Wolfson Electrostatics
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Electrostatic Hazards in the Home and Car
Graham Hearn, Technical Director of Wolfson Electrostatics, reviews two serious everyday hazards, Dangerous dusts & Forecourt fires which may have electrostatics as their cause and cure.
Dangerous Dusts
In our homes, places of work, and even during travel between the two, we are constantly inhaling microscopic particles. Indoors, these dust particles can include pet allergens and house dust mite faeces. Out of doors, fine airborne dusts may include diesel emission particles and pollen. Because these particles are very fine (many of them less than 10 microns in diameter) they are capable of travelling deep into the lung causing asthma and other bronchial problems. The scale and extent of these problems is enormous. In 1993, for example, the cost of pharmaceuticals to the National Health Service in the UK for the treatment of asthma alone was 347 million pounds, representing 11% of the total cost of all prescriptions. In Western Europe, there is an alarming upsurge in the development of asthma, especially in children, and in the UK alone approximately one in every seven infants is now likely to develop the illness.
Modern homes usually maintain a relative humidity of between 50% and 80% and temperatures of around 25C. These conditions, combined with the proliferation of fitted carpeting, central heating, sealed double glazed windows and upholstered bedding, create ideal conditions for mite colonisation. Mites feed on human skin flakes and adults are typically 400 microns in length. Each may produce up to 20 faecal pellets a day, ranging in size from 10 to 40 microns. It is surprising to many that most common dwellings are colonised by mites and a typical infestation level is 100,000 mites in one square metre of carpet. One of the most likely places for dust mite infestation is in carpeting, with the fine dust settling deep down into the carpet pile on or near the upper surface of the backing. Simply walking on a carpet can disturb these dust particles sufficiently for them to become airborne, with the subsequent risk of inhalation. Paradoxically, cleaning the carpet with a vacuum cleaner can exacerbate the problem by agitating this deposited layer (some particles containing allergens are so fine that they will pass through the bag and filters in the vacuum cleaner and be dispersed into the air). Even wet cleaning systems are not recommended, as the increase in humidity actually makes conditions favourable to dust mites.
All of this leads to the need for a totally different philosophy towards allergen avoidance in the home. In the Electrical Engineering Department of the University of Southampton work has been undertaken to develop ways of preventing allergen inhalation as an alternative to the use of drugs1.
One method, which has so far given promising results, is to use electrostatic forces of attraction to anchor the allergenic particles within the carpet thus preventing the dust from becoming airborne. This immediately reduces the risk of inhalation. Work at the University of Southampton has looked at the possibility of using electrets in order to establish a permanent electrostatic charge in the carpet fibres or backing. An electret may be compared to a permanent magnet but instead of displaying permanent magnetic polarisation, electrical charge within the volume of a polymer is permanently displaced to create an electrical dipole. This technique is already widely used in filtration and is particularly well suited to the precipitation of respirable dusts. Preliminary tests at the University of Southampton have already shown that the addition of electret fibres to the backing web of a tufted carpet enhances particle retention with a corresponding improved retention of the allergen content1.
Obviously the continuous retention of particles within the carpet inevitably leads to a build up of dust, which may eventually present a general domestic hygiene problem. One suggested way of addressing this problem is to move to the concept of a low allergy electret carpet as a disposable item - just like a filter. Such a carpet need not necessarily be of high quality and a low cost disposable product may well be acceptable, especially for use in high-risk areas such as children's bedrooms, nurseries and hospitals.
Footnote:
It is well known that electrostatic forces can be used to precipitate airborne dust particles from the atmosphere and an example of this precipitation effect can be found in almost every home. Television sets generate high voltages which set up an electric field between the TV set and surrounding surfaces. This electric field influences dust particles in the air and tends to deposit them on the screen, the TV housing and any nearby surfaces. It is quite common for a very fine layer of dust to accumulate on the screen of a TV after it has been switched on for a few hours. If this dust were analysed it would almost certainly contain fragments of dust mite faecal pellets - a material, which has been identified as one of the main asthma allergen carriers.
