IEEE Std C57.12.20 pdf free download

IEEE Std C57.12.20 pdf free download.Overhead-Type Distribution Transformers 500 kVA and Smaller; High Voltage, 34 500 V and Below; Low Voltage, 7970/13 800Y V and Below.
7.2.1 Tap changer for de-energized operation
(See item 3 in Figure 7 through Figure 14.) Tap changers for de-energized operation shall be provided on tapped transformers. Each tap changer position and the tap voltage or percentage associated with it shall be clearly identifiable by reference to nameplate information. All positions of the dc-energized tap changer shall be operative positions. Transformers may have either an internal de-energized tap changer or an externally operated dc-energized tap changer unless one or the other is specified.
The internal dc-energized tap changer shall have the operating handle inside the tank above the liquid level. The tap changer shall rotate in a clockwise direction from a high tap voltage to a lower tap voltage in the high- voltage winding. The tap changer shall be provided with mechanical stops to identify the highest and lowest tap positions.
The handle of the externally operated dc-energized tap changer shall be brought out through the side of the tank in or near segment 3, as shown in Figure through Figure 14. It shall he designed to help prevent accidental operation by requiring a preliminary step before the tap setting can be changed. Position numbers or letters corresponding to the nameplate shall be clearly identifiable near the handle. There shall be located on or adjacent to the operating mechanism a written caution statement to de-energize the transformer before operating.
7.2.2 Series-multiple connections
(See item 14 in Figure 7, Figure , and Figure II.) The series-multiple voltage-changing device may be an internally mounted terminal board or externally operated switch, unless one or the other is specified. Either device is for dc-energized operation.
Internal terminal boards with movable links shall have all studs identified on the board, and the nameplate shall show the connections to be made for each voltage.
The handle of the externally operated switch shall be brought out through the side of the tank in or near segment 3, as shown in Figure 7, Figure , and Figure 11. It shall be designed to indicate voltage position and to help prevent accidental operation by requiring a preliminary step before the voltage setting can be changed. Actual voltages or position numbers corresponding to the nameplate shall be clearly identifiable near the handle. There shall be located on or adjacent to the operating mechanism a written caution statement to deenergize the transformer before operating.
7.2.3 Liquid-level marking
(See item 2 in Figure 7 through Figure 14.)A suitable marking inside the tank shall indicate the correct liquid
level at 25 °C.
7.2.4 Lifting lugs
(See item 5 in Figure 7 through Figure 14.) The lifting lugs shall be permanently attached to and arranged on the tank to provide a balanced lift in a vertical direction for the completely assembled transformer and shall be designed to provide a satiety factor of five (NEMA MG2 [B3]). The safety factor of five is the ratio of the ultimate stress to the working stress of the material used. The working stress is the maximum combined stress developed in the lifting lugs by the static load of the completely assembled transformer(MIL-STD-209K[B2].)IEEE Std C57.12.20 pdf download.