Electricity at work- health and safety
David Rogers looks at how the control of static electricity in the workplace has evolved, and discusses what equipment is currently best suited to protect workers from the risks it poses.
Static electricity is often viewed as the hidden enemy of health and safety professionals, particularly in the manufacturing and processing industries.
As an invisible force, it is often not identified as a problem until an incident occurs. However, the consequences may be serious for both the personnel involved and the company. Practitioners need to protect the well-being of their workforce from the possibility of electric shocks and subsequent injuries, and ensure that the working environment is adequately protected against the risk of fire and explosion caused by electrostatic discharge.
What is static electricity?
It was in 600BC that the philosopher and mathematician Thales of Miletus first described rubbing amber on the fur of a cat and consequently being able to pick up feathers. This is the first known account of the natural force of static electricity, and the word electrostatic, meaning ‘electricity at rest’, was introduced.
When a material holds a net electrical charge, either positive or negative, it is said to have a static charge. In many cases, this charge will decrease slowly over time — with the actual length of time depending on the resistance of the material. For practical purposes, the two extreme examples can be taken as plastics and metal. Plastics generally have high resistances, allowing them to maintain static charges for long periods. Metals, on the other hand, have low resistance, which means an earthed metal object will discharge almost instantaneously.
Static in industry
Until about 50 years ago, static was not a significant problem in industrial processes, as many of the key elements that influence the generation and maintenance of a static charge — such as types of material, ambient temperature, humidity, and repeated actions like friction or separation — were not present in the manufacturing environment. In the early days of textiles manufacture, for example, the raw natural products wool and cotton absorbed water from the atmosphere and so had a very low static charge. Also, the old factory environment, with its steam pipes providing both heat and humidity, meant that the likelihood of generating static charges was low.
However, raw materials and manufacturing processes have changed substantially over the last 50 years, as has the factory. Firstly, man-made materials such as nylon, polyester, and various plastics now dominate and are more readily charged than natural materials.
Furthermore, the widespread introduction of air conditioning and dry air heating has led to both drier atmospheres and reduced humidity levels, bringing static issues to the fore. Industrial processes suffer to a greater or lesser extent from static-related problems of product contamination, slow machine speeds, shocks to operators, and risk of fires or explosions.
Shocks in the workplace
Many people have experienced the uncomfortable feeling of an ‘electric shock’. Everyday activities such as closing the car door, pushing a trolley around the supermarket, or shutting the dishwasher can result in voltages of more than 10,000 volts. However, as the current flows for such a short time, it is rare that any serious harm occurs.
In a factory environment, a machinery operator can receive a shock directly from the product they are working with, or become charged through induction while they are standing in the electric field of a charged object — for example, while working in a converting plant, adjacent to a plastic web during the unwinding and rewinding process. The charge builds up until the operator touches an earthed part of the machine, and then discharges, resulting in a shock. Again, it is unlikely to lead to any serious harm, but if the shock is strong enough to cause a recoil reaction, then accidents may occur as the operator collides with a colleague, with a machine frame, or stumbles into the path of other machinery, resulting in injury.
Eliminating the risk of static shock is important to avoid injury and to show due care and attention to staff welfare. There are, however, other underlying aspects. Firstly, when someone receives a shock, the machine involved must be stopped to investigate the cause. This will result in downtime and lost production. Should the shock be strong enough for the recipient to require medical attention then the machine cannot be restarted until it is checked by health and safety experts.
Production can be further affected if these shocks are happening to operators on a regular basis, as people will intuitively work at a slower pace if they perceive themselves to be at risk of a shock. No one likes to experience pain and the constant threat of shocks will result in a dissatisfied and unhappy workforce. Also, in today’s litigious age, a company’s failure to “provide a safe working environment” may result in legal action.
Passive to active control
One of the earliest ways to control static charge in industry was by using ‘passive’ ionisation in the form of conductive materials, such as carbon fibre brushes. This method reduced the level of static charge but didn’t remove it completely.
Static control took a major step forward in the 1960s with the introduction of ‘active’ electrical ionisation systems. These systems relied on normal AC mains voltage being boosted to about 5kV through a special transformer and being carried to sharp emitter pins, where the high energy generated a large number of negative and positive ions. A statically-charged surface of either polarity passing close to this ion cloud would then be neutralised. Such systems worked well but were limited in terms of working distance (typically 20mm). They also had a relatively short working life and caused electrical shocks if the pins were inadvertently touched.
Progressive enhancements to this AC technology have improved system performance. In particular, the recent development of resistively coupled systems has improved reliability, provided shockless operation, and extended the working range up to 150mm.
The relatively recent introduction of pulsed DC technology for static control provided not only an effective alternative to AC systems but has also optimised solutions for specific materials and more demanding processes.
What can be done?
A wide range of specialist equipment is available to control static within the workplace. Conductive wristbands and floor mats work well in many areas but are impractical in environments where the workforce needs to move around a lot.
Ionising blowers are widely used throughout the manufacturing industry to prevent shocks. They work by combining an ionisation head with an integrated fan system and can generate a high-volume flow of ionised air over a wide work area, leaving the workforce comfortable and static-free.
Ionising bars can be mounted directly on to machinery to control static and safeguard operators. They differ in terms of performance and range of ionisation, depending on the particular application. Static-control specialists will be able to advise on the optimum solution and positioning of the ionising bar to give the best results.
Mounting ionising equipment in hazardous environments requires special attention. This is particularly important in factory environments, where solvents are used as part of the process. For example, in the printing industry, solvent-based inks are a potential source of fire and high-speed web presses charge up as they pass through successive print heads, leading to the danger of a static discharge igniting the flammable solvents and vapours. In these circumstances, companies must ensure that their static-control solutions are certified by an approved testing body to comply with Article 9 of the Council Directive 94/9/EC (ATEX).1
Thermoforming is another manufacturing process in which there is an increased risk of uncontrolled static causing a potentially fatal incident. Both flammable solvents and gases are present, doubling the risk of fire, which can be easily caused by a spark resulting from a static discharge.
Owing to the high costs that may be incurred as a direct result of static-related fires or accidents, static-control systems employed should be the most effective available. At the extrusion stage of the thermoforming process, safety can be further enhanced by a new development that automatically controls and optimises the removal of static charge.
Specifiers, therefore, need to take advice from their static-equipment providers to ensure that they can differentiate between products that merely provide the reporting of static, and systems that can optimise static charge removal by sensing the residual charge on the target material, and reacting to it. This means that the static-control system can operate at optimum performance under a range of conditions without the need for operator intervention.
As manufacturing processes continue to develop, so the associated effects of static charge will present bigger challenges. Whereas, in the past, the main focus of static control was to improve productivity and profitability in industry by reducing the build-up of static-charge on processed materials, now industry is seeing health and safety concerns being discussed more openly. The signs are good, as static-control companies are now working together with health and safety professionals to help reduce the hazard risk from static discharges and prevent operators from suffering static-related shocks.
1 European Parliament and Council, Directive 94/9/EC Equipment intended for use in Potentially Explosive Atmospheres (ATEX)
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