Electrostatic Solutions Ltd |
Hazards & nuisance |
| Electrostatic Technologies | Electronics manufacture | R&D services |
Welcome to Electrostatic Solutions, the UK based on-line static electricity R&D and consultancy center. Please browse around, and get in touch if we can be of assistance.
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Static electricity can cause sparks to occur unexpectedly. In some industries such sparks cause the risk of fire or explosion, and it is important to avoid this by reducing static electricity build-up to safe levels.
Ignition is a possible risk if;
If electrostatic ignition hazard is suspected, then it is important to get the expert advice needed to assess the situation. Determining the need for preventative measures, and how to reduce electrostatic ignition risk to negligible levels, is best done by an experienced specialist. Good general guidance is given in BS5958, a key UK standard dealing with electrostatic hazards, but no standard can adequately cover all possible situations.
Typical measures aimed at preventing ignition include;
The Minimum Ignition Energy of a flammable mixture is used as an indicator of the possible spark ignition sensitivity of the fuel under a given set of test conditions. The risk of ignition of a flammable mixture is a complex function of the fuel ignition sensitivity and the igniting ability (incendivity) of the spark.
Electrostatic discharges occur in many forms, and this incendivity factor in itself is highly complex. The most hazardous type of ESD is one which occurs between metal objects. This has the highest incendivity. A discharge from an insulating surface is less incendive, but can easily ignite some sensitive fuel mixtures. Another type of discharge, the corona discharge, tends to occur from sharp edges of metals at high voltages. This is generally excepted to be not incendive under most circumstances, although the situation where highly sensitive atmospheres are present is less clear.
Most people know from their own experience that when you move around in everyday life, static electricity can cause sparks to occur unexpectedly. This effect can appear and disappear inexplicably - some days, little static is noticed, but other days sparks seem to fly regularly. The effects literally change with the weather - on humid days, there is generally little noticable static electrical activity. Static is most often noticed in a very dry atmosphere, especially in centrally heated buildings during cold dry weather.
One problem that is often experienced is that when you get out of the car, you get a shock on closing the door. The source is usually static charges which build up between your body and the car seat while you are in the seat, but remain harmlessly neutralised until you get up. At that time, you take considerable electric charge with you as you get out of the car. If the charge has no discharge path, then a very high voltage (several thousand Volts) can build up very quickly. When you reach for the door, the high voltage causes a spark which discharges you quickly to the car - giving you a shock in the process.(Many people cannot feel shocks with discharges of less than about 4000 Volts!). One solution is to make sure you hold onto the metal door frame as you get out of the seat, allowing yourself to harmlessly discharge slowly as you get up.
Static electric charges can build up on trolleys as you push them around a shop. The charge is usually generated by the movement of the trolley wheels, and your foot action as you walk. Once again, thousands of Volts may be built up on your body and on the trolley. The charge remains unnoticed until you touch something, giving the characteristic shock.
A similar thing happens when you walk across a floor, when high levels of electric charge often building up on your body as you walk. The shock comes when you touch a filing cabinet, door knob, or other substantial conductive object, which may or may not be electrically grounded. (Sometimes people assume that the object gave them a shock, when in fact they themselves were the source). If the object happens to be a computer or other electronic system, the system can experience an electrostatic discharge (ESD) which can cause the system to crash if the shock is great enough. I once measured the voltage on about ten computer users after they had entered a computer room, and before they sat at their consoles. Few of them had a voltage of less than 4kV (1kV=1000V) before the sat down. Their body voltage normally increased substantially as they sat down, unless they happened to touch a conductive discharge path as they sat.
Photocopiers use static electricity in their operation, and also generate a fair amount of static on the paper or film as it runs through the machine. A person operating the machine for some time may find that some of this static charge builds up on them as the unload the paper from the output trays. This can, in extreme cases, cause unpleasant shocks to be experienced. A simple temporary solution ( which may or may not be acceptable!) is for the operator to take their shoes off! This often allows the static charges to drain to ground before they can build up to significant levels.