risk of lightning strikes to a structure or system. The Ng used to be calculated from the annual thunderstorm days TD. A more accurate method of determining the Ng is by the use of a lightning detection system (LDS).
Malaysia is situated in a very active region of thunderstorm activity. Figure 2 shows a keraunic level map for Peninsular Malaysia. For further understanding about lightning behavior at that location, normally a study was done in term of temporal analysis and spatial analysis.
Figure 2
In 1995, Peninsular Malaysia was estimated to have average ground flash density (Ng) 4.40 flashes/sq.km/year[1]. 4% of the flashes were positive flash as shown in Table 1
Table 1
METHODS OF LIGHTNING PROTECTION
Lightning protection is essential for the protection of humans, structures, contents within structures, transmission lines and electrical equipment from thermal, mechanical and electrical effects caused by lightning discharges. Lightning cannot be prevented, but it can with some success be intercepted, and its current can be conducted to a grounding system without side flashes where it is harmlessly dissipated.
The methods of lightning protection are well described using the zone of protection concepts for a structure. The zone of protection is the space within the volume to be protected and where the lightning protection air termination is located. Air termination can have different forms such as:
• Rods;
• Stretched wires; and
• Meshed conductors.
In order to place the air termination in a proper position on the building to be protected, the following methods can be used:
• The protective angle method.
• The rolling sphere method.
• The mesh method.
• Faraday cage method.
Protective Angle Method
The structure to be protected lies within an imaginary cone ABC with the highest point of the structure providing protection as shown in Figure 3. Table 2 provides the recommended values for the cone angle, α (in degrees).
Table 2
The protection angle method is suitable for simple buildings. If the height is larger than the rolling sphere radius, R, the protection angle method cannot be used.
The zone of protection offered by an air termination system is considered to be 45º for heights up to 20m. Above this height, the zone of protection is determined by the rolling sphere method.
Figure 3
Rolling Sphere Method
The rolling sphere method is used when we are dealing with complex structures. When the use of the protective angle method is excluded, the rolling sphere method is used to identify the protected volume of the structure.
This involves rolling an imaginary sphere of radius given in Table 3 over a structure. The areas touched by the sphere are deemed to require protection as shown in Figure 4. On tall structures, this can obviously include the sides of the building.
Table 3
Figure 4
If two parallel horizontal LPS air terminations are placed above the horizontal reference plane as shown in Figure 5, the penetration distance p of the rolling sphere below the level of the air terminations is calculated as follows:
Equation 1
p is the penetration distance of rolling sphere, R is the radius of rolling sphere and d is the distance separating two air terminations.
Figure 5
1 – Two air termination, 2- horizontal reference plane, 3- whole protected area, ht- physical height of the air termination above the reference plane. This height is given in Table 1.
The Mesh Method
The mesh method is suitable for protecting flat surfaces. BS 6651 recommends that on high risk structures such as explosive factories and thatched roofs, no part of the roof should be more than 2.5m from an air termination conductor. This is generally achieved by applying a 5m x 10m mesh to the roof.
However, for most structures, a mesh of 10m x 20m is considered sufficient, giving a maximum distance from any part of the roof to the nearest conductor of 5m. All metal structures on the roof would be interconnected using this mesh.
The recommended sizes for the mesh according to their protection level are listed in Table 4.
Table 4
Faraday Cage Method
Electromagnetic radiation is associated with lightning and can cause damages to the sophisticated electronics in a building. Prevention of such intrusion of undesired electromagnetic impulses through both radiation and conduction is termed Electro Magnetic Compatibility (EMC). “Faraday Cage” is the usual method of radiation prevention, in which building will be screened with the conductive materials so that the structure behaves like cage as shown in Figure 6. The interior of a completely enclosed metal shell is free from the effects of any external changes of electric field. This method of protection is provided to the part of the building where sophisticated electronics are installed (such as computer rooms, medical theatres and scanning rooms, control chambers of power plants, airports, military bases, and communication bases etc.).
