Prevention of Electrostatic Risks

Static electricity is an everyday phenomenon – there can be few of us who have not experienced a static shock after walking across a room and touching the doorknob, or on getting out of a car.

Other static nuisance effects include the clinging of some fabrics to the body, the sticking of a plastic document cover, or the attraction of dust to a TV or computer screen.

Static electricity also helps parts of our everyday life by its applications: photocopying, painting, pulverisation, de-dusting, etc. Directly related to the atomic structure of the materials used or treated in industry, it is created spontaneously, under certain conditions, during operations of manufacture or handling. Static electricity can build up rapidly on objects, in unexpected ways, to produce surprisingly high voltages.

Risks of accidents

Invisible and discrete, static electricity often appears in an alleviating but always insidious way. It can cause accidents (fires, explosions, etc) with disastrous consequences: deaths, serious injuries and burns, considerable damage to property. Among the principal causes of accidents one can quote:

• Transfer operations of oil liquids (discharge, transfer from one tank to another)
• Cleaning of cisterns and the non appropriate use of extinguishers
• Powder discharge in an explosive atmosphere (gas or vapour of flammable liquids)
• Disruptive phenomena due to people electrically charged but isolated from the ground
• Use of solvents, in particular of toluene, in installations presenting isolated metal parts or insulating parts
• The absence of equipotentiality between materials

European regulation

Three directives known as “new approach directives “ related to the free movement of products in the European Union, deal with problems of prevention and protection of explosive atmospheres:

MACHINERY DIRECTIVE 2006/42/EC

This directive adopted on May 17, 2006 replaces directive 98/37/EC of June 22, 1998.

As in the previous version, two Essential health and Safety Requirements (EHSR) cover these risks:

EHSR 1.5.2.

Static electricity Machinery must be designed and constructed to prevent or limit the build-up of potentially dangerous electrostatic charges and/or be fitted with a discharging system.

EHSR 1.5.7.

Explosion Machinery must be designed and constructed in such a way as to avoid any risk of explosion posed by the machinery itself or by gases, liquids, dust, vapours or other substances produced or used by the machinery. Machinery must comply, as far as the risk of explosion due to its use in a potentially explosive atmosphere is concerned, with the provisions of the specific Community Directives.

The electric material used in these machines must in addition be in conformity with the specific directives into force with regard to the risks of explosion.

ATEX DIRECTIVE 94/9/EC

concerning equipment and protective systems intended for use in potentially explosive atmospheres:

Adopted 23 the Mars 1994, it applies as well to the electric as non-electric equipment intended to be used in all the types of explosive atmospheres (gas, vapour, fogs, dust).

The EHSR 1.3.2

is specific to the dangers arising from static electricity: “Electrostatic charges capable of resulting in dangerous discharges must be prevented by means or appropriate measures.”

DIRECTIVE 89/686/CEE related to the design of the Personal Protective Equipment

Adopted on December 21, 1992, it specifies the procedures for the conformity assessment and the essential health and safety requirements applicable to all the individual protection equipment for professional use, sports, DIY and leisure.

One specific requirement ( EHSR 2.6 ) is dealing with PPE for use in explosive atmospheres.

“PPE intended for use in explosive atmospheres must be so designed and manufactured that it cannot be the source of an electric, electrostatic or impact-induced arc or spark likely to cause an explosive mixture to ignite.”

This means that PPE intended to be used in an explosive environment, needs to:

Have antistatic properties which remain effective during all its service life when used and maintained correctly in accordance with the manufacturer instructions

Be made of materials which are known not to cause sparks e.g. by impact, shock or friction

Have no components likely to create sparks by shock or friction

Not include unprotected electric components or parts, which do not comply (where relevant) to directive 94/9/EC of 23 March 1994 on the approximation of the laws of the Member States concerning equipment and protective systems intended for use in potentially explosive atmospheres

Two additional Directives known as “Social directives” on the minimal regulations aiming at improving the protection of workers as regards health and safety treats also these questions:

ATEX DIRECTIVE 1999/92/CE concerning the protection of workers likely to be exposed to the risk of explosive atmospheres.

This directive adopted on 16 December 1999, lays down minimum requirements for the safety and health protection of workers potentially at risk from explosive atmospheres and in particular the obligations of employers.

