Sparks spell danger – The Physics Congress 2003
An effective new approach to preventing sparks when an electrically charged object is earthed could prevent explosions and save lives following a motorway smash involving fuel tankers or other hazardous vehicles. Dr Klaus Schwenzfeuer of the Electrostatic Laboratory at the Swiss Institute for the Promotion of Safety & Security in Basle, will reveal the method at the Institute of Physics Congress at Heriot-Watt University on Monday 24 March.
Earthing a conductive and charged object will almost always produce a spark discharge just before the earth contact is made. This spark can be enough to ignite any combustible gases or vapours in the vicinity. According to Dr Schwenzfeuer, there are several approaches to reducing the risk of sparking but some are more effective than others.
He explained that conductive objects such as metal fuel containers are usually earthed before they are used. This reduces the chances of a spark. However, while this approach is safe for fuel storage or transport, it assumes that the conductive object will never become charged with static electricity. That might happen in an accident when the earth connection could easily be detached. During an accident, Dr Schwenzfeuer pointed out, a flammable atmosphere might be present too. “In this case it is essential that an earthing mechanism is known, which can guarantee a spark free contact and thus a non incendiary contact between the earthing clamp and the conductive and charged object.”
The situation described above may typically be present if, for instance, the fire brigade were to empty a fuel tanker involved in an accident. Among fire brigades there are different opinions on how to avoid making sparks during the earthing procedure. Ideas such as earthing the truck with a cable not connected to earth or earthing the truck through an electrical resistor have been proposed in the past. Dr Schwenzfeuer suggested that the most effective method is to force a corona discharge, rather than a spark to occur. This is a well known method used for example in the printing or coatings industry, where rapidly moving machinery can carry a high electric charge and potentially release a dangerous spark in a flammable atmosphere.
A corona discharge releases the pent up electrical energy in a diffuse manner and does not reach the thousands of degrees temperature of a spark so that flammable vapours or gases in the area are very unlikely to ignite.
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