--- title: "Ring bus rearrangement and zones" subject: "Switchgear and Protection" chapter: 1 key: "semester_6/switchgear_and_protection:ring-bus-comparison" source: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/ring-bus-comparison site: BCT Engineering --- # Ring bus rearrangement and zones Switchgear and Protection · Chapter 1: Principle of Power System Protection - Asked in: 2078 Chaitra, 6 marks - Video: [6 Electrical Substation Bus Schemes Explained](https://www.youtube.com/watch?v=ZU4y6vsabP4) — TestGuy - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/ring-bus-comparison - Whole chapter: https://www.bctengineer.com/md/semester_6/switchgear_and_protection/chapter-1 ## Question The system of fig. 1(a) is rearranged as shown in fig. 1(b) where the bus arrangement at B has been changed to a ring bus arrangement. (i) Compare the operation of the two systems when a fault occurs on line BC with a. a failure of breaker 7, b. a failure of breaker 6. (ii) Sketch the zone of protection. [Fig. 1(a): Bus A is fed through breaker 1. Two parallel lines run from A to bus B: upper line with breaker 2 at A and breaker 4 at B, lower line with breaker 3 at A and breaker 5 at B. Load 1 is supplied from bus B through breaker 6. Line BC leaves bus B through breaker 7 and ends at breaker 8 at bus C; Load 2 is supplied from C through breaker 9.] [Fig. 1(b): Same system, but B is a ring bus with four breakers 4, 5, 6 and 7 in the ring. The line from breaker 2 joins the ring between breakers 4 and 5, the line from breaker 3 joins between breakers 5 and 6, Load 1 is taken from the ring between breakers 4 and 7, and line BC leaves the ring between breakers 7 and 6, ending at breaker 8 at bus C; Load 2 is fed from C through breaker 9.] ## Short answer For fault on BC: if breaker 7 fails, single bus trips 4,5,6 losing load 1; ring bus trips 6,8 and 4, also losing load 1. If breaker6 fails, single bus is unaffected; ring bus trips5 and remote3, load 1 stays. Ring bus zones overlap at breakers, no separate bus zone. ## Answer The ring at B (Fig. 1(b)), taken in the order 4 → 5 → 6 → 7 → 4: the line from breaker 2 joins between 4 and 5, the line from breaker 3 between 5 and 6, line BC between 6 and 7, and load 1 between 7 and 4. #### (i) Comparison of operation for a fault on line BC **Normal clearing** - Fig. (a): breakers 7 and 8 trip. Load 1 continues. - Fig. (b): line BC is fed from the ring through 6 and 7, so breakers **6, 7 and 8** trip. The ring is opened, but load 1 is still fed through 4 from the line of breaker 2; no load is lost. **(a) Failure of breaker 7** | Arrangement | Back-up action | Load 1 | |---|---|---| | Fig. (a) single bus | Breaker-failure relay trips all bus-B breakers 4, 5, 6 (or remote 2, 3 trip) | Interrupted | | Fig. (b) ring bus | The breaker on the other side of 7 in the ring, i.e. **4**, trips (with 6 and 8) | Interrupted, because load 1 lies between 7 and 4 | In the ring bus fewer breakers trip and the lines from A remain connected to the ring, but load 1 is still lost because it is placed next to breaker 7. **(b) Failure of breaker 6** | Arrangement | Back-up action | Load 1 | |---|---|---| | Fig. (a) | Breaker 6 is the load-1 breaker and is not involved