With a view to preventing and providing protection against explosions, the employer shall in particular:

– assess the specific risks arising from explosive atmospheres , taking account at least of the likelihood that ignition sources, including electrostatic discharges , will be present and become active and effective…(Art 4) – take technical and/or organisational measures appropriate to the nature of the operation… (Art3) – provide workers with appropriate working clothes consisting of materials which do not give rise to electrostatic discharges that can ignite explosive, … where workers or the working environment act as charge carrier or charge producer. (Annex 2.3)

DIRECTIVE 89/655/EEC concerning the use of work equipment by workers at work

In this directive adopted on 30 November 1989, applicable to work equipment in use at workplaces, a general minimum requirement related to risk of explosion is laid down.

2.18. All work equipment must be appropriate for preventing the risk of explosion of the work equipment or of substances produced, used or stored in the work equipment.

Prevention of electrostatic risks

General measures

The way of an efficient prevention of the risks is to integrate safety at the earliest stage of design of equipment and installations . There isn’t a standardised unique solution but several possible solutions which can be separately or simultaneously applied according to the identified risks and to the characteristics of the installation.

Among possible technical measures of prevention, one can distinguish those intended to:

• Remove or limit the explosive atmosphere : ventilation, sealing of containers, air intakes of the installations of pneumatic transport, inertage of containers, etc
• Avoid the appearance of disruptive discharges : ensure equipotentiality and earthing of the electrically conducting elements, use of conductive materials, pipes, aspiration tubes, limit the use of non homogeneous materials such as composite materials (conductive/insulating), etc
• Limit production and accumulation of electrostatic charges : reduction of friction between elements of different nature and reduction of the contact pressure, earthing, increase of conductivity of the insulating bodies by addition or application of “anti-static “ products (e.g. polyols, sulphur organic compounds, phosphoric and phosphorous acids, humidification of atmosphere, elimination of charges generated on non-conducting bodies (use of inductive, electric or radioactive eliminators), elimination of charges (generators of ions, air ionisers)
• Maintain at optimum level the mastery of the risks due to static electricity (control and routine checking)

In addition to these measures of technical nature, information and training actions to the risks due to static electricity and its control, constitute also an essential step of prevention.

The use of personal protective equipment

General issues: The use of antistatic personal protective equipment is advisable where risks cannot be adequately controlled by other means which are equally or more effective, or in addition to organisational and technical measures.

According to the Directive 89/656/EEC on the use of PPE, the employers shall:

– Provide to workers PPE appropriate for the risks involved, without itself leading to any increased risk and shall – Provide PPE in compliance with the relevant provisions of the Directive89/686/EEC on the design of PPE in particular with the Essential Heath and Safety Requirement 2.6 related to PPE for use in explosive atmospheres, – Ensure its good working order and satisfactory hygienic condition by means of the necessary maintenance, repair and replacements.

Several European standards on PPE having antistatic properties (shoes, gloves, clothing) have been already published. They contain measurement methods to assess their electric resistance other terms their capacity to eliminate, dissipate electrostatic charges. Limit values for surface resistance and vertical resistance between 10 8 and 10 11 ohms are generally required in these standards, which is appropriate to cover all ATEX situations (gas, vapour and dust). An other induction charging method is proposed to assess the charge decay (migration of charge across or through a material leading to a reduction of charge density or surface potential at the point where the charge was deposited) and the shielding factor ( related to the electric field strength).

Among all EN standards used as a reference for the CE certification of PPE one can quote:

Standards related to protective clothing:

– EN 1149 “Protective clothing. Electrostatic properties”: Part 1: 2006 Test method for measurement of surface resistivity, (Supersedes EN 1149-1:1995) This European Standard specifies a measurement method of the surface resistivity of materials intended to be used in the manufacturing of electrostatic dissipative protective clothing (or gloves) to avoid incendiary discharge. This test method is not applicable for materials to be used in the manufacturing of protection clothing or gloves against mains voltages.

Part 2: 1997 Test method for measurement of the electrical resistance through a material (vertical resistance) This European Standard specifies a test method for measuring the electrical vertical resistance for materials intended to be used in the manufacturing of protective clothing. This test method is not applicable for materials to be used in the manufacturing of protection clothing or gloves against mains voltages.

Part 3: 2004 Test methods for measurement of charge decay This European Standard specifies methods for measuring the dissipation of electrostatic charge from the surface of materials for garments. The test methods are applicable to all materials, including homogeneous materials and inhomogeneous materials with surface conducting fibres and core conducting fibres.

