--- title: "Single line diagram from 11 kV to end use" subject: "Industrial Power Distribution and Illumination" chapter: 4 key: "semester_6/industrial_power_distribution_and_illumination:sld-11kv-to-end-use" source: https://www.bctengineer.com/q/semester_6/industrial_power_distribution_and_illumination/sld-11kv-to-end-use site: BCT Engineering --- # Single line diagram from 11 kV to end use Industrial Power Distribution and Illumination · Chapter 4: Distribution Substation of Industrial Plant - Asked in: 2075 Baisakh, 4 marks; 2072 Asoj, 8 marks; 2071 Bhadra, 8 marks (asked 3 times) - Video: [Single line diagram of pole mounted substation l 11kv/415 v substation](https://www.youtube.com/watch?v=wlgcF4ynTB8) — ELECTRICAL ENGINEER - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/industrial_power_distribution_and_illumination/sld-11kv-to-end-use - Whole chapter: https://www.bctengineer.com/md/semester_6/industrial_power_distribution_and_illumination/chapter-4 ## Question Draw a neat single line diagram of a 11 KV start from 11 KV to end use 230 V. Explain briefly about each component. ## Short answer The SLD shows 11 kV feeder, lightning arresters, AB switch, drop-out fuse, 11/0.4 kV Dyn11 transformer, LT breaker, MDB, SDBs, and final circuits. Components: LA diverts surges, AB isolates, DO fuse protects transformer, transformer steps down, LT breaker protects secondary, MDB distributes, SDBs feed loads, APFC corrects power factor. ## Answer The single line diagram (SLD) shows, with one line for all three phases, how power flows from the 11 kV utility line through the distribution substation to the 230 V end users. #### Single line diagram ```text 11 kV overhead feeder (utility, NEA) | [LA] lightning arresters (3 nos) | [AB] air-break / gang-operated isolator | [DO] drop-out fuse (or HT VCB + relay) | [HT metering CT/PT] (for HT consumers) | (TR) 11/0.4 kV, Dyn11 transformer | neutral + body earthed [ACB / MCCB] LT main breaker + LT meter | ======== MDB 400 V, 3-ph 4-wire busbar ======== | | | | MCCB MCCB MCCB MCCB | | | | SDB-1 SDB-2 SDB-3 APFC panel (motors) (lighting) (sockets) (capacitors) | | | MCB/MPCB MCB MCB/RCCB | | | 400 V M 230 V lamps 230 V outlets ``` #### Function of each component 1. **11 kV incoming feeder**: overhead line (or HT XLPE cable) from the utility (NEA) substation. 2. **Lightning arresters (LA)**: one per phase, connected between line and earth; divert lightning and switching surges to ground and protect the transformer insulation. 3. **AB switch / isolator**: gang-operated, off-load switch for isolating the substation for maintenance; operated only after the load is removed. 4. **Drop-out fuse (DO) / HT VCB**: protects the transformer and the 11 kV line from transformer faults. Large industrial substations use a VCB or SF₆ breaker with overcurrent and earth-fault relays. 5. **HT metering (CT/PT)**: for HT consumers, measures energy in kWh, kVAh and maximum demand for billing. 6. **Distribution transformer 11/0.4 kV, Dyn11**: steps voltage down to 400 V line / 230 V phase; delta HV blocks triplen harmonics, star LV gives a neutral for single-phase loads. Neutral and tank are earthed separately. 7. **LT main breaker (ACB/MCCB)**: main switch and overload/short-circuit protection on the secondary; LT energy meter, ammeter and voltmeter. 8. **Main distribution board (MDB)**: copper busbars that feed outgoing circuits through MCCBs or switch-fuse units. 9. **APFC panel**: automatic capacitor bank that keeps power factor near 0.95–0.99, reducing current and penalty charges. 10. **Sub-distribution boards (SDB)**: located near load centres (each shop or floor); they have MCCB/MCB incomers and MCB outgoings. 11. **Final circuits**: MCBs, RCCBs/ELCBs, motor starters (DOL, star-delta) for 400 V motors, and 230 V lighting and socket circuits between one phase and neutral. 12. **Earthing system**: separate earth electrodes for transformer neutral, body/equipment and lightning arresters; protects people and lets protection operate. Single-phase end users are balanced across R, Y and B phases so the transformer is equally loaded. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is the function of lightning arresters in the SLD? A. Divert lightning and switching surges to earth to protect transformer insulation B. Interrupt short-circuit current C. Measure energy for billing D. Correct the power factor Answer: A. Arresters are connected between line and earth, one per phase, and carry surge currents to ground. (Key point: Lightning arrester) 2. How should the AB (air-break) switch be operated? A. On load, to break fault current B. Only during a short circuit C. Only after the load has been removed, since it is an off-load isolator D. Automatically by relays on overload Answer: C. The AB switch is a gang-operated off-load isolator used to isolate the substation for maintenance. (Key point: AB switch) 3. Why is delta used on the HV side and star on the LV side? A. Delta blocks triplen harmonics; star provides a neutral for single-phase loads B. Delta gives the neutral; star blocks harmonics C. Delta raises the voltage; star lowers losses D. Both reduce the need for earthing Answer: A. Delta HV traps triplen harmonics and star LV gives 230 V phase-to-neutral supply. (Key point: Dyn11 reasoning) 4. What does the APFC panel do? A. Protects against lightning surges B. Isolates the substation off-load C. Switches capacitor banks automatically to keep power factor near 0.95-0.99 D. Steps 400 V down to 230 V Answer: C. Better power factor reduces current and penalty charges. 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