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.
No comments:
Post a Comment