Figure 6
LIGHTNING PROTECTION - ROD
The standard LPS which are comply with the national or international technical standard that has been scientifically proven to provide safety to users. The standard LPS component consists of conventional air terminal, down conductor and earth terminal [2].
The main component of standard LPS is conventional air terminal (Franklin rod) which has been invented by Benjamin Franklin in 18th century. After more than 350 years, the Franklin rod is still in use throughout the whole world and has been scientifically validated by two major studies group which was AGU and Federal Interagency Lightning Protection User Group. It is a passive device and serves as a sacrificial device when the lightning strikes it rather than building structure. It can be installed at various locations on the rooftop that are likely strike by lightning. Therefore, the building is protected by direct lightning strike [2]. Figure 7 shown Conventional lightning rod installed on the building rooftop.
Figure 7
The ESE Air Terminal is a device which creates an upward propagating streamer faster than conventional lightning rod (Franklin rod). When downward leader from the thunderstorm is approaching to ground surface, ESE Air Terminal has short initiation time in microseconds of creates an upward streamer compare to conventional lightning rod. There are different types of ESE Air Terminal on the market today. Each type of ESE Air Terminal has a different protective radius stated by its manufacture [3] [4]. Figure 8 below shown ESE Air Terminal lightning rod installed on the building rooftop.
Figure 8
FACTORS AFFECTING SOIL RESISTIVITY
Factors that affecting soil resistivity may be summarized as below:
a) Type of earth (eg, clay, loam, sandstone, granite).
b) Stratification; layers of different types of soil (eg, loam backfill on a clay base).
c) Moisture content; resistivity may fall rapidly as the moisture content is increased, however after a value of about 20% the rate of decrease is much less. Soil with content greater than 40% does not occur very often.
d) Temperature; above freezing point, the effect on earth resistivity is practically negligible
e) Chemical composition and concentration of dissolved salt.
f) Presence of metal and concrete pipes, tanks, large slabs, cable ducts, rail tracks, metal pipes and fences
g) Topography; rugged topography has a similar effect on resistivity measurement as local surface resistivity variation caused by weathering and moisture
SOIL RESISTIVITY TEST METHOD
Wenner Method
Figure 9
In the Wenner method, all four electrodes are moved for each test with the spacing between each adjacent pair remaining the same. The Wenner array is the least efficient from an operational perspective. It requires the longest cable layout, largest electrode spreads and for large spacings one person per electrode is necessary to complete the survey in a reasonable time. Also, because all four electrodes are moved after each reading the Wenner Array is most susceptible to lateral variation effects.
However the Wenner array is the most efficient in terms of the ratio of received voltage per unit of transmitted current. Where unfavorable conditions such as very dry or frozen soil exist, considerable time may be spent trying to improve the contact resistance between the electrode and the soil.
Equation 2
Where ρaw = apparent resistivity (Ω) I = injected current (Amps)
a = probe spacing (m) R = measured resistance (Ω)
Δv = voltage measured (volts)
Schlumberger Method
Most widely used in electrical prospecting. Two current electrodes may be placed a large distance apart and the potential electrodes moved along the middle third of the line.
Economy of manpower is gained with the Schlumberger array since the outer electrodes are moved four or five times for each move of the inner electrodes. The reduction in the number of electrode moves also reduces the effect of lateral variation on test results.
Considerable time saving can be achieved by using the reciprocity theorem with the Schlumberger array when contact resistance is a problem. Since contact resistance normally affects the current electrodes more than the potential electrodes, the inner fixed pair may be used as the current electrodes, a configuration called the ‘Inverse Schlumberger Array’. Use of the inverse Schlumberger array increases personal safety when a large current is injected. Heavier current cables may be needed if the current is of large magnitude. The inverse Schlumberger reduces the heavier cable lengths and time spent moving electrodes. The minimum spacing accessible is in the order of 10 m (for a 0.5m inner spacing), thereby, necessitating the use of the Wenner configuration for smaller spacings.
