Country of Origin: United Kingdom

Static Electricity Introduction ......................................................................................................................... 1 What is Static Electricity? .................................................................................................... 2 Causes of Static Electricity .................................................................................................. 2 Flammable or Explosive Atmospheres – The Need for Risk Assessment ......................... 3 Ways to Reduce or Control Static Electricity ..................................................................... 3 Summary .............................................................................................................................. 3 References and Additional Information Sources .............................................................. 3

Introduction

Many fires have been caused by static electricity which can be more fully and usefully referred to as “undesirable electrostatic discharge”, with some fires resulting in major property losses and subsequent significant interruption to business continuity. Static electricity is generated by many industrial processes and is actually used as part of the process in many cases. This Risktopic deals with the type of static electrical discharge which represents an inadvertent and undesirable source of ignition leading to property loss fires, explosions and accidental damage. This accidental electricity presents a real risk of significant loss unless the risk is adequately recognised and controlled.

This Risktopic has the objective of providing some background information about undesirable electrostatic discharges and giving guidance to help ensure that, in the case of relatively minor usages of flammable liquids, static electrical hazards are being adequately addressed. However, in the case of flammable gases and dusts and in the case of significant industrial installations, the advice of a competent and suitably qualified professional adviser will normally be required to address individual circumstances.

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What is Static Electricity?

Electricity is normally used in purpose provided circuits of conductors in which electrical charge passes around the circuit components and carries out work, for example, at light bulbs or in electric motors before returning to the source of electrical power. The driving force of the movement of the electricity is a potential difference or voltage. With this type of use of electricity, the movement of the electricity is intentional and controlled. Often, insulators are used to prevent the movement of the electricity to any other point outside the circuit (an insulator is any material that does not conduct electricity). In the case of static charging, there is normally an abrasive action between two surfaces. The abrasive action produces friction that results in surface electrons being stripped from the substance, whilst the electrical charge which results remains in place on the surface of the substance. An electrical charge can accumulate in this way in proportion to the frictional abrasion applied. If the electrical potential keeps on accumulating without any ready means of dissipation it is possible for the charge to produce a discharge spark. This is typically a spark across a gap to another surface at a lower level of electrical potential – probably a conductor. It is this type of discharge which might have sufficient energy to ignite a flammable atmosphere. The following are just some examples where static electricity is likely to be a problem in terms of fire and explosion, causing property losses:-

Moving parts of machines such as rollers – particularly in machines such as fast moving printing machines e.g. of the rotogravure type, where volatile solvent inks are in use for printing onto insulating plastic sheets – a static discharge may set fire to any uncontained flammable ink and air vapour mixture.

Organic powders being transported in plastic tubing - a static discharge could lead to an explosion within the tubing.

People moving around a factory but not wearing anti-static clothing and/or footwear touching micro-electronic circuits which, during manufacture, are not yet suitably protected. Damage to the delicate electronics may occur when the static induced discharge passes through the micro-circuits.

Flammable liquids such as non polar hydrocarbon based solvents, which tend to be insulators, dropping through gravity from one container not physically joined to the receiving container below – a fire starts when a spark from the liquid jumps to the lower vessel.

Causes of Static Electricity Static can be created in a number of ways, the main two causes being: friction and separation. There are a number of other factors that can contribute to an increased risk of static being created, namely: the type of material being used or processed, environmental conditions and repetitive actions. Friction When two surfaces come into contact with each other and friction is caused there is a risk of an electrostatic discharge caused by electrons attached to one of the surfaces transferring to the other surface. Consequently the power of the static charge increases in relation to how hard or fast the two surfaces are rubbed together. Separation Static discharge is created in a similar way to friction, except this time the charge is created when the two surfaces are separated. Again, the charge is created by electrons transferring from one surface to the other causing a positive charge on one surface and a negative charge on the other. As with friction, the faster that materials are separated the higher the charge is likely to be. Type of Materials

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The type of material has a major effect on the production of static electricity since some materials are more prone to becoming electrically charged than others. Plastics, for example, can maintain static charges for a long period of time allowing a higher charge to build up. Environmental Conditions The condition of the atmosphere surrounding materials or surfaces has an effect on static electricity generation. Drier air is more conducive to providing an environment for the creation of static electricity. Conversely, higher humidity is likely to make it more difficult for static charges to be produced. Temperature fluctuations can also help to create the right environment for static. Materials that go through a cooling cycle are more prone to generating an electrical charge. Repetitive Actions Repeated actions can alter the amount of an electrical charge. For example, a process which involves the coming together of two surfaces repeatedly will build up a bigger charge through friction. Equally, a material which is charged while moving over a series of rollers will increase its electrical charge after each roller.

Flammable or Explosive Atmospheres – the need for risk assessment

Fires and explosions can only occur in the event of a discharge of an accumulated static electrical charge in the presence of a flammable or explosive atmosphere in areas where the following are in use and are allowed to mix with air :-

Flammable liquids.

Flammable and explosive gases.

Combustible and flammable dusts.

Various flammable substance and air mixtures will have different minimum ignition energies. These energies are measured in miilijoule (mj) units. Below the specific energy level any discharge spark may not have sufficient energy to ignite the atmosphere concerned. The range of concern is from 1mj for gases to 5000 mj for some dust and air mixtures.

