Circuit-breakers

During overloading and short circuits, we require circuit-breakers to safeguard electric circuits. Circuit-breakers are automatic devices capable of making and breaking of electric circuits. Insulating fluids are used here to extinguish the arc produced due to the fault current interruption and to provide total insulation between the contacts and from each contact to earth. Some examples of insulating fluids are air, compressed air, hydrogen producing oil and sulphur hexafluoride gas. Insulating materials used in circuit-breakers should have high dielectric strength, thermal and chemical stability, high thermal conductivity, low dissociation temperature, should be non-flammable, should not produce carbon during arcing and should be commercially available. The arc between the circuit-breaker consists of ionized gas particles. By deionizing, we can execute arc interruption. This can be done by high pressure development, arc splitting or forced convention and turbulence. Switchgear can be used to execute current interruption.

Biological oxygen demand

This gives an idea about the water purity and quality. It is a measure of the amount of oxygen needed by bacteria and other microorganisms to oxidize the organic material present in a water sample over a period of 5 days. The biological oxygen demand (BOD) of drinking water should be less than 1. Nitrates and phosphates in water contribute to BOD. These are plant nutrients and facilitate plant and algae growth in water. When these plants die, it becomes an organic waste which gets decomposed by bacteria resulting in a high BOD level. When water temperature is increased, rate of photosynthesis increases. This enhances plant and algae growth. On death, they are decomposed and BOD level gets increased. Biological oxygen demand (BOD) can be determined experimentally by a method known as Winkler’s method. The basic idea employed here is that in alkaline medium, dissolved oxygen(DO) oxidizes manganese-2+ ions to manganese-4+ ions and in acidic medium, DO oxidizes iodine ions to free iodine.