Electrical Fundamentals

Introduction  

The paper first discusses the difference between conductors, semiconductors and insulators. The paper goes on to explain what is static electricity, the danger situations caused by static electricity and offers some and information on the origin of the static electricity in different environment is also highlighted. The paper gives some less known facts about the always present danger and illustrates static electricity, its dangers and consequences on aviation as well as preventive measures for fire and explosion protection.

Molecular structure of conductor, semiconductor and insulators

The difference between conductors, semiconductors and insulators lie in their molecular structure. Conductors are the best conductors of electricity while insulators do not conduct electricity at all under normal circumstances (Robertson, 2001). The conductivity in the semiconductors is between that of insulators and the conductors. The molecules of different materials are made up of atoms. The atoms are made up of electrons which surrounds the nucleus of the atoms.  There are two bands surrounding the nucleus which the electrons occupy. These are the conducting band and the non conducting band. The molecules in conducting materials have electrons in the conducting band. This makes it possible for the electrons to move freely making them good conductor of electricity. On the other hand the insulators have their electrons in the non conducting zone.

In these materials also the gap between the two bands is very big. It therefore follows that this materials are not able to conduct electricity. For the semi conducting materials the gap between conducting band and the non conducting band is small and the electrons can be easily excited to move to the conducting band. This can be done by increasing the temperatures of the material and this is the reason why the conductivity of semi-conductors improves with the increase in the temperature. Semiconductor devises consists of diodes, transistors, SCR’s LED’s and triacs. Pure semiconductors are usually referred to us intrinsic semiconductor. The conductivity (the ability to conduct electricity) of the semiconductor material is drastically changed by adding other elements. This other elements are usually referred to as impurities and the process of adding is called doping. Examples of semiconductors are silicon, selenium and germanium while the impurities are phosphorus and boron (Alexander and Sadiku, 2005).

Turboelectric effect is a phenomenon where materials with weakly bound electrons tend to loose them ,while materials with sparsely filled outer shells tend to gain them resulting in one of the material being positively charged while the other being negatively charged. The polarity and strength of the charge on a material once they separated depends on the relative positions of the material in the turboelectric series.

Static electricity in aircrafts

Static electricity is generated whenever two substances, one of which is an insulator, are split up if they were initially held together. Initially the object will be electrically neutral but when the objects are moved this brings about a difference in electrical charge between the objects. One of the substances becomes positively charged while the other will have negative charges. The objects electrical charge depends on the speed of surface friction, size of the contact surface and the humidity. The substance which has been charged tends to loose the charges to become neutral. This can be very dangerous especially in an explosive atmosphere as it is accompanied by production of sparks. With enough energy from the spark detonation will occur resulting in fire.

One of the likely cause of the static electricity in the aircrafts both at the ground and when in the air is the movement of fuel in the pipelines. When the fluid circulates in the pipeline there is friction between the pipe surface and the fluid layer also between the different layers of the fluid. The fluid particle in the middle of the pipeline has the highest speed. Because of the difference in the speed of the fluid and the friction fluid bubbles and also due to the poor conduction of the fluid we have the fluid splitting into ions which in turn result in the accumulation of static electricity.  When the poor conduction goes through a grounded pipeline where there is positive electrical charge in the pipeline and near the entrance to the ungrounded fuel tank. Inside the tank fuel tank there will arise a negative electrical charge while outside the tank being positively charged. When the electrical charge accumulates and electric potential in the fuel tanks to an appreciable value the following will occur

• A spark jumping from outside fuel tank to the ground
• A spark jumping from inside surface of the fuel to the surface of the tank.

Because of this free voltage at the surface of the fluid, fuel tanking with grounded fuel-carrying vehicle isn’t a guarantee to eliminate static electricity. The spark can jump from surface of the fuel to the side of fuel tank, and if energy of the spark is big enough, it can to start detonation in explosive environment (Alexander and Sadiku, 2005).

         Aircrafts always have a positive charge when they are flying. At the normal altitude of the 10 000m where commercial jet aircraft fly, the atmosphere naturally carries a positive charge of approximately 100, 000V. This is caused by the existence of an electrical field that exists between the Earth and the ionosphere, the ionosphere being approximately 300 000V. The positive which is induced by friction between the plane and the air as it flies is added to the existing 100,000V. The low air humidity at the altitudes is a perfect environment that fosters huge electrostatic charges. The plane can acquire a charge of up to one million Volt or even more, which shares with the passengers inside the pane. During landing in dry conditions, huge sparks are usually released to Earth as soon as the aircraft touches the ground

Static electricity and lightening problems

Electronic components devices which are used in the field of electronics are extremely sensitive to the presence of static electricity and can be damaged by static discharge. Discharge of static charges has been known to create severe hazards in industries dealing with flammable substances, where a small electrical spark can ignite explosive mixtures. The flowing movement of finely powdered substances or low conductivity fluids in pipes or through disturbance has been known to create a build up of static electricity. Dust clouds of finely powdered substances can become combustible or explosive and when there is a static discharge in a dust or vapor cloud explosions has resulted (Joseph, 1999).

         Lightening is the dramatic natural example of static discharge. The details of the phenomenon are still unclear and they remain a subject of debate, the initials details charge separation is thought to be associated with contact between ice particles within the storm clouds. Significant charge usually persists only in regions of low electrical conductivity where very few free charges are able to move in the surroundings. The flow of neutralizing charges often results from neutral atoms and molecules in the air being to form separate positive and negative charges which travel in opposite direction of the electric current, neutralizing the original accumulated charge. At about 30000 volts per centimeter (30Kv/cm) the static charge breaks down in this way depending on the humidity.

