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Jeremy Smallwood's static electricity page
Jeremy
Smallwood's
Static
Electricity
Page
Contents
About
static electricity
Preventing
ESD damage in electronic manufacture
Materials
for electrostatic solutions
Reducing
static electricity nuisance in shops, offices, homes and
cars
Avoiding
electrostatic spark explosion hazards
Avoiding
electrostatic problems in tools and machinery
Using
electrostatics in technology
Purifying
and recycling waste materials using electrostatic
separation
Capacitive
sensor solutions
Biographical
Information
Contact
Information
Comments,
Questions and Suggestions
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.
Back to
Top
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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Top
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;
material resistivity and resistance up to 1015 ohms
electric field and potential (Voltage) measurements from <100V to
greater than 20kV
charge decay time measurements from 30ms to 102 seconds or
greater
triboelectrification (charge generation) measurements
dielectric permittivity and loss measurements
breakdown voltage measurements
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Top
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.
Back to
Top
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.
Back to
Top
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;
a fuel is present and a potentially flammable atmosphere could arise (many
dust clouds, solvent vapours and fuel mists commonly form sensitive flammable
mixtures with air)
there is air or another oxidising material which could react with the fuel
there is the possibility of static electric build-up leading to an
incendive spark
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
preventing the occurrance of a flammable atmosphere, e.g. inerting with
nitrogen
preventing dangerous build-up of static charges by grounding metallic
objects, and replacing insulating materials with static dissipative materials.
Some consultancies offer specialist measurements for assessment of
electrostatic ignition risk and explosion hazard associated with powders or
other materials;
Minimum Ignition Energy measurements, especially for dust clouds
Maximum Explosion Pressure, Explosion violence (Kst), Minimum
Explosible dust concentration and other hazard assessment measurements
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.
Back to
Top
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.
Back to
Top
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 paint spraying. Static electricity is used
to attract the paint to the target, reducing paint wastage and improving
coverage of the target.
Electrostatic precipitation. Electrostatic forces are
used to precipitate smoke particles and clean the gas emitted from a chimney.
Electrostatic atomisation. Static electric forces are
used to form a fine spray of liquid particles.
Photocopiers and laser printers. These use electrostatics
in the printing process.
Electro-osmotic dewatering. Electrostatic forces speed up
the settling of fine particles suspended in water, and can be used to dewater
a sludge.
Dielectrophoresis. Particles in a non-uniform electric
field can be made to be repelled from, or attracted to, regions of high field
strength.
Corona treatment of plastic surfaces. Corona discharges
can be used to improve the ability of a plastic surface to receive print or
adhesive treatments.
Electrostatic technologies often feature low energy consumption as one of the
benefits.
Back to
Top
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;
Separation of metal from cable scrap
Separation of plastics for recycling
Separation of papers and plastics from a waste stream
Back to
Top
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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Top
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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Top
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.
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