Electro Static Discharge: Controlling static electricity

Wrist straps are one solution to control electrostatic discharge (ESD) Technicians can test to make sure of proper grounding. Controlling static electricity By Jim Sparks Foreign Object Damage (FOD) is something that has been drilled...


Electrostatic Discharge Wrist straps are one solution to control electrostatic discharge (ESD)

Technicians can test to make sure of proper grounding.



By Jim Sparks

Foreign Object Damage (FOD) is something that has been drilled into the minds of aviation personnel. We are all aware of the result when a foreign object enters the inlet of a turbine engine or when the wing of a moving aircraft comes in contact with a stationary rigid object like the hangar wall.

One form of FOD that has been all but overlooked is electrostatic discharge or ESD. Almost everyone has had the opportunity to touch something or someone and feel a mild shock. This is a form of ESD. Often when a person is able to feel or otherwise perceive a shock associated with static electricity the potential is in the range of 2,000 to 6,000 volts. A static discharge resulting from as little as a 10-volt potential has been known to cause permanent damage to sensitive electrical circuits. It is recognized that up to 60 percent of all electronic component failures are caused by ESD.

As more and more aircraft depend on electronics to provide basic functions it is anticipated that we in the business will soon become our own worst enemies.

To understand is to control


Can formation of electrostatic charges be realistically eliminated? Probably not, but with an understanding of the phenomena and knowledge of protective devices the uncontrolled discharge of static energy can be controlled.

Static electricity is often referred to as "triboelectricity." Tribo is a Greek word meaning, "to rub". Therefore triboelectricity is a form of electricity produced by rubbing. Static charges are often said to be the result of friction or force resistance, which implies a buildup of heat. In reality it is the contact or touching of materials that causes the buildup of electrical charges and not the effect of heat. A contact potential will occur almost anytime two solid objects make contact. This voltage is generally quite low but varies with material types. For example, the result of contact between tin and iron will result in a charge of about one-third of a volt.

To understand static charges we have to first have a grasp of the atom. The hydrogen atom is the simplest as well as the lightest. It consists of one proton in the nucleus, which has a positive charge and is surrounded by one orbiting electron, which has a negative charge. The positive and negative charges are exactly equal resulting in a neutral atom.

Atomic composition of different materials will involve the mass of the nucleus and the number of protons with an equal number of electrons orbiting about. Electrons in the outermost orbit may be either firmly or weakly attached. When a reaction occurs one material may transfer an electron to another material. This results in a change in the state of charge. The material that looses the electron becomes a positive ion while the material gaining the electron will become negative.

Conductors and insulators


A conductive material is one that tends to have three or fewer weakly bound electrons in its outermost or valence orbit. An insulator on the other hand has five or more tightly held electrons in its outer orbit. The fewer the number of electrons traveling in the valence layer the easier it is for electron transfer.

Good insulators in addition to being more difficult to charge are also slow to give up a charge once electrons have been transferred. This effect can often be demonstrated by observing someone attempting to clean the acrylic windshield of an aircraft shortly after a flight. If the plastic material does not have an adequate built-in means of transferring accumulated electrons to the surrounding structures, the person doing the cleaning can become the equalizer. Please do not attempt this at home.

The effect on aircraft


Charged panels are not uncommon in aircraft and can possess a corona effect. This is where electrons transfer from a surface to the air. The result is a ghostly glow visible often in low light conditions.

Saint Elmo's Fire is the common name for this corona discharge effect. Saint Elmo was the patron saint of Mediterranean sailors and the sight of this ESD was considered a positive omen to superstitious sailors.

Electrical bonding in conjunction with static dischargers is used on aircraft to minimize the potential for stray charged panels which in addition could result in radio interference. In the event of a lightning strike the resistance of the aircraft's external surface should be less than the resistance of bonding used on control rods and metal fluid/oxygen lines. Aircraft electrical racks also have a very low resistance path to the aircraft structure.

Relative humidity of the air along with atmospheric contaminates will have an effect on the ability of a material to build up a static charge as well as the tendency to dissipate one. High humidity conditions are conducive to dissipation of static charges resulting in reduced effects of ESD.

Damage to components

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