Hazardous Area

Hazardous areas are classified into zones according to the nature of the gases present in the potentially explosive atmosphere, and the likelihood of that atmosphere being present. The nature of the atmosphere is characterized by the chemical composition of the gas and its ignition temperature, and the notions of gas grouping and temperature classification have been developed in order to formalize this.

A useful concept in the consideration of how explosions occur is the hazard triangle. The sides of the triangle represent fuel, oxygen and a source of ignition, all of which are required in order to create an explosion. For the purposes of this chapter, a fuel is considered as a flammable gas, vapor or liquid, although dust may also be a potential fuel. Oxygen is of course present in air at a concentration of about 21 per cent. The ignition source could be a spark or a high temperature. Given that a hazardous area may contain fuel and oxygen, the basis for preventing explosion is to ensure that any ignition source is either eliminated or prevented from coming into contact with the fuel-oxygen mixture.

Zone classification

The zone classification defined in IEC 79 is used in Europe and most other parts of the world; it is summarized in Table 16.1. Various types of explosion protection are available, and their suitability for the different zones is shown in the table.

Zone	Suitable protection
Zone 0 Areas in which hazardous explosive gas atmospheres are present constantly or for long periods, for example in pipes or containers	Ex ‘ia’ Ex ‘s’ (where specially certified Zone 0)
Zone 1 Areas in which hazardous explosive gas atmospheres are occasionally present, for example in areas close to pipes or draining stations	Ex ‘d‘; Ex ‘ib’; Ex ‘p’; Ex ‘e’; Ex ‘s’; Ex ‘o’; Ex ‘q’; Ex ‘m’; Equipment suitable for Zone 0
Zone 2 Areas in which hazardous explosive gas atmospheres are rare or only exist for a short time, for example areas close to Zones 0 and 1	Ex ‘N’/Ex ‘n’; Equipment suitable for Zones 1 & 0

In the USA hazardous areas are classified in a slightly different way, according to the National Electrical Code. In brief, hazardous areas are classified either as Division 1, where ignitable concentrations of flammable gases or vapors may be present during normal operation, or as Division 2, where flammable gases or vapors occur in ignitable concentrations only in the event of an accident or a failure of a ventilation system.

Gas grouping and temperature classification

The energy required for ignition differs from gas to gas, and the grouping of gases together with their classification by temperature is used in Europe to describe the suitability of a piece of electrical equipment for use with explosive atmospheres of particular gases.

Table below lists common industrial gases within their appropriate groups. Gas group I is reserved for the classification of equipment suitable for use in coal mines. Gas group I1 contains those gases found in other industrial applications, and it is subdivided according to the relative flammability of the most explosive mixture of the gas with air.

Group	Representative gases
I	Methane
IIA	Acetone, ethane, ethyl acetate, ammonia, benzol, acetic acid, carbon monoxide, methanol, propane, toluene, ethyl alcohol, I-amyl acetate, N-hexane, N-butane, N-butyl alcohol, petrol, diesel, aviation fuel, heating oils, acetaldehyde, ethyle ether
IIB	Town gas, ethylene (ethene)
IIC	Hydrogen, acetylene (ethyne), hydrogen disulphide

Temperatures are classified from T1 to T6, as shown in Table below 3. The levels show the maximum surface temperature permitted for equipment which has been assigned that temperature class, and the common gases for which each class is appropriate are also shown.

North American practice is to define hazardous materials in classes. Flammable gases and vapors are Class 1 materials, combustible dusts are Class 2 materials and ‘flyings’, such as sawdust, are Class 3 materials. Class 1 is subdivided into four groups depending on their flammability: A (including acetylene), B (including hydrogen), C (including ethylene) and D (including propane and methane). The subgroup letters are in the opposite order of flammability to the IEC groupings shown in Table 2. The North American temperature classification is similar to the IEC system shown in Table 3, but the classes are further subdivided to give more specific temperature data.

