Conor Glettenberg English 101
Professor Turner
23 July 2014 Lightning Rod Attraction: Blunt Tip vs. Pointed Tip
Abstract:
This paper explores the effects that lightning rod tip geometry has on lightning. To determine the most effective tip geometry various types of rods including pointed, concave, blunt, flat, and conical were tested with electric pulses in a lab and in real lightning situations. It was observed that the blunt shaped rod was the most efficient shape for creating a path for lightning to the ground. In real lightning situations a smaller surface area generates far more corona emissions than a larger area. The increased corona emission almost prevents lightning from traveling the directed path. In the case of the blunt rod, the lesser emissions provided a desired path for the lightning to travel down. A lot of research was put in to find the ideal tip shape for lightning rods, and it was determined that the increased surface area directly correlates to how strong the electric field is at the tip of the lightning air terminal (LAT).

Introduction:
It is a beautiful and deadly natural occurrences. With a shock of several million volts per strike, and the capability to reach temperatures between fifteen-thousand and sixty-thousand degrees, lighting is one of the most powerful forces known to man. With all of that power lightning can cause a lot of damage. In North America lightning damage to equipment results in losses exceeding twenty-six billion dollars annually. Lightning strikes the ground around 8.6 million times a day making it nearly impossible to avoid, no matter where you live. A great deal of effort is put into protecting people and property from lighting strikes. One of the simplest and best known devices developed for lightning protection is the lightning rod. A lightning rod is a small metal rod that is normally attached to a high elevated structure. The rod is then connected to a wire that runs down the side of the building into a conductive grid. It is a common misconception that lightning rods attract lightning. The real purpose of these devices is to provide a low-resistance path to ground that can be used to conduct the enormous electrical currents when lightning strikes occur. The rod attempts to direct any destructive currents away from the structure and into the ground.

History of Lightning Rods:
Figure 1 shows the different geometries of the lightning rods used
Figure 1 shows the different geometries of the lightning rods used Lightning rods are not a newfound technology. Lightning research dates back to as early as the beginning of the 18th century. The first protection system was invented by Benjamin Franklin in the 1700s. Franklin thought he could discharge all the lightning from a storm cloud by placing a metal rod at the top of a tall building. This proved not to be true, as it is known that lightning rods merely provide an easier path for lightning to travel to the ground. Even though Franklin’s original hypothesis was incorrect he did discover an effective way to prevent lightning from striking buildings. Since Franklin’s discovery the design of lightning rods has not changed that much, but a lot of research has been put in make it even more effective. One thing that has been studied a lot over the years is the tip geometry of rods. A large amount of this research was a result of Nikola Tesla patenting the lightning rod on May 14, 1918. Tesla was able to patent the rod over Franklin because he knew that a larger area on rods was better than the original pointed design. In Fig. 1 the five common shapes for lightning rods; the standard pointed shape, concave, blunt, flat, and conical shapes are shown.

Materials:
Lightning rods are mostly made out of copper and other copper alloys like tin and zinc. It is important that the rods are very conductive so that the lightning has the most direct path. The most conductive metal known is silver. Though using silver rods would be extremely effective the cost to produce mass amounts of the rods would be too expensive. Copper is used because it is the second most conductive metal and it is much cheaper to buy. The copper rod is then connected to a huge copper or aluminum wire that travels all the way down the side of the structure to a conductive grid buried in the ground. A lot of metal is required to ensure a rod works efficiently. The copper materials found in lightning rod designs are cost efficient and conductive making it ideal for the best protection.

Setup:
Figure 2 setup of the control room
Figure 2 setup of the control room
The setup to create continuous corona emissions consists of an oval aluminum high-voltage electrode (HVE) connected to a two-nylon string attached to a two-pulley system (TPS). This two-pulley