Lower voltage readings are obtained when using Schlumberger arrays. This may be a critical problem where the depth required to be tested is beyond the capability of the test equipment or the voltage readings are too small to be considered.
Figure 10
With the Schlumberger array the potential electrodes remain stationary while the current electrodes are moved for a series of measurements. In each method the depth penetration of the electrodes is less than 5% of the separation to ensure that the approximation of point sources, required by the simplified formulae, remains valid.
Schlumberger array
Equation 3
Where ρas = apparent resistivity (Ωm)
l = distance from centre line to inner probes (m)
L = distance from centre line to outer probes (m)
R = measured resistance (Ω)
One rod method
This is the simplest method compared to Wenner and Schlumberger method above. The test is based on measuring the resistance of a single rod that is driven into the ground for a known depth. The resistance measurement and rod dimensions are then used to calculate the average soil resistivity required to produce the measured resistance.
The earth resistance of a rod will usually reduce as its driven depth is increased. The resistance of a rod should never increase with driven depth. It is the rate at which the resistance decreases with depth that allows the soil structure and layer resistivity to be determined. Soil structure where the deeper layer has a lower resistivity than the upper, will produce sudden changes in the gradient of the rod’s resistance curve. Where the top layer has a lower resistivity than the lower layers then the structure is more difficult to determine, as the test current will tend to continue to flow in the top, lower resistivity layer. The resulting low current density in the higher resistivity layer has little influence on the measured of the rod. Where a very high resistivity stratum is penetrated, the rod resistance may remain virtually constant with increasing depth. If a further lower resistivity layer is penetrated beneath this, then the rod resistance will again begin to decrease with increasing depth.
CASE STUDY 1: RISDA building, Jalan Ampang
The building was struck by lightning at the corner roof structure, where the ESE air terminal installed about 30metres away at the highest location from the corner roof structure. This was primarily due to air terminal is not effective in providing sufficient protection on all the roof structure.
Figure 11
From our observation recently, the defect at the building had been rectified by RISDA management. To improve the lightning protection, the management had increased the number of lightning conductor at strategic point on top of the building to enlarge the protection radius in case if the lighting strikes to the building. A lightning conductor mounted on top of a building and electrically connected to the ground through a wire, to protect the building from damage due to lightning strikes. In case if the lightning strikes to the building it will preferentially strike the rod and be conducted to ground through the wire, instead of passing through the building.
Figure 12
Figure 13
To measure soil resistivity of the RISDA building, we had used driven rod method or also called the three probe method or three pin method. The purpose of resistivity testing is to obtain a set of measurements which may be interpreted to yield an equivalent model for the electrical performance of the earth, as seen by the particular earthing system.
From the investigation of soil resistivity of the RISDA area it shows that in many locations, the types of soil are sand and it can be quite homogenous. A set of readings taken with various probe spacing gives a set of resistivity (see Appendix 1). The range of soil resistivity is between 371 Ωm – 445 Ωm. The factors chiefly affecting soil resistivity is type of soil, climate, seasonal conditions and others factors. From all of these factors, there is a large variation of soil resistivity between different soil types and moisture contents.
In addition, the weather was around 32 ºC measurement reading taken and it was contributed to a low resistance value since the condition of the soil was dry with low percentage of moisture content.
CASE STUDY 2: The Mall, Kuala Lumpur
The Mall is located at Jalan Putra, Kuala Lumpur. The Legend Hotel is located beside The Mall. Currently, the lightning struck below the edge of the roof still visible from the distance can be shown in Figure 12 and haven’t repaired it. So, our group member will use KYORITSU (Model 4102) to measure the grounding resistivity on 25-9-2010 Saturday. After measurement has been done, we record our results on the earth resistivity measurement report.
