ABSTRACT
The voltage level of a distribution system can be anywhere from about 5 kV to as high as 35 kV with the most common voltages in the 15 kV class. The average short-circuit level at a distribution substation has been shown, by survey, to be about 10,000 A. There are few things more confusing in distribution engineering than trying to find out rules of over-current coordination, that is, what size fuse to pick or where to set a relay, etc. The loading of distribution transformers varies more widely than substation units. When the transformer is overloaded, the high temperature decreases the mechanical strength and increases the brittleness of the fibrous insulation. Continuous thermal rating factor (TRF)—Continuous TRF is normally designated for current transformers and is the factor by which the rated primary current is multiplied to obtain the maximum allowable primary current without exceeding temperature rise standards and accuracy requirements.