Part 4: Garment test (item deleted in 2005 from the work programme of CEN/TC 162)

Part 5: Performance requirements (prEN under approval) This draft European Standard specifies requirements for electrostatic dissipative protective clothing to avoid incendiary discharges. The requirements may not be sufficient in oxygen enriched flammable atmospheres. This draft standard is not applicable for protection against mains voltages.

Standards related to protective gloves

– EN 420:2003 – protective gloves – General requirements and test methods According to clause 4.5 of this standard the electrostatic properties shall be tested according to the test method described in EN 1149-1 or EN 1149-2 or prEN 1149-3. Electrostatic pictograms shall not be used for this property. In this standards ist is stated that these tests are designed for garments and have not been validated for gloves. Some interlaboratory trials have shown significant discrepancies in test results for the triboelectric charging method ( method 1 of EN 1149 part3). It is thus essential to give comprehensive information about the test parameters used along with any test result.

Standards related to safety, protective and occupational footwear

In the three following EN standards, Antistatic footwear are footwear whose resistance, when measured according to EN ISO 20344:2004, 5.10, lies above 100 k 2 and is less than or equal to 1 000 m 2 :

– EN ISO 20345:2004 – Safety footwear for professional use – Specifications (the safety footwear has antistatic properties if the marking contains symbol A , or categories S1, S2, S3 or S4)

– EN ISO 20346:2004 – Protective footwear (the protective footwear has antistatic properties if the marking contains symbol A , or category P1, P2, P3 or P4 )

– EN ISO 20347:2004 – Occupational footwear (the occupational footwear has antistatic properties if the marking contains symbol A , or category O1, O2, O3, O4 or O5)

According to these three above standards, each pair of antistatic footwear shall be supplied with a leaflet containing the following important statement:

“Antistatic footwear should be used if it is necessary to minimise electrostatic build-up by dissipating electrostatic charges, thus avoiding the risk of spark ignition of, for example flammable substances and vapours, and if the risk of electric shock from any electrical apparatus or live parts has not been completely eliminated. It should be noted, however, that antistatic footwear cannot guarantee an adequate protection against electric shock as it introduces only a resistance between foot and floor. If the risk of electric shock has not been completely eliminated, additional measures to avoid this risk are essential. Such measures, as well as the additional tests mentioned below, should be a routine part of the accident prevention programme at the workplace.

Experience has shown that, for antistatic purposes, the discharge path through a product should normally have an electrical resistance of less than 1000m2 at any time throughout its useful life. A value of 100k2 is specified as the lowest limit of resistance of a product when new, in order to ensure some limited protection against dangerous electric shock or ignition in the event of any electrical apparatus becoming defective when operating at voltages of up to 250V. However, under certain conditions, users should be aware that the footwear might give inadequate protection and additional provisions to protect the wearer should be taken at all times.

The electrical resistance of this type of footwear can be changed significantly by flexing, contamination or moisture. This footwear will not perform its intended function if worn in wet conditions. It is, therefore, necessary to ensure that the product is capable of fulfilling its designed function of dissipating electrostatic charges and also of giving some protection during the whole of its life. The user is recommended to establish an in-house test for electrical resistance and use it at regular and frequent intervals.

Classification I footwear can absorb moisture if worn for prolonged periods and in moist and wet conditions can become conductive.

If the footwear is worn in conditions where the soling material becomes contaminated, wearers should always check the electrical properties of the footwear before entering a hazard area. Where antistatic footwear is in use, the resistance of the flooring should be such that it does not invalidate the protection provided by the footwear.

In use, no insulating elements, with the exception of normal hose, should be introduced between the inner sole of the footwear and the foot of the wearer. If any insert is put between the inner sole and the foot, the combination footwear/insert should be checked for its electrical properties”.

Use of working clothing in potentially explosive atmospheres: Antistatic shoes and clothes must be used. The floor must also be antistatic otherwise the wear of antistatic shoes is useless.

Underwear or warm clothing made of insulating synthetic fibres could be used as far as workers do not remove them in the explosive atmosphere zone.

Other nuisances arising from static electricy and their prevention

In addition to the risks of fire and explosion, static electricity may provoke nuisances both for workers and manufacturing processes.

Physiological effects

If the nuisance for workers is not dangerous in itself due to the weak energies involved (some millijoules), it can become painful by its repetition and even have very serious consequences by effect of surprise (e.g. falls from ladder). Most people do not feel an ElectroStatic Discharge shock unless they are charged to over 4000 V (the sensitivity threshold varies between people, and even over parts of the body).

