We will just see how the sectional clearance is calculated. Sectional clearance, it is here, 33 kV, then 11 kV. The insulator bottom to the top of plinth level is as standard as per the Indian Electricity Rules. We maintain 2,440 mm irrespective of voltage level. Then the phase to earth clearance minimum. In the case of 33 kV it is this, 320 mm. Here, we normally keep 150 mm. Sectional clearance should be minimum sum of these two, you can see 2,760 mm should be minimum. What we have followed here is adopted 2,800 mm and 2,600 mm. This is what indicated in the table which we will see. We will now start at Section AA to see how the bus bar heights are arrived at. The bus bar height is maintained, you see 5,500 mm, from the top of the plinth level. This is the height of this trunk bus bar measured from the top of the plinth. Now, if you see this bar length of this particular distance is 13 meters. We can consider approximately 3 percent of the span length, the sag has 3 percent of the span length. 13 meters, if it is a span, 3 percent to sag would be approximately 390 millimeter. The phase to earth clearance for the equipment interconnections for 33 kV will be 320 mm. Let us also consider answer question of about amount 1,000 mm for the working clearances. The total requirement that we need for the phase to this height will be, for example, this is at a height of 3.8 meters. This ground clearance height is kept at 3,800 mm. The sag of this span, let us take three percent of the span length, that is 390 mm. There is a face-to-face clearance between this conductor and this conductor. Because this contractor is going like this, this conductor is running perpendicular. There will be face-to-face clearance between these two conductors. The minimum face-to-face clearance is 320 mm. Then we should have another at least 1,000 mm clearance. If we add all these you will get 5,510 mm. We get 5,500 mm, which is okay for this height of bus bar, height at 5,500 mm. If you see this line gantry alone, this is terminated at a height of 6,500 meter. The reasons could be normally this bar length is higher. It could be even 50 meter or 60 meter and all. The sag of the line also would be higher. Also, if you see there is a suspension string which is there below. For example, 33 kV, if you take normally if you are adapting disc insulators, you will have about four disks, and spacing between each disk will be about 145 millimeter normally we will use. This incoming line gantry is normally terminated at a higher height compared to main bus bar height. It is kept at 6,500 mm. Mainly from the consideration of a longer span length. What is bay rate? The line where each bay rate, I will explain here in the plan. This distance is called as the bay rate. We're keeping that as 5,500 mm. This is nothing but center-line of this column to this center-line of the B phase, then B phase to Y phase, Y phase to R phase. Again, R phase to center-line of the structure. This distance is 1,250 mm. Phase-to-phase distance is 1,500 mm. If we add 150 plus 1,500 plus 1,500 plus 1,250, it will work out to 5,500 mm. This clearance is normally higher than what we keep as the known phase to phase requirement of 320 mm and all, because of the following aspects. One is there will be a short circuit forces between the conductors. There will be wind forces also acting between the conductors and mechanical aspect for the movements within the substation. We need to keep normally higher clearances between the conductors in our layout so that we are up there. These clearances minimum 5,500 mm, as far as their 33 kV system is concerned. Now, if you take the transformer bays into consideration, if we say the normal bay width is 5.5 meter. If you see here, this is two bays of transformer bays. The distance should be accordingly 5.5 plus 5.5, it should be 11 meters. But we are keeping 30 meters here, why? The reason is because of the two transformers. If we keep lower meter, the spacing between the two transformers will become very less. We have seen that the line bay width is kept at 5,500 mm. If you see these two transformer bays, transformer bay 1, bay 2, then, accordingly, this bay width should be 5.5 plus 5.5 should have been 11 meters. But we are keeping 13 meters, purely from legal point of view. If we keep 11 meters here, these two transformers will be much more again closer. Right now the physical gap is four meters around approximately. If this layout had been made with 11 meters, then this physical distance between these two transformers would be only about two meter or so. You'll have to have workable clearances so that people are able to walk freely between these firewall and this marshaling box and all that. We need a higher distances. Sectional clearance also should be kept in mind. You can also see here why this additional distance is required. Because these two bay, for example, we told that this is R phase, this is Y phase, this is B phase. Similarly, this is R phase. This is a B phase of one bay. This is the R phase of another bay. When one bay is under shut down or something, we should ensure minimum sectional clearance between these two bays, so that people will be able to work on the bay which needs re-maintenance. That is also another reason we should keep a higher distance for easy work-ability and maintaining the clearance aspects. That is why this distance of 30 meter is kept instead of 11 meters. This should be borne in mind when you are designing your layers where in transformers are also involved to maintain sufficient sectional clearance between the two adjacent bays into account. Continued in next part.
