How a spark forms

1. Static charge builds up whenever insulating materials rub against each other, and it stays where it is because there is no conducting path away.
2. Liquid fuel flowing quickly along a hose or pipe rubs against the inside wall, so electrons are transferred between the fuel and the pipe and both become charged.
3. Powders such as flour, sugar or grain dust charge up in the same way as they are poured, tipped or blown along ducting.
4. Clothing, plastic containers and vehicle bodywork all charge up through ordinary movement and contact.
5. As the charge collects, the potential difference between the charged object and nearby earthed metal grows larger.
6. When the potential difference across the air gap becomes large enough, the air stops behaving as an insulator and charge crosses it.
7. That brief flow of charge through the air is a spark, and it lasts only a tiny fraction of a second.
8. All of the stored energy is transferred in that very short time, so the air in the spark channel reaches a high temperature and glows.

Why sparks are dangerous

1. A spark is dangerous mainly because it is a source of ignition in places where flammable substances are present.
2. Petrol vapour, diesel vapour, solvents, aerosol propellants, methane and fine dusts all catch light readily once they are ignited.
3. A vapour or dust mixed with air burns far more easily than the liquid or solid it came from, because the fuel is already spread through the oxygen it needs.
4. Even a small spark can transfer enough energy to start that reaction.
5. If the mixture burns inside an enclosed space, the hot gases expand rapidly and the result is an explosion rather than a steady fire.
6. Filling a car with petrol is a familiar risk, because petrol evaporates easily and vapour collects around the open filler neck.
7. Charge can build up on the driver, on the car body, on the nozzle or on a plastic fuel container during filling.
8. Transferring fuel between a road tanker and an underground storage tank moves large volumes very quickly, so far more charge can be separated.
9. Aircraft refuelling, solvent and paint handling, and grain, flour and sugar silos all carry the same hazard.
10. Sparks are also a hazard around sensitive electronics, because a discharge through a microchip can destroy it permanently.

Preventing the build-up by earthing

1. Earthing deals with the hazard at its source by stopping charge from collecting in the first place.
2. A conducting path lets electrons flow to or from the earth as quickly as the charge is separated, so the charge never reaches a dangerous level.
3. With no large build-up, the potential difference across any air gap stays small and no spark can form.
4. A road tanker is connected to an earthed point with a bonding cable before any fuel is transferred, and the cable stays attached until the transfer is complete.
5. Aircraft are earthed and bonded to the fuel tanker for the same reason before refuelling begins.
6. Metal pipework, nozzles and filters in a fuel system are bonded together with conductors, so that no two parts of the system can sit at different potentials.
7. Fuel must be dispensed into a metal container standing on the ground rather than a plastic one resting on a car boot, because the metal container can be earthed and the plastic one cannot.
8. Drivers are advised to touch the metal bodywork before picking up the nozzle, so any charge on them is removed before they are close to the vapour.
9. Antistatic additives in the fuel, conducting hoses and conducting flooring all do the same job by making otherwise insulating parts of the system slightly conducting.
10. Limiting the flow rate of the fuel also helps, because slower flow rubs less and separates less charge.
11. Humidifying the air in a factory reduces static build-up as well, because a thin film of moisture on surfaces lets charge leak away steadily.