1-The type of oil that has been used in virtually all oil circuit breakers is one where naphthenic base petroleum oils have been carefully refined to avoid sludge or corrosion that may be produced by sulfur or other contaminants.
The resulting insulating oil is identified as type 10-C transformer oil. It is characterized by an excellent dielectric strength, by a good thermal conductivity (2.7 x l0-4 cal/sec cm ˚C) and by a high thermal capacity (0.44 cal/gm ˚C).
The purity of the oil usually can be judged by its clarity and transparency. Fresh oil has a clear amber color, while contaminated oil is darkened and there are some black deposits that show signs of carbonization. The condition of the oil normally is evaluated by testing for its withstanding capability. The tests are made using a spherical spark gap with two spheres 20 mm in diameter and at a gap of 3 mm.
Fresh oil should have a dielectric capability greater than 35 kV. For used oil it is generally recommended that this capability be no less than 15 kV.
2-UL 489 encompasses circuit breakers “intended for installation in a circuit breaker enclosure or as parts of other devices, such as service entrance equipment and panel boards.” According to UL terminology, devices meeting this standard are considered listed products.
For approval, UL 489 requires the device pass a series of calibration, overload, endurance and short-circuit tests. The minimum short-circuit test must be performed at 5000A. Overload tests are performed at six times the current rating of the device or 150A minimum. Devices rated up to 600V and
6,000A are covered in this standard. Additionally, most UL 489 devices are used in electrical distribution panels; therefore, the minimum current ratings available are seldom less than 15A. During UL 489 testing, the device must survive short-circuit testing and continue to provide future overload protection.
In service entrance panels, available short-circuit fault currents measure 50,000A or greater. However, as power is distributed throughout a building, the available short-circuit currents diminish. If an electrical outlet is just 20 feet away from the power source, Ohm’s law states even with limitless available short-circuit current AWG 14 copper wire limits the maximum available fault current at the outlet to no more than 1200A at 120V.
-----------------------------------------------------------------
Creative thinking is enjoyable, Then think about your surrounding things and other thought products.
http://electrical-riddles.com