system protrudes out from both sides of the HVE as shown in Fig. 2. The pulley either enlarges or narrows the air-gap distance between the HVE and the lightning air terminals (LAT) through the TPS aids air-gap clearance variation. The air is humidified through a nozzle-type sprinkler attached to an electric pump, to simulate the weather conditions of a real lightning storm.
Figure 3 Camera set up for corona emission measurements
Figure 3 Camera set up for corona emission measurements
To capture the corona emission activity the high-voltage electrode is connected to a 230V/100 kV high-voltage generator system. The generator consists of a 5 kilo-volt-ampere transformer and a rectifier circuit with two 140 kV, 8 kW rectifier diodes and a 25,000 pF smoothing capacitor, used to generate a high-voltage direct current (HDVC). The output of the HVDC generator must be controlled by a variable voltage regulator (VAR) since the voltage is so high. This is done by using a variable voltage regulator (VAR), which is connected at the low-voltage side of the supply system. The corona emission current measurements are carried out through a 1.2 k resistor connected to a lead that joins the tested LATs to the grounding system of the experimental rig. To protect the current-measuring system a metal-oxide varistor is used in case the system suffers from an unexpected breakdown of air insulation caused by fluctuations in the voltage. Varistors are useful because they divert the current created by the spike in voltage from important components. A LeCroy LT344L 500-MHz digital storage oscilloscope (DSO) is used to capture the corona emission signatures as shown in Fig. 3. Oscilloscopes are widely used for lightning research. The instruments are extremely precise, allowing for the best data when measuring corona emissions. The LeCroy DSO consists of four channels. These channels are used for detection and measurement purposes as well as a general-purpose interface bus (GPIB) port for external connection.

Procedure:
In this system three types of corona discharges are involved; onset streamers, negative glow, and pre-breakdown streamers. For this experiment the pre-breakdown streamer mechanism was studied. The standard pointed shape, concave, blunt, flat, and conical LATs were all placed under the high-voltage side plane electrode one at a time. The variable voltage regulator controlled the output HDVC by adjusting the power frequency main voltage. The voltage is stopped once a continuous discharge of the pre-breakdown streamers starts. The peak corona discharge was observed and recorded at this point.
A close loop equation is obtained through the use of dimensional analysis. The corona discharge current relies on numerous parameters that include permittivity, time, area of the tip that produces a corona, distance, and input voltage. The four fundamental dimensions, length, mass, time, and electric charge, may represent these parameters in their respective equations. These equations can also be represented in a dimensional matrix by using dimensional analysis to determine the relationships of the quantities. The relationships then provide a general form for equations that describe the occurrence. In experiments there are often variables that are more cared about than others. The Buckingham theorem states that the dimensionless parameters can be related. The theorem allows researchers to solve for the most relevant quantities in a particular experiment thus, providing a trusted general model of corona discharges.

Purpose of Experiment: The purpose of these experiments is to study the effect of lightning on the different tip geometries. Until recently, it was thought that the standard pointed rod was the best protection for structures. As lightning research became more and more in depth this proved not to be the case. In field tests the blunt rod appeared to be the preferred path for lighting. Scientists wanted to try and replicate these results in a lab. Testings done in the High Voltage Labratory of Universiti Teknologi Malaysia were able to generate an electric pulse in a lab since attracting real lighting to a specific spot is not possible with current technology. Researchers had to assume that the air electrification at the rod tip caused by the electric impulse was similar to that of real lightning. Without this assumption the experiment would not yield usable results. Many variables had to be considered during the experiments. Two important variables studied are the rods being ionized or not and they performed competitive test between different rod tips.
Results
Table 1 Correlation among various LAT tips with electric field strength | Standard | Concave | Blunt | Flat | Conical | E(kV/cm) | I(mA) | E(kV/cm) | I(mA) | E(kV/cm) | I(mA) | E(kV/cm) | I(mA) | E(kV/cm) | I(mA) | 5.65 | 5.83 | 5.35 | 4.31 | 7.35 | ≈0 | 8.16 | 14.75 | 8.00 | 14.75 | 5.32 | 5.53 | 4.70 | 4.00 | 7.12 | ≈0 | 7.41 | 14.17 | 7.18 | 13.83 | 4.94 | 5.07 | 4.39 | 3.86 | 6.78 | ≈0 | 6.92 | 14.00 | 6.57 | 13.08 | 4.53 | 4.39 | 4.17 | 3.76 | 6.53 | ≈0 | 6.39 | 12.33 | 6.17 | 11.50 | 4.38 | 4.13 | 3.95 | 3.65 | 6.31 | ≈0 | 6.03 | 11.08 | 5.79 | 10.42 |