Whilst the free release of the relevant substances during normal working conditions can be reduced by containment and other means, the risk of combustible and explosive atmospheres forming will usually still remain.

The risk of ignition will be significantly reduced by ensuring that adequate precautions are taken to eliminate the risk of an undesirable electrostatic discharge in appropriate areas or zones following the recognition and classification of such zones in a risk assessment.

Dangerous Substances and Explosive Atmospheres Regulations 2002 (DSEAR)

Guidance on the zoning of workplaces, for the purposes of identification of potentially flammable and explosive atmospheres, is included within these regulations, under which employers at workplaces need to:-

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Carry out a risk assessment under Regulation 5, in which it is necessary to consider various relevant factors including the hazardous properties of the substances in use, the circumstances of the work and the likelihood that an explosive atmosphere will occur and its persistence. It is acceptable to take into account the measures which have been or will be taken to eliminate or control the risk.

It is necessary under Regulation 5 (g) for the employer to consider “the likelihood that ignition sources, including electrostatic discharges, will be present and become active and effective”.

Use the Schedule of Zone definitions in Schedule 2 of the Regulations as an aid to the classification of places where explosive atmospheres may occur. Initially this requires a determination of a hazardous place and subsequently zoning of a hazardous place based on the occurrence, frequency and duration of explosive atmospheres.

Zone definitions for explosive atmospheres, ranging from continuously present to not likely to occur in normal operations, are given titles: Zone 0, Zone 1 and Zone 2. For dusts, similar definitions are found and given corresponding titles: Zone 20, Zone 21 and Zone 22.

More information on zoning and risk assessment is given in the more specific Risk Topics covering selection of suitable electrical equipment in potentially flammable atmospheres and in the titles given under Additional Information.

Ways to Reduce or Control Static Electricity

Earthing and Bonding The risk can be controlled by making sure that, as far as possible, all relevant parts of machinery and equipment are joined together with equipotential earthing cables. Where portable vessels or drums are used, earthing and bonding should be provided to make sure that, at relevant times, an electrical potential difference across the gap cannot be created during pouring or decanting. Connection should be by means of fly-leads held in place as long as necessary by crocodile or screw on clamps during the decanting or other process. Electrostatic Charge Eliminators The neutralisation of the charge on materials such as paper or plastic films can be assisted by the ionisation of the atmosphere around the charged material. Ionisation is achieved by the use of various types of devices, known as static eliminators and includes combs and radioactive devices. Safer Plant Operation and Protection of People In certain situations, the generation of static electricity can be controlled by the design of the plant and / or by the method in which a particular process is carried out. For example:-

As far as is practicable any operation which may cause generation of static electricity should be undertaken outside those areas where flammable atmospheres may be present, e.g. the removal of dry raw materials from plastic or plastic lined containers, the mixing of raw materials etc.

Static generation increases with the rate of flow of liquids, therefore, the flow velocity of a liquid through

a pipe should not exceed a specified level, based on the properties of the liquid and the process involved. As a guideline the normally accepted pumping speed for non-polar solvents is 1 metre per second.

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The splashing of a liquid allowed to "free fall" into a tank is a source of static generation. This should be avoided, either by arranging for bottom entry into a tank or by the use of a fill pipe, which extends to the bottom of the tank.

Mechanical mixing or agitation of a liquid can cause static generation and should be kept to a minimum. A person insulated from earth can accumulate an electrostatic charge which will produce sparks on

discharge with sufficient energy to ignite a flammable atmosphere.

In many situations the resistance of the footwear and flooring is low enough to provide adequate earthing, however, where a potential hazard is identified this can generally be controlled by the provision of suitable “anti-static” footwear.

In certain situations it may also be necessary for anti-static clothing to be provided. Clothing should not be removed in areas where a flammable atmosphere is present.

Maintenance of Control Measures Once suitable risk control measures have been implemented, procedures should be established for the regular testing and maintenance of static control equipment. Earthing and bonding systems should be regularly inspected for mechanical integrity and continuity tests should be carried out at least once every 12 months. With some processes, good housekeeping may be particularly important to ensure that deposits of materials being handled (for example, paints, adhesives, on floors and equipment) do not act as insulators and prevent risk control measures from being effective. Summary Static electricity not only has the potential to cause electric shocks, in certain atmospheres static discharges can cause explosions or fires to occur. It is important to recognise that certain materials, such as plastics, have an increased ability to create static electrical discharges if suitable precautions are not taken. The guidance in this Risktopic has been designed to help identify how static electricity is generated, factors that can increase the risk and measures that can be taken to reduce or control the risk of static discharge. Zurich strongly recommends that a thorough risk assessment is undertaken where there is a risk of static electricity being generated within a flammable or explosive atmosphere. References and Additional Information Sources

1. Dangerous Substances and Explosive Atmospheres Regulations 2002. (DSEAR)

2. BS 5958: 1991 -1: Code of Practice for control of undesirable static electricity. General Considerations.

3. PD CLC/TR 50404:2003: Code of practice for the avoidance of hazards due to static electricity. (Available from the British Standards Institute)

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