Jeremy Smallwood's

Static Electricity

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The same phenomena which create lightning and thunderstorms are around us every day, creating incredibly high voltages which cause sparks and shocks.

Static electricity is most often experienced as nuisance shocks, but can in some circumstances cause explosions. Static electricity can also be used to our advantage in designing paint spray systems, electrostatic separators, and the ubiquitous photocopiers and laser printers.....


About static electricity

Static electricity forms part of our daily experience. Most people have noticed from time to time electric shocks when getting out of their car, or after walking across a carpet and touching a metal door handle. This electrostatic discharge nuisance is normally a minor inconvenience, but in some cases static electricity can cause more serious problems. Static electricity can also be harnessed to good effect in technologies as diverse as photocopiers, dewatering and flue emissions control.

Static electricity is in fact generated whenever two materials touch and then separate. One material charges positively, and the other negatively. The charge which is generated may not be noticed, as often there is an electrically conductive path which can dissipate the static electricity harmlessly away. The build-up of static electricity depends on whether the rate of charge dissipation is greater than the rate of charge generation. If it is, there is no problem. If charge generation is greater than the charge dissipation rate, static electric charges build up. A very high voltage can be produced quite rapidly, leading to electrostatic discharge (ESD) sparks and shocks.

If there is a flammable atmosphere present, such as a solvent vapour or dust cloud, then the risk of explosion hazard may cause concern. Less dramatic, but also very important, is the possibility of ESD damage in electronic manufacture. Here, even a small amount of static can destroy a sensitive electronic component, resulting in component losses reduced reliability and increased rework costs.

Where static electricity is controlled to reduce these effects, it is usually by choice of materials specified as having certain electrical properties. In many cases, these will be specified to particular national and international test standards.

Static electricity can also be useful in new technology. An example is in the application of electrostatic separation in recycling of materials. Electrostatics also forms the basis of some sensor solutions, for example where the level of a liquid or size or proximity of an object may be detected.

Email me to discuss static electricity issues, or visit Electrostatic Solutions site.

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Preventing ESD damage in electronic manufacture

Electronic equipment manufacturers know that small electrostatic discharges (ESD), which can occur during assembly and handling of printed circuit boards and modules in electronic equipment. ESD damage can cause failed components leading to equipment test failures and rework costs, or latent component failures which could cause failures in equipment in the field.

Whilst it is difficult to attribute specific failures to ESD damage, most manufacturers prefer to prevent possible damage and reliability problems by assembling equipment under electrostatic safe conditions in an electrostatic protected area (EPA). In Europe, guidance is given in the EN100015 series of standards, and many manufacturers adhere to this as part of their Quality Assurance procedures. Whilst installation of static preventative equipment can provide good protection against static damage, the effectiveness of these measures can be compromised by working practices, materials and equipment allowed into the EPA.

Good quality advice on effective implementation of EN10015 or in-house standards, tailored to suit the manufacturer’s needs, can lead to significant savings in equipment outlay as well as product failure and rework costs.

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Materials for electrostatic solutions

In many cases, the choice of material and its electrical properties is a key factor in controlling the generation and the safe dissipation of static electricity.

Conductive polymers are available which have properties suitable for a wide range of applications. Sometimes the use of more traditional materials may be the most suitable choice to avoid static problems.

The electrical properties of materials, such as resistivity and charge decay properties, are very important in the specification of materials for electrostatic uses. Often these must be measured according to recognised industrial or national standards.

A good electrostatics Consultancy and Test House will often offer material and electrical property measurements capability such as;

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Electrostatics national and international test standards.

There are many national and international standards relating to electrostatics measurements, and there is not space to review them all here. There are some key standards relevant to UK and European electrostatics:

Electronics industry applications

EN100015 Basic specification: Protection of electrostatic sensitive devices

This is currently the key standard applicable to electronics manufacture in Europe. It is in 4 part, of which Part 1 General requirements is the most useful to most manufacturers implementing electrostatic protected manufacturing areas (EPAs).

Electrostatic hazards

BS5958

This is a key standard dealing with electrostatic hazards avoidance in industry. Part 1 gives a good general introduction to electrostatic hazards issues. Part 2 gives a range of measures applicable to various industrial situations such as petrochemical installations, and flammable powder handling.

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Reducing electrostatic discharge nuisance in shops, offices, homes and cars

Most people know from their own experience that when you move around in everyday life, static electricity can cause sparks to occur unexpectedly. This effect can appear and disappear inexplicably - some days, little static is noticed, but other days sparks seem to fly regularly. The effects literally change with the weather - on humid days, there is generally little noticable static electrical activity. Static is most often noticed in a very dry atmosphere, especially in centrally heated buildings during cold dry weather.

One problem that is often experienced is that when you get out of the car, you get a shock on touching the door to close it. The source is usually static charges which build up between your body and the car seat while you are in the seat, but remain harmlessly neutralised until you get up. At that time, you take considerable electric charge with you as you get out of the car. If the charge has no discharge path, then a very high voltage (several thousand Volts) can build up very quickly. When you reach for the door, the high voltage causes a spark which discharges you quickly to the car - giving you a shock in the process.(Many people cannot feel shocks with discharges of less than about 4000 Volts!). One solution is to make sure you hold onto the metal door frame as you get out of the seat, allowing yourself to harmlessly discharge slowly as you get up.

Static electric charges can build up on trolleys as you push them around a shop. The charge is usually generated by the movement of the trolley wheels, and your foot action as you walk. Once again, thousands of Volts may be built up on your body and on the trolley. The charge remains unnoticed until you touch something, giving the characteristic shock.