Table 2: Type of comparison | Type of Rod | Number of strikes | Percentage of strikes (%) | Standard and Blunt | Standard | 8 | 40 | | Blunt | 12 | 60 | Conical and blunt | Conical | 6 | 30 | | Blunt | 14 | 70 | Concave and blunt | Concave | 11 | 55 | | Blunt | 9 | 45 | Flat and blunt | Flat | 13 | 65 | | Blunt | 7 | 35 |
Table 1 shows the size of the electric field used and the size of the corona that was a resultant. The standard rod had coronas that were of equal value of the electric field strength. The concave rod also had coronas of equal value to the field strength as well. The flat and conical rods had coronas greater than the standard and concave corona size because the surface area is greater. It can be concluded that the larger the surface area of the tip of the lightning rod the smaller the corona will be. This is backed by the results from the blunt rod. Since the blunt rob is of spherical nature, the surface area of the tip is greater than most of the other rods. With an average size electric field introduced, the corona was almost zero. With a small amount of corona the blunt rod generated less strikes than the other rod in the lab. When brought into real lightning situations however the blunt rod performed much better. Table 2 shows comparison tests between the blunt rod and the other various rods used in real lightning situations. It is logical to compare each rod to the blunt rod to determine which rod is indeed the most ideal. In the table, the comparison test is broken down into results of each individual rod. The results include the number of strikes each rod received and what that number percentage is of the total strikes created. In the end, the blunt rod attracted more total strikes than all the other ones combined.
Result Discussion: From the experiments performed above, it is obvious to conclude that the blunt rod is the new way we should be protecting ourselves from lightning strikes because of their effective ness to refrain from forming coronas and providing the most ideal path to the ground of the mass flow of electrons. The results should not be taken lightly. The standard lightning rod used all across the world should be replaced with blunt ones, it’s the new gold standard.

Figure 4
Figure 4

The smaller surface area is ideal for keeping coronas from forming, as shown in Fig. 4 above. The design of the experiment did not perform as well as scientists hoped in the lab, but it performed perfectly in field tests. Blunt rods are the best solution to lightning strike redirection. An issue that remains is the unpredictability of where lightning will strike. Scientists have not been able to accurately generate a true lightning strike in lab situations. This means that for accurate results scientists must take the risk of doing research in the field. Even then, it is impossible to determine where lightning will strike. One issue that researchers are trying to solve is designing a system that can attract lightning to a single spot every time. If this technology was made then the world’s structures would be much safer from lightning damage. We can combine strategies from both types of experiments to have the safest most effective lightning protection.

Ionization of Rods:
The first test studied was what effect ionized rods had on the experiments. For the experiment they set up a rod with the tip one meter from the ground. The electrode for the impulse is place two meters above the tip of the rod. First, the unionized rod is tested with multiple electric impulses in this setup. The rods are then illuminated for 5 minutes with Ultra-Violet lamps to create an ionized zone around the tip of the rods before any more pulses are sent. Multiple impulses are used to see the effects on the ionized rods. For all these experiment they use a digital impulse analyzing system (DIAS) to measure the actual voltage breakdown value for each experiments.

Competitive Testing: When the rods were tested against each other the set up remained mostly the same. The only difference is that the rods are place two meters apart from other and the electrode is in the center of the two tips. Two different shaped rod tips were placed together and twenty electrical pulses were sent out. The goal was to find which path the generated lightning preferred. After the tenth electric pulse the rods are switched places to ensure there is no position biased. The procedure is repeated until all the pairs were tested against the blunt rod, both ionized and unionized. They counted the amount of time the rods attracted the electric pulse and recorded the data. The goal was to see which tip would be most effective if real lightning were to strike. The scientists hoped that the larger area of the blunt tip would attract the lightning better than tip with a smaller area. After all tests were performed the blunt rod appeared to be the worst of the bunch, generating the least amount of strikes of any tip. However, most of the experiments performed in the lab could not be validated due to the main assumption researchers made claiming their generated pulses were similar to that of real lightning. Field tests would show that in real lightning situations much more coronas were generated. The high generation of these coronas formed somewhat of a shield around the other shaped tips. The shield prevented a clear path for lightning to travel to the ground. In both the lab and field tests it was observed that the blunt rod generated less coronas than the other rods. The absence of coronas on the blunt rod allowed for the lightning to travel more efficiently to the ground than the other rods. In real lightning situations the blunt rod proved most effective and is used on most structures around the world today.

Conclusion:
In an increasingly urbanized society it is important to ensure that structures created are protected from natural phenomena like lightning. The design the of the lightning rod has not changed much since its introduction in the 1700, but a lot of research has been put in to make it more efficiently direct lightning to the ground. The results of lab tests showed that the blunt shaped rod was not the ideal shape for an effectual path. The other shaped rods were able to generate more corona emissions than the blunt rod with the electric pulse. In a side by side comparison test with the other rods, the blunt rod only received a maximum of eight strikes out of the twenty generated in each test. The blunt rod also received less voltage with and without ionization. When brought into actual lightning conditions the fewer corona emissions actually proved to be useful. In fact, the other shaped rod generated far too many emissions, creating a sort of shield around the tip. This shield would sometimes prevent the lightning from traveling down the path to the ground. The tips of the rods differ slightly in size, but the addition of even a few more millimeters of surface area makes a huge difference in lightning protection.

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