The discharge superheats the surrounding air resulting in a bright flash and production of a shock wave that causing the clicking sound. The lightening is simply a scaled up version of the sparks which are normally seen in domestic occurrences of static discharge. The flash occurs because of the being heated to a high temperature that it emits light by incandescence while the clap of thunder is as a result of the shock wave created as the superheated air explosively. When a plane passes through a zone in the air where this is occurring it is likely to be struck with a chance of it being disastrous. This is however minimized by the fact that most aircraft have their skin made of aluminum which is a very good conductor of electricity. The secret of safety when lightening hit is the allowing of current to flow through the skin from the point of impact to the other point without interruption or diversion to the interior of the aircraft.

Lightening also can have a problem on the flight instrument and the computers. These is usually taken care of by use of surge suppressors and shielding to insure that electrical transients do not threaten the on board avionics and the miles of electrical wiring found in the modern aircraft. It is also a requirement to make sure that all components that are vital to the safe operation of an aircraft are certified to meet the stringent regulation of the FAA for planes flying into United States [Robertson, 2001].

Preventive and protective measures

Due to the negative effects caused by electrostatics in navigation and other instruments, aircrafts have installed systems that eliminate the harmful effect of the static charges. On the wings and flaps of some civil aircrafts certain extensions similar to antennas whose functions is reduction of the effect of the static charges. The extensions (antennas) provide additional path for discarding the electron surpluses from the aircraft structure back to the atmosphere.

The opening of the wing fuel tank on the big aircraft is situated at the bottom side of the aircraft wing. This is in order to reduce the harmful influence of static electricity during the pouring of the fuel. Connecting of the aircraft with the tank track using a cable in order to equalize the electrostatic potential is obligatory in addition to the fuel containing antistatic additives. This practice of using a connecting cable is however questionable as grounding creates the possibility for a sudden or cumulative discharge that is sparkling which may be a crucial factor for detonation of inflammable or explosive mixture of fumes of inflammable liquids or gases and the air in the flammable atmosphere. Workers in charge of the air craft tanking take precaution by wearing antistatic clothes and shoes in addition to training on secure performance of the work [University of Colorado].

Solutions by A33O and B777

To tackle the problem caused by electrostatics, airlines such as A330 and B777 have taken measures to avoid the hazards that come with static charges. One of the measures as noted by Braunović et al. (2006) is to recognize that the aircraft fuel system is very sensitive spot and any damage causes fire. During design and construction, these systems are under the special care of the constructors. They are made of special material which do not produce sparks and reduce the harmful influence of the static electricity, though sometimes this is not good enough to ensure complete safety and security of the system.

Also the important thing they do is to emphasize the role that can be played by the pilot and the navigation team at the ground in avoiding disasters. The aircraft avoid flying into lightening storms or flying through storms or areas where lightening is likely to occur. This can be possible by the proper coordination of the weatherman who predict violent where storms may likely to be and the pilot who makes the final decision on what do.

As discussed earlier in the text lightening is a massive flood of electrons seeking equilibrium either from ground to cloud or cloud to cloud. In either case there is an accumulation of electric charge at the edge of the cloud and this finds its way from one place to another, through what is called a step leader. The sheer power of the cloud starts attracting electrons from the ground. The electrons gather on anything like trees, a person and telephone poles. The electric charge tries to work its way through the air ionizing it, until it meets the leader. When they do finally meet there is lighting with the step leader being a victim of the lightening. An aircraft flying between the highly charged portions of the cloud will act as a conduit (Robertson, 2001). The aircrafts have static wicks which dissipate the charges. The static wick is a metal connected electrically to the frame of the aircrafts with one or two spikes or needles on the end. Because the spikes concentrate the electric charge around them, and they are connected to the airframe, the aircraft is able to dissipate any static electricity that may be buildup out into the air. Also if the lightening happens to hit the aircraft, chances are that the electricity will go through the dissipater and not the aircraft.

Conclusion

Static electricity is usually considered as being very dangerous. As a rule once static electricity appears it is known to be accompanied by so many unwanted effects which include: electroshocks, leading to secondary injuries as a consequence of the uncontrolled movements and in uncontrolled atmosphere it leads to fires and explosions. On the other hand when discussing how aircrafts should be protected from a lightening strike, the best safety feature is the pilot who should check the weather before flying and to make smart decisions about where to fly. The planes are forced to fly through a storm, the static wicks on the wings trailing edges might be of help to the plane.

References

Charles K. Alexander, Matthew N. O. Sadiku (2005), Fundamentals of Electric Circuits. New York, ‎ McGraw Hill

Christopher R. Robertson (2001) Fundamental Electrical and Electronic Principles.‎ Elsevier, Elsevier Science publishers

Djovčoš Martin, Dangers of static electricity and protection in Aviation. Retrieved March 5, 2009 from

 http://www.forensicexpvojvodina.org.yu/radovi/Dangerous_of_static_electricity.pdf

Joseph J. De France (1999). Electrical Fundamentals‎. New York, Prentice Hall

1. Braunović et al. (2006). Electrical Contacts: Fundamentals, Applications and Technology.‎ London, CRC Press

Peter Staheli, Retrieved March 5, 2009 from

 http://www.users.bigpond.com/lifelogics/static_electricity.htm

University of Colorado: An online textbook US Navy Electrical Engineering Training Series. Retrieved March 5, 2009 from

http://www.physlink.com/Education/AskExperts/ae568.cfm