Forecourt Fires
Electrostatic safety audit on plastic fuel tank
During the early 1990's, health and safety officials in Germany were notified of over thirty fires occurring during the refuelling of cars at roadside service stations3. Similar incidents have occurred in other European countries, although there have been few such incidents reported in the UK. Subsequent investigations by Wolfson Electrostatics and PTB in Germany have attributed the cause to static electricity with factors including fuel pump nozzle design, vehicle construction materials and forecourt surfaces all contributing to the hazard. Recently, a similar incident, which occurred in the UK, was captured on a forecourt security video. From the video is it impossible to ascertain the cause of the ignition. All that can be seen is a sheet of flame issuing from the vehicle fuel tank causing the person filling the car considerable alarm, but no physical injuries. An examination of the incident by Wolfson Electrostatics produced some interesting findings.
Most people have experienced annoying shocks from static electricity with modern vehicles and most people blame the car. It is true that vehicles can rise to several thousand volts while in motion. However, tyres and road surfaces are, in electrostatic terms, relatively electrically conductive. Collaborative work by Wolfson Electrostatics and the Motor Industry Research Association (MIRA) involved measurements on a vehicle on the MIRA test track and recorded transient voltages of 5000 volts or greater. This electrostatic potential, however, decayed to zero in less than 1 second once the car became stationary. The electric shock that occupants experience getting out of the car is due to electrostatic potentials developed on them rather than the car body. Reaching out and touching the metal door of the car causes a spark to occur from their charged body to the car at ground potential, rather than vice versa. In this situation static electricity is generated by frictional contact between the clothing of the vehicle occupant and the synthetic seat fabric. Since the occupant is entirely isolated from contact with earth (plastic steering wheel, plastic dash, synthetic carpets, synthetic seats) there is no opportunity for the charge to leak away. If the person is wearing synthetic footwear or the forecourt flooring is electrically insulating, they can walk from the vehicle to the fuel pump carrying a high level of static.
The spark energy required to ignite petroleum spirit vapour is around 0.25 milliJoules. Work undertaken at MIRA has indicated that sparks generated by people leaving a vehicle can have an energy content of up to 100 milliJoules. An obvious hazard is now created.
There is an important difference between roadside service stations in the UK and those in continental Europe. In the UK it is impossible to latch the pump on, therefore during the filling of the car, the person is maintained at earth potential by virtue of continuous contact with the earthed nozzle and trigger. In some European countries, it is possible to leave the pump while it is still delivering fuel and return to the interior of the car, thus regenerating static electricity on the body by sliding into and out of a seat. By returning to the pump a spark can then be produced in the presence of petrol vapour. Some of the aforementioned incidents in Germany have been attributed to this chain of events.
An increasing number of vehicle parts are now constructed from plastics. This applies to both interior and exterior components. One of the largest single plastic items is now the fuel tank. There are considerable engineering advantages in constructing a fuel tank from plastic rather than metal. However, the use of highly electrically insulating material, such as plastic, in close proximity with the sensitive flammable atmosphere produced by the fuel raises the question of electrostatic ignition hazards. In this case, the generation of electrostatic charge is due not to the vehicle or the occupant but results from the high speed pumping of petrol into the tank. Many unleaded fuels have an electrical conductivity below 100 picoSiemens per metre. This places them in the optimum range for electrostatic charge generation. Relatively insulating liquids, such as petroleum spirit pumped through pipes, constrictions and filters, can develop surprisingly high levels of static.
A plastic fuel tank assembly, when installed in a vehicle, is a relatively complex structure and there are many metallic fuel tank components such as pumps, springs, valves and clips. Static electricity generated by the flow of fuel into the tank can easily be transferred to these items if they are ungrounded.
Sparks from ungrounded metal components in and around the fuel tank have been responsible for a number of vehicle fires and this appears to be the case in the recent UK incident. Leading car companies, having realised the threat of static electricity, have now established an audit procedure aimed at evaluating and eliminating ignition hazards from plastic tanks. Each new design is rigorously tested.
The general question of static electricity and forecourt fires, however, has yet to be fully addressed.
References
- Hughes J F "Electrostatic allergen control", Electrotechnology, Oct/Nov 1996.
- Gaynor P T and Hughes J F, "Dust anchoring characteristics of electret fibres with respect to Der p I allergen".
- Von Pidoll et al "Avoidance of electrostatic hazards during refuelling of motor cars", J Electrostatics, 40 & 41 (1997), 523-528.
For further information on possible electrostatic hazards in your environment contact Wolfson Electrostatics.