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Fowler Associates Labs

 

 

Static Fire Stories Articles & Technical Papers Current News
Shock horror probe - the world of a static electricity consultant

Dr. Jeremy Smallwood
Electrostatic Solutions Ltd. www.static-sol.com

Mention static electricity and most people think of dusty physics school books with odd experiments - the Van de Graff generator, long sparks and hair standing on end….. rubbing things with cats fur…gold leaf electroscope and electrophorus…..I ask you, what relevance has it to industry and every day life in the real world?

A great deal, actually. Every material and object is made of electrical charges, and some electrical charges are separated whenever two materials in contact are separated. Charges are being separated all the time in many different situations around us. We probably didn't notice this much until we started using highly insulating materials, such as plastics and rubber, in our homes, offices and workplaces, preventing charges from dissipating harmlessly and encouraging them to build up as electrostatic voltages. Who hasn't experienced a static electric shock? Nowadays we put insulators on our feet and lay highly insulating floor materials, use man-made fiber clothes and furnishings. We put plastic wheels on carts and make our machines out of engineering plastics.

A shock often shows we had charged to about 4kV or more before we released that charge in an electrostatic discharge (ESD). In the electronics industry most semiconductor devices are susceptible to ESD damage. A human body charged to 100 V or less can destroy some sensitive components. Circuit boards have to be handled and assembled in ESD Protected Areas where electrostatic fields and voltages are kept to a low level. A voltage susceptible device may suffer breakdown of a thin insulating layer (e.g. gate oxide of a mos transistor) - it takes only a small amount of charge to charge up the small gate capacitance ( a few picoFarads) to the breakdown voltage (a few volts). An energy susceptible device may fail by a high ESD current of a few amps passing through a micron size device junction or interconnect metallization, bringing it to melting point. Outside the EPA susceptible circuit boards and devices must be protected against electrostatic fields and ESD currents by shielding packaging. A whole industry has grown up supplying ESD protective equipment and packaging to the electronics industry - for my part I research ESD topics, provide consultancy services and advise on best practice, run ESD training seminars and help write standards such as ESD S20:20 and IEC 61340-5-1 to provide guidance to industry.

Small consultancy jobs can be extremely varied. Often the solutions are simple in principle, and in practice with a little knowledge the situation could have been avoided, but remedial action may be difficult or expensive. In one case, the client reported drivers in their car park experienced severe shocks when pulling parking tickets from a dispenser. A site visit showed that a new epoxy floor covering had been fitted up a long ramp that brought cars to the barrier -and all the charge stored on the car (say 800 pF capacitance, charged to several kV?) would be discharged through the driver's arm. The cure - fit conductive floor next to the ticket barrier so that the car could dissipate its charge through its tyres.

An embassy in a northern climate complained that the ambassador was getting shocks when she ascended stairs and touched her office doorknob. The stairs were made of glass, a good insulator and at the other end of the triboelectric series from most shoe sole materials. Ideally, I would recommend not fitting a highly insulating glass floor. The remedial action - a surface treatment - is probably unreliable and requires regular refreshment. Similarly, in a new prestige UK site severe shocks were experienced on the stair wells. The architect had specified beautiful but highly insulating tread tiles that charged a typical person to over 5 kV within a few steps. A well earthed stainless steel banister ensured that they would get a good shock. It is difficult to specify low cost and reliable remedial action in the face of such built-in electrostatic problems.

In a high street retail site staff complained that "the lift was giving (them) shocks". It had a metal fascia that had been shown to be well earthed. The Saturday lad had taken to wearing rubber washing up gloves, he suffered so much. The lad's duties included loading a large mobile metal rack (on insulating wheels of course) with highly charged garments in polythene covers from a lorry, and then wheeling them along a highly insulating carpet to the lift, both lad and trolley reaching over 8 kV.

In the USA, many petrol fueling facilities have latching nozzles that allow the user to go away while the vehicle is filling. Many people return to wait in their car when the weather is cold. When they get out, their body voltage may rise to around 10 kV. If they do not ground themselves before they pick up the fuelling nozzle to remove it from the tank aperture, the resulting spark can ignite the emerging petrol vapour causing fire and injury.

Many fine dusts can be ignited by electrostatic sparks. Factories have been destroyed, and avoiding fires and explosions is a real issue in chemical industries. With solvents and flammable vapours the risk of ignition is even greater. It may only take 0.2 mJ energy to ignite a vapour - the equivalent of about 10 kV on a drinks can, or a few kV on a charged person. Vapours can also be ignited by brush discharges from charged insulating surfaces. Insulating solvents charge to high levels when running through pipes, splashing or during filtering. Conductive objects (including people!) must reliably earthed, and plastic surfaces are typically limited to less than 100 cm2 area. Insulating powders charge highly during transport. The charge builds up when they are deposited in a silo, creating a highly charged pile with potentially flammable fine dust cloud above. The CENELEC TR50404 standard has been written to give guidance covering a wide range of industrial circumstances.

Static electricity can also be used for our benefit. Electrostatic separation can be used to separate some materials that are difficult to separate by other means. A mixture of plastics chips from recycled wasted can be separated on the basis of their triboelectric charging properties. An insulating material such as rubber or plastic can be separated from a conductor such as wire fragments, paper or card.

The architect with me on the car park visit, faced with several kV measured on a car, commented incredulously "How could we have predicted this?" Actually, even a little knowledge of static electricity applied during the design stage would avoid many electrostatic problems. Unfortunately it is rarely considered until problems show, by which time it is too late. Far from being irrelevant, static electricity is highly important in a very wide range of modern home and industrial environments and processes and is an everyday fact of modern life.

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