STATIC ELECTRICITY

Static electricity is a voltage charge which builds up to many thousands of volts between two surfaces when they rub together. A dangerous situation occurs when the static charge has built up to a potential capable of striking an arc through the air gap separating the two surfaces.

Static charges build up in a thunderstorm. A lightning strike is the discharge of the thunder cloud, which might have built up to a voltage of 100 MV, to the general mass of earth which is at zero volts. Lightning discharge currents are of the order of 20 kA, hence the need for lightning conductors on vulnerable buildings in order to discharge the energy safely.

Static charge builds up between any two insulating surfaces or between an insulating surface and a conducting surface, but it is not apparent between two conducting surfaces.

A motor car moving through the air builds up a static charge which sometimes gives the occupants a minor shock as they step out and touch the door handle.

A nylon overall and nylon bed sheets build up static charge which is the cause of the ‘ crackle ’ when you shake them. Many flammable liquids have the same properties as insulators, and therefore liquids, gases, powders and paints moving through pipes build up a static charge.

Petrol pumps, operating theatre oxygen masks and car spray booths are particularly at risk because a spark in these situations may ignite the flammable liquid, powder or gas.

How to reduce Static Electricity?

Bonding surfaces together with equipotential bonding conductors prevents a build-up of static electricity between the surfaces. Use of large-diameter pipes, reduce the flow rates of liquids and powders and, therefore, reduce the build-up of static charge. Hospitals use cotton sheets and uniforms, and use bonding extensively in operating theatres. Rubber, which contains a proportion of graphite, is used to manufacture antistatic trolley wheels and surgeons ’ boots. Rubber constructed in this manner enables any build-up of static charge to ‘ leak ’ away. Increasing humidity also reduces static charge because the water droplets carry away the static charge, thus removing the hazard.

Hazardous area classification & Type of Electrical equipment usage

            As  per  standard,  the risk associated with inflammable gases and  vapours are classified into 3 classes or zones. 

    a)   Zone 0 is the most hazardous, and is defined as a zone or area in which an explosive gas–air mixture is continuously present or present for long periods. (‘Long periods’ is usually taken to mean that the gas–air mixture will be present for longer than 1000 h per year.)

Ordinary electrical equipment cannot be installed in Zone 0, even when it is flameproof protected. However, many chemical and oil-processing plants are entirely dependent upon instrumentation and data transmission for their safe operation. Therefore, very low-power instrumentation and data transmission circuits can be used in special circumstances, but the equipment must be intrinsically safe, and used in conjunction with a ‘safety barrier’ installed outside the hazardous area. Intrinsically safe equipment must be marked Ex ‘ ia ’ or Ex ’ s ’ , specially certified for use in zone 0.

b)   Zone 1 is an area in which an explosive gas–air mixture is likely to occur in normal operation. (This is usually taken to mean that the gas–air mixture will be present for up to 1000 h per year.)

In Zone 1 all electrical equipment must be flameproof and  marked Ex ‘ d ’ to indicate a flameproof enclosure.

C)    Zone 2 is an area in which an explosive gas–air mixture is not likely to occur in normal operation and if it does occur it will exist for a very short time. (This is usually taken to mean that the gas–air mixture will be present for less than 10 h per year.)  

The electrical equipment used in zone 2 will contain a minimum amount of protection. For example, normal sockets and switches cannot be installed in a zone 2 area, but oil-filled radiators may be installed if they are directly connected and controlled from outside the area. Electrical equipment in this area should be marked Ex ‘o’ for oil-immersed or Ex ‘p’ for powder-filled.

If an area is not classified as zone 0, 1 or 2, then it is deemed to be nonhazardous, so that normal industrial electrical equipment may be used.