Figure14
Figure15
From the report that we recorded (see Appendix 2), we found that the grounding resistivity is relativity small. It means that The Mall of the grounding resistivity is in good condition. Since The Mall did not install with any lightning rod at their rooftop, The Legend Hotel was installed with ESE Air Terminal on their rooftop which was higher height than The Mall can be shown in Figure 13. Although The Legend Hotel was installed ESE Air Terminal, but the lightning struck below the edge of the roof at The Mall and The Legend Hotel. The reason is because The Legend Hotel installed with non-standard LPS. The non-standard LPS which are did not comply with the national or international technical standard that has been scientifically proven to provide safety to users. Besides that, the non-standard LPS are usually easier and cheaper to install compared to standard LPS [2]. So, the consultant of the building installed the ESE Air terminal at The Legend Hotel did not follow according to the MS IEC 61024-1-1:2001 Malaysia Standard.
CASE STUDY 3: General Hospital, Kuala Lumpur
In set picture taken in 2006 shown that the edge of the building was struck by lightning that cause a physical damage to the building as shown in Figure 14. From our observation in a visit recently to the building found that the building was left unoccupied for quite some time, this could justify the damage remains unrepaired. It was understand that the building will undergo major renovation to improve services.
Figure 16
Figure 17
Figure 18
Figure 18, shown that no lightning protection was installed on top of that old building. Alternatively, water piping was used to measure soil resistivity at that area and the result shows it has a good soil resistivity (see appendix 3).
CASE STUDY 4: SuCasa Service Apartment, Kuala Lumpur
SuCasa Service Apartment is located at Jalan Ampang, Kuala Lumpur. The soil resistivity test was carried out on October 9, 2010 using one-rod method. The lightning protection system installed at the building is a conventional franklin rod type. We found that this building is exposed to the lightning strike because of the earth resistivity is quite high (see Appendix 4). This may be regarding to the soil type at that area (sand/gravel).
Figure 19
CONCLUSION
As a summary to the case study:
1) Case Study 1: Risda.
• Lightning protection was installed but not according to the standard, hence insufficient protection to the building.
• An improvement was done by increasing the number of air terminals at a strategic location.
2) Case Study 2: The Mall.
• Grounding measurement shows a relatively small resistivity and good for conductivity.
• No lightning protection system installed has exposed the building to lightning strike.
3) Case Study 3: General Hospital.
• Grounding measurement shows a relatively small resistivity and good for conductivity.
• No lightning protection system installed has exposed the building to lightning strike.
4) Case Study 4: Sucasa Service Apartment.
• Lightning protection was installed but not according to the standard, hence insufficient protection to the building.
• High grounding resistivity at that area also contribute to the failure of lightning protection.
Based on the case study, it can be concluded adhering to the standard is important when installing lightning protection to ensure the building is fully protected. All figures above shown that the lightning will strike at the edge due to charge density is high at a sharp edge. Hence by placing the air terminal at a strategic location also can reduce a risk of being struck by lightning.
REFERENCES
[1] The Study of Lightning Ground Flash Phenomena in Peninsular Malaysia, MD Ahsanul Alam, 1996.
[2] Conventional and Un-conventional Lightning Air Terminal: An Overview, Hartono Zainal Abidin & Robiah Ibrahim, 8th January 2004.
[3] Evaluation of Early Streamer Emission Air Terminal, Scott D. Mclvor, Roy B. Carpenter, Jr., Mark M. Drabkin.
[4] Eritech System 1000: ESE Lightning Protection Products.
APPENDIX
Appendix 1: measurement taken at RISDA
Appendix 2: measurement taken at The Mall
Appendix 3: measurement taken at HKL
Appendix 4: measurement taken at Sucasa Service Apartment.
GREAT EFFORT: MMSC Consultancy Team Member
MMSC member
Plate No: 1: Lightning struck below the edge of the roof at The Mall, Kuala Lumpur where that area is not protected by lightning protection system.
(a)
(b)
Plate No. 8: Lightning struck at the roof edge of Kuala Lumpur General Hospital