A similar thing happens when you walk across a floor, when high levels of electric charge often building up on your body as you walk. The shock comes when you touch a filing cabinet, door knob, or other substantial conductive object, which may or may not be electrically grounded. (Sometimes people assume that the object gave them a shock, when in fact they themselves were the source). If the object happens to be a computer or other electronic system, the system can experience an electrostatic discharge (ESD) which can cause the system to crash if the shock is great enough. I once measured the voltage on about ten computer users after they had entered a computer room, and before they sat at their consoles. Few of them had a voltage of less than 4kV (1kV=1000V) before the sat down. Their body voltage normally increased substantially as they sat down, unless they happened to touch a conductive discharge path as they sat.

Photocopiers use static electricity in their operation, and also generate a fair amount of static on the paper or film as it runs through the machine. A person operating the machine for some time may find that some of this static charge builds up on them as the unload the paper from the output trays. This can, in extreme cases, cause unpleasant shocks to be experienced. A simple temporary solution ( which may or may not be acceptable!) is for the operator to take their shoes off! This often allows the static charges to drain to ground before they can build up to significant levels.

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Avoiding electrostatic spark explosion hazards

Most people know from their own experience that static electricity can cause sparks to occur unexpectedly. In some industries such sparks could cause the risk of fire or explosion, and it is important to avoid this by reducing static electricity build-up to safe levels.

Ignition is a possible risk if;

If electrostatic ignition hazard is suspected, then it is important to get the expert advice needed to assess the situation. Determining the need for preventative measures, and how to reduce electrostatic ignition risk to negligible levels, is best done by an experienced specialist. Good general guidance is given in BS5958, a key UK standard dealing with electrostatic hazards, but no standard can adequately cover all possible situations.

Typical measures aimed at preventing ignition include

Some consultancies offer specialist measurements for assessment of electrostatic ignition risk and explosion hazard associated with powders or other materials;

The Minimum Ignition Energy is used as an indicator of the possible spark ignition sensitivity of the fuel under a given set of test conditions. The risk of ignition of a flammable mixture is a complex function of the fuel ignition sensitivity and the igniting ability (incendivity) of the spark.

Electrostatic discharges occur in many forms, and this incendivity factor in itself is highly complex. The most hazardous type of ESD is one which occurs between metal objects. This has the highest incendivity. A discharge from an insulating surface is less incendive, but can easily ignite some sensitive fuel mixtures. Another type of discharge, the corona discharge, tends to occur from sharp edges of metals at high voltages. This is generally excepted to be not incendive under most circumstances.

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Avoiding electrostatic problems in tools and machinery

Modern plastic materials find an ever increasing range of application in tools and machinery. As parts made from traditional materials are replaced by polymers, static electricity can unexpectedly lead to problems in a new design. High levels of static can cause shocks to operators, or cause materials to stick to machine parts.

The solution often lies in the choice of material used in carefully identified key parts, to provide an effective path for it to safely discharge.

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Using electrostatics in technology

While static electricity is often experienced as an unwanted phenomenon, it can also be harnessed in technology to achieve effects which may be difficult to achieve in other ways. Examples are;

Electrostatic technologies often feature low energy consumption as one of the benefits.

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Purifying and recycling waste materials using electrostatic separation

Electrostatic separation is a highly versatile range of techniques which can be used to sort or purify many different materials. It has been used for many years in the mining industries for ore separation. The following applications in materials recycling have been developed in recent years;

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Capacitive sensor solutions

Capacitive sensors find a wide range of applications such as non-contact sensors and proximity detectors. Design of a sensitive sensor to reliably and reproducibly detect the item of interest, whilst rejecting background fluctuations, requires an electronic circuit tailored with an optimised electrode design. Many different techniques are available which can be used as the basis of a sensor design.

Electrode structures can be modelled using Finite Element computer techniques to optimise the design.

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Biographical Information

Dr Jeremy Smallwood

I trained initially in electronics, achieving a BSc in 1979 at the University of Southampton before spending seven years in small companies in electronics research and development. I then returned to the University of Southampton to research ignition of materials by electrostatic discharges, achieving my PhD in this subject in 1993. Since then I have worked substantially in electrostatics consultancy and R&D across a wide range of electrostatics topics. I have contributed to British Standards Committees on electrostatics measurements, hazards, and the prevention of damage in the electronics industry.

In March 1998 I formed a specialist company, Electrostatic Solutions Ltd, to provide top level electrostatics expertise in R&D and consultancy services.

Visit Electrostatic Solutions site

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Comments, Questions and Suggestions

If you have any problems with static which you may be concerned about, or ideas about how static electricity might be useful, please contact me.

I would be interested to know what you would like to see in a specialist electrostatics web site, and what information or service you would find most useful. I would also like to hear how the site could be improved in any other ways.

I look forward to hearing from you.

Jeremy.

Email: jeremys@static-sol.com

Please note: Static electricity is a highly variable phenomenon. Whilst the information here is provided in good faith, expert advice should be sought before applying electrostatic techniques or preventative measures. I accept no liability for matters arising from the application of the information in these pages. In particular, I recommend that any suspected electrostatic hazard should be fully assessed by a competent consultant as a matter of the highest priority.

Contact Information

Dr Jeremy Smallwood

13 Redhill Crescent, Bassett, Southampton, Hants, SO16 7BQ, UK.

Tel. +44 (0)23 8090 5600

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Copyright Jeremy Smallwood 1998.
Last revised: August 05, 2000.