The electrostatic properties of protective clothing
The rubbing of a synthetic material against the skin or undergarments is sufficient to permit electrostatic charges to build up on the fabric. These triboelectric effects of a fabric can generate thousands of volts and a charge dissipation via a tiny spark from a coverall to a surface of opposite electrical potential in a flammable, gaseous, or dust-charged atmosphere could result in a catastrophic explosion.
Safety in explosive environments
Companies operating in sectors such as the chemical, pharmaceutical, industrial coatings and gas supply industries use combustible materials that can potentially create explosive atmospheres. These ‘explosive protection zones’ or ‘EX-Zones’ are classified into various categories depending on the frequency and length of time that the hazard exists. Combustible gases and vapours are classified into three explosion groups (IIA, IIB and IIC) according to the minimum amount of energy required to ignite them. The most easily ignitable group is class IIC.
Antistatic features in protective clothing
Antistatic finishes for limited-use garments generally work by using the moisture of the air to turn the finishing compound into a charge-conductive surface. This means, that if there is enough moisture in the air – typically above 25% RH – the antistatic property is ’active’. If however the moisture level is below 25% RH, the antistatic property will be either reduced or perhaps completely absent, depending on the prevailing humidity level.
Earthing
In order to avoid the creation of sparks (that might ignite an explosive atmosphere or cause operator discomfort), the garment and the wearer need to be properly grounded. This means that both the clothing and the wearer must be continuously earthed, taking care to ensure that the correct fabric side (inner or outer) is grounded in those cases where the garment’s antistatic treatment is limited to one side. Special attention must also be paid to garments with attached socks or overshoes.
There are some essential rules for the safe discharge of static electricity:
• Both wearer and garments must be correctly and continuously grounded via conductive safety shoes, floor and/or grounding cable.
• Electrostatic charges may build up on ancillary equipment. Breathing apparatus and other devices must therefore be separately grounded when being worn in conjunction with a garment.
Single-sided versus double-sided
Some fabrics, particularly multi-layer, coated and coloured fabrics, may be antistatic treated on one side of the material only. An antistatic coating on both sides of a garment will reduce antistatic build-up and the attraction of particulates. However, neither single- or double-sided coatings will necessarily prevent the risk of ignition in highly explosive conditions such as hydrogen atmospheres and oxygen-enriched air. In these cases the garment manufacturer must be consulted for guidance. In all situations the garment must be adequately grounded. With one-side treated garments care must be taken to ensure that it is the surface of the clothing which has been given antistatic treatment that is earthed.
Atex Directives
For standard chemical protective clothing it is not a compulsory requirement for garments to be antistatically treated or have antistatic features. However due to the prevalence of operations and applications being managed under ATEX controls it is a much-requested feature. Organizations in the EU must follow the ATEX[1] Directives to protect employees from explosion risk in areas with an explosive atmosphere. There are two ATEX directives:
• The new ATEX Directive 2014/34/EU[2] is for equipment manufacturers and covers equipment and protective systems intended for use in potentially explosive atmosphere.
• The ‘ATEX 137’ workplace directive 99/92/EC[3] provides minimum requirements for improving the safety and health protection of workers potentially at risk from explosive atmospheres.
Antistatic certifications
In order to compare antistatic properties of chemical protective clothing on a standardised level, there are several norms which manufacturers can use. With such norms the surface resistance and the charge-decay properties of fabrics can be measured and/or assessed. The surface resistance is covered by EN 1149-1 and the charge-decay is covered by EN 1149-3. EN 1149-1 is mostly used for finished fabrics, whereas EN 1149-3 is used when surface resistivity cannot be used because the dissipation of charges is based on induction. In addition to these test-method standards there is a further standard, EN 1149-5:2018[4] which provides the performance requirements for anti-static PPE.
[1] ATEX – The abbreviation derives from ‘ATmosphères EXplosibles’.
[2] Directive 2014/34/EU on equipment and protective systems intended for use in potentially explosive atmospheres.
[3] Directive 99/92/EC Minimum requirements for improving the safety and health protection of workers potentially at risk from explosive atmospheres
[4] EN 1149 – 5:2008 Protective clothing with electrostatic properties