Class	Highest permissible surface temperature (oC)	Representative gases
T1	450	Acetone, ethane, ethyle acetate, ammonia, benzol, acetic acid, carbon monoxide, methanol, propane, toulene, town gas, hydrogen
T2	300	Ethyl alcohol, (-amyl acetate, N hexane, N-butane, N-butyl alcohol, ethylene
T3	200	Petrol, diesel, aviation fuel, heating oils
T4	135	Acetaldehyde, ethyl ether
T5	100
T6	85	Hydrogen disulphide

Refferrence

Newnes Electrical Engineer’s Handbook; D.F. Warne; 2000

Introduction to Health and Safety at Work, Fourth Edition 

Pregnancy By Trimester

Fetal Growth at the end of the (A) third, (B) sixth, (C) ninth month, and (D) just before birth

1st Trimester (Weeks 1-12)

During the first trimester, a woman may not even know she is pregnant. The first sign of pregnancy that she may encounter is her missed period. At this point she should take a pregnancy test to confirm that she is pregnant. After her missed period a woman may begin to experience the emotional aspects of the pregnancy such as mood swings and exhaustion. These emotional changes are usually accompanied by physical changes such as morning sickness, an increase in breast size and tenderness, and a decrease in sexual desire. There is considerable variability in the ways women experience these things.

Not only does the mother go through multiple changes during the first trimester, the baby does as well. By the end of the twelfth week the baby's bones have begun to ossify, and the eyes and ears have formed. The baby's external genitalia will begin to form, but they will not be complete until well into the second trimester. By the end of the first trimester, a woman should be able to listen to her baby's heart beat.

2nd Trimester (Weeks 13-26)

The second trimester is a little bit more difficult for the mother than the first. Although a lot of the physical symptoms present during the first trimester —tenderness of the breasts, morning sickness, and cramping—have started to go away during the second trimester, there are a few new physical changes that the mother will experience. She will start to notice that her belly is growing and she is starting to look more like a pregnant woman. She will start to feel the baby moving, and may even begin to have some Braxton-Hicks contractions. . Braxton-Hicks contractions are relatively painless contractions experienced throughout pregnancy that are not associated with going into labor. A woman usually experiences more Braxton-Hicks contractions as her pregnancy progresses. The mother will also notice that she has to go to the bathroom more often and her heart is beating more rapidly. She may also begin to have heartburn. These are all very common and normal symptoms of pregnancy, hence there is no cause for concern. The mother will probably still have mood swings during this period, but they are not as severe as in the first and third trimesters.

By the end of the second trimester the baby will be moving around in the amniotic fluid, and the baby will begin breathing. Eyebrows, fat deposits, and gender are visible in an ultrasound, and fingers, toes, and teeth are growing nicely.

3rd Trimester (Weeks 27-42)

The third trimester is usually the hardest of the three trimesters for the woman. Most women begin to feel very uncomfortable because the uterus is growing rapidly. They continue to have the Braxton-Hicks contractions, and most women start to get continuous backaches. The fact that they are so uncomfortable and have such painful back problems makes it hard for many women to sleep during this period. This adds to their exhaustion and mood swings. The heartburn usually continues during the third trimester, so women should watch what they eat during these three months. Also, most women will start to experience some droplets of fluid coming from their nipples. This is normal when a woman is close to the end of her pregnancy and she is ready to start breast-feeding.

The third trimester is also one of the more uncomfortable trimesters for the baby. The space in the uterus becomes cramped as the baby grows and the mother will notice more sharp movements. The baby will also begin to turn upside down in the uterus. The baby's eyes will start to function better, and he or she will be able to sense changes in light. All the parts of the baby, including the hair and fingernails, continue to grow and develop. By the end of the third trimester the baby will be ready for life outside his or her mother's uterus.

All of the symptoms and changes mentioned above are natural and expected for a pregnant woman and her fetus. If you are pregnant and are experiencing abnormal abdominal pains or cramps, spotting, or extremely intensified versions of the symptoms described above, you should consult a doctor as soon as possible. You should also make sure to get regular examinations during the entirety of your pregnancy.

Never Feel Sick Again

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