Tapping off

Short-lime rating of the tap-offs It is possible that in certain installations as a result of system needs it m iy not be possible to limit the fault level of the system within a desirable limit. A power-generatina station, connected to an external grid, to... 

Through the tap-offs of the bus, the unit auxiliary transformers (UATs) are connected to feed the station auxiliary services. For more clarity we have taken out the portion of the tap-offs from Figure 13.21 and redrawn it in Figure 13.18 to illustrate the above system and its interconnections. The tap-offs are now subject to the cumulative inOuence of the two supply sources. In the event of a fault on this section, both the sources would feed the same and the fault current through the tap-offs would add up. The tap-offs should thus be designed for the cumulative effect of both fault levels. For the sake of an easy reference, Table 13.8 suggests a few typical values of fault currents, worked out on the basis of data considered for the G and GT. One such example is also worked out in Example 13.3. 

Figure 13.18 Fault level of tap-offs of a bus system under cumulative influence of two power sources...

Accordingly, the fault level for which the tap-offs should be designed,... 

Main bus, its feeding tap-offs and interconnecting links or cables etc. and all outgoing links, interconnecting devices that are also connected on the main bus such as shown at section F. Figure 13.29. 

All the joints, particularly power joints at the main busbars, tap-offs and cable ends, should be checked... 

Installation ot overhead bus system with tap-off boxes in a large assembly shop... 

Figure 28.4(b) An overhead bus system shown with tap off boxes (Courtesy GE Power)... 

Figure 29.7 Tap-off connections from a large section of busbar showing the overlap as equal to tbe width of the smaller section lap-off links...

Tap-offs with lee joints from the main run to the two UATs. 

For each phase on the neutral side, I CT for differential protection, two sets of three CTs each for metering, in the tap-offs for the two DATs and two sets of three CTs each for protection in the tap-offs for the two UATs. 

Tap-offs with a neutral CT, from star point of the generator, to the neutral grounding transformer (NOT). 

VTs and surge protection tap-offs for each phase from the main run, for metering and surge protection. 

The terminal ends are provided with terminal palms or tap-off palms, as required, to bolt them to the other sections. 

The cross-sections of the conductor and the enclosure arc then checked for their adequacy to dissipate the heat generated. The cross-section is then adjusted suitably by permuialions and combinations to arrive at the most appropriate size, keeping in mind the available extruded sections, accounting for all such factors that may affect the rating or the fatilt level of the enclosure and the conductor, such as tap-offs, which are subjected to a cumulative faull level, openings in the conductor for heat dissipation or the enclosure for inspection windows that can infhience their ratings. A calculation in Example 31.1 will clarify the procedure to establish the size of the... 

Endurance of trunking system with trolley-type tap-off facilities 32/958... 

For the tap-offs in a power-generating station, connecting a VAT through the main bus section between the generator and the generator transformer... 

For the tap-offs, connecting a UAT through the main bus section between the generator and the generator transformer, however, as discussed above, the momentary peak current will depend upon the short-time rating of such tap-offs. The likely ratings are noted in Table 13.8. 

In practice, the dimmer switch incorporates a small variable resistor into the switch box, which allows differing amounts of voltage to be tapped off. ... 

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