This phenomenon of discharge is related to the accumulation of charges by the persons themselves by friction on the ground and on the seats, in particular in very dry weather conditions.

To prevent this phenomenon, which is frequent in offices and working rooms, several simple means exist:

– Use of anti-static materials avoiding the appearance of charges or use materials facilitating the flow of the charges. – Maintenance of a high relative air humidity, in all cases higher than 50% and in certain severe cases higher than 65%. – Application of anti-static or dissipating aerosols (effect limited in time)

Physical effects

– the energy transported by an electrostatic discharge or the corresponding electromagnetic field can damage electronic components or produce dysfunction of electronic or data processing materials. Indeed, current electronic components (ex : MOS) are very sensitive to static electricity; electrostatic discharges of very weak energy can deteriorate them. – The electronics components may also be disturbed; electrostatic discharges being able to cause in addition to the deterioration of component, the loss of data-processing data. Requirements related to this type of risk are laid down in the European standard EN 61340-5-1 “Electrostatics. Protection of electronic devices from electrostatic phenomena – General Requirements “ and in User Guide EN 61340-5-2. – another effect of the electric field can be the attraction of dust or particles. This phenomenon may cause problems in the manufacture of the integrated circuits or printing of circuits.

In these cases as in the precedents, it is necessary to fight against the static electricity by:

– avoiding the generation of electrostatic charges: use of antistatic materials, wearing of antistatic shoes and clothing, – allowing the dissipation of the charges: earthing of the conducting or dissipating materials, earthing of operator by means of bracelet, eliminators…

Bibliography

(The full text of the Directives can be consulted free of charge on the EUR-LEX European law website: :http://europa.eu.int/eur-lex/en/search/search_lif.html.)

Published: 01st Jul 2008 in Health and Safety International

• 89/391/EEC Council Directive 89/391/EEC of 12 June 1989 on the introduction of measures to encourage improvements in the safety and health of workers at work (OJEC L 183, 29.6.1989, p. 1)
• 89/655/EEC Council Directive 89/655/EEC of 30 November 1989 concerning the minimum safety and health requirements for the use of work equipment by workers at work (second individual Directive within the meaning of Article 16 (1) of Directive 89/391/EEC) (OJEC L 393, 30.12.1989 p. 13)
• 94/9/EC Directive 94/9/EC of the European Parliament and the Council of 23 March 1994 on the approximation of the laws of the Member States concerning equipment and protective systems intended for use in potentially explosive atmospheres (OJEC L 100, 19.4.1994, p. 1)
• Guidelines on the application of Council Directive 94/9/EC of 23 March 1994 on the approximation of the laws of the Member States concerning equipment and protective systems intended for use in potentially explosive atmospheres, May 2000 (published by the European Commission, 2001). ISBN 92-894-0784-0
• 1999/92/EC Directive 1999/92/EC of the European Parliament and of the Council of 16 December 1999 on minimum requirements for improving the safety and health protection of workers potentially at risk from explosive atmospheres(15th individual Directive within the meaning of Article 16 (1) of Directive 89/391/EEC) (OJEC L 23, 28.1.2000, p. 57)
• Guide of good practice for implementing Directive 1999/92/EC of the European Parliament and of the Council on minimum requirements for improving the safety and health protection of workers potentially at risk from explosive atmospheres, COM(2003) 515 final, 25.08.2003 67/548/EEC Council Directive 67/548/EEC of 27 June 1967 on the approximation of laws,
• 2006/42/EC Directive of the European parliament and of the of 17 May 2006 on machinery, and amending Directive 95/16/EC, (OJ L 157/24,09.06.2006)
• 89/656/EEC Council directive of 30 November 1989 on the minimum health and safety requirements for the use by workers of personal protective equipment at the workplace (third individual directive within the meaning of Article 16 (1) of Directive 89/391/EEC), OJEC L 393, 30/12/1989, p. 0018 – 0028
• 89/686/EEC Council Directive of 21 December 1989 on the approximation of the laws of the Member States relating to personal protective equipment (OJEC L 399, 30/12/1989, p. 0018 – 0038)
• Guide “Static electricity”, ED 874, 107 pages, INRS, 2004 ( available in French at http://www.inrs.fr/htm/electricite_statique.html)