Lightning Rod Attraction: Blunt Tip Vs. Pointed Tip

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:

        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. [pic 1][pic 2]

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:

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 [pic 3][pic 4]

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.

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[pic 5][pic 6]

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.

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