1. Field of the Invention
The present invention relates to the field of lightning protection, and in particular to a method and apparatus for mobile lightning protection.
2. Background Art
Some vehicles are designed with sufficiently sized and proportioned metal frames, or metal skins, to protect occupants against the effects of lightning strikes. This is not typically a design goal of the vehicle. Rather it is one inherent and unintended result of the metal skin forming an approximation of a Faraday cage around the occupant. Other vehicles lack such protection. Prior art methods of insuring occupant protection are prohibitive in some applications due to size, weight and cost concerns. This problem can be better understood by a review of basic electricity of relevance to lightning protection.
Basic Electricity of Relevance to Lightning Protection
The passage of electricity through matter is caused by an electrical potential difference between two points. It causes electrical current to flow from one point to the other. The potential difference is termed the voltage and is measured in volts. In electrical engineering, one point is typically designated as a common reference for voltage and is called the xe2x80x9ccommonxe2x80x9d or xe2x80x9cgroundxe2x80x9d. It is said to have a voltage of zero, and the voltage at any other point is the potential difference between it and ground. Electrical current is measured in amperes.
When a constant voltage is applied between two points, the magnitude of the current that flows depends on the resistance of material between them. This is called resistive current. The resistance is the voltage divided by the current and is measured in ohms. One ohm is one volt divided by one ampere. If the resistance is very low, the material is said to be a good conductor. Metals are good conductors and typically have resistances much less than one ohm. If the resistance if very high, for example much greater than one million ohms, the material is an insulator. Air and solid polymers are good insulators.
When current flows through a conductor, a potential difference is developed between points along its length. This difference in potential is termed a voltage drop. If the current is constant in magnitude, the voltage drop equals the product of the current and the resistance, and it is called the resistive voltage drop. If the current magnitude varies with time, there is an additional voltage drop equal to the product of the rate of change of current and the inductance. This additional voltage drop is termed the inductive voltage drop. For lightning currents flowing through good conductors, the inductive voltage drop is always very much larger than the resistive voltage drop.
The application of constant voltage to a good insulator normally causes very little current to flow through it. However, if the voltage is increased to a very large value, an electrical discharge arc occurs. It then becomes a very good conductor and so very large discharge current flows through the arc in what was preciously an insulator. There is also another mechanism by which a small but significant current can flow through an insulator at voltages less than the discharge voltage. If the applied voltage varies with time, there is an additional current equal to the product of the rate of change of voltage and the capacitance. For voltages caused by lightning currents and good insulators, this capacitive current is always much larger than the resistive current.
The fundamental parameter which controls the flow of current through materials is the electric field. The average electric field is the applied voltage divided by the length of the material and so is measured in volts per meter. In a metallic conductor, the electric field is low, but it still has sufficient magnitude to impose a drift velocity on the free electrons that normally move at random in the metal, rather like a gas confined in a container. The drift imposed by the electric field is the source of the current flow through the conductor. In contrast, the material of a good insulator has no free electrons and that is why there is very little resistive current flow when normal voltage is applied. However, if the voltage is increased sufficiently to cause a discharge, the electric field becomes large enough to detach large numbers of electrons from the insulator""s atoms and molecules. These are now free electrons and so create the conducting discharge arc.
There is another fundamental parameter associated with the flow of electric current in a good conductor. It is the magnetic field. If the current is constant in magnitude, the magnitude of the magnetic field that surrounds the conductor is constant, much like that produced by a magnet. However, if the current varies with time, the magnetic field also varies with time. Such a time-varying magnetic field will induce a voltage in the materials that surround the conductor. If the surrounding material is also a conductor, the inducted voltages will cause circulating currents to flow in it. For lightning currents, the rates of change of current and of magnetic field are very large, and so the magnitudes of induced circulating currents will be significant. These circulating currents can be harmful to people who are close to the lightning current.
Lightning Protection
Lightning occurs when there is charge separation and positive and negative electrical charges accumulate in different parts of a cloud, like a giant battery. When the difference in electrical potential between and area of a cloud and the ground is great enough, an electrical current will arc through the air to neutralize these charges. Lightning produces an electrical discharge and results in a large current arc. Since relatively small amounts of current passing through a human being causes injury or death, it is desirable to protect against the very large currents caused by lightning strikes passing through the human body.
In some vehicles (e.g., cars and trucks), people inside the vehicle are protected against lightning strikes. Lightning strikes the vehicle, passes through the metal skin and frame of the vehicle and travels to the ground through the tires without passing through the passengers of the vehicle. The resistance of air is high enough to make the air an effective insulator if the electric field inside the vehicle is small. The mostly enclosed metal skin of the vehicle acts approximately like a Faraday cage. A Faraday cage is a conductive sphere. Inside a conductive sphere, there can be no electrical or magnetic fields. The skin of a car is neither completely enclosed nor a sphere. However, it is close enough to approximate a Faraday cage that the electric and magnetic fields inside the car are relatively small. Thus, the current does not arc to the occupants of the vehicle. Instead, the current passes through the conductive skin of the vehicle to the ground.
Some vehicles (e.g., golf carts) are not sufficiently enclosed to approximate a Faraday cage and do not provide a safe path for current to flow to ground during lightning strikes. Lightning strikes to such vehicles can arc to a passenger either directly or by a xe2x80x9cside-flashxe2x80x9d from some part of the vehicle. Additionally, strong and abrupt changes in electromagnetic fields are present in the vehicles. These fields are believed to have harmful effects on passengers.
One prior art solution is to increase the size of the frame to make the vehicle more enclosed. However, a more enclosed vehicle design is undesirable for vehicles such as golf carts because it would add expense, increase difficulty boarding and exiting the vehicle and unacceptably increase the weight and cost of the vehicle.
The present invention provides a method and apparatus for mobile lightning protection. In one embodiment of the present invention, a lightning interception rod is positioned above a vehicle. The rod is designed to intercept lightning before it strikes the vehicle. In one embodiment, the rod connects to four down-conductors to conduct the current supplied by the lightning towards the ground. One fourth of the total current supplied by the lightning flows through each down-conductor. Since each down-conductor carries less current than the total lightning strike, the voltage drop is smaller and so a passenger can be closer to each down-conductor without generating a side-flash than if the total lightning strike passed through only one down-conductor.
In one embodiment, the four down-conductors are positioned in a rectangular configuration. One down-conductor is positioned on the right side of the front of the vehicle. A second down-conductor is positioned on the left side of the front of the vehicle. A third down-conductor is positioned on the right side of the back of the vehicle, and a fourth down-conductor is positioned on the left side of the back of the vehicle. In this configuration, the down-conductors are located at a safe distance from a typical passenger. Additionally, the magnetic fields generated by the flow of current through the down-conductors act against each other. Thus, the magnitude of the magnetic field at locations inside the vehicle is reduced or eliminated, as are its harmful effects on the passenger.
In one embodiment, down-conductors are covered by a insulating material. The insulating material allows passengers to be closer to the down-conductors without a side-flash occurring. In one embodiment, the insulating material is a layer of polymer of at least 3 mm thickness.
In one embodiment, one or more chains are attached to the vehicle and allowed to contact the ground. The chains provide a path for current from the lightning strike to reach ground. In one embodiment, two chains are attached to the vehicle. One chain is positioned at the front of the vehicle and another chain is positioned at the back of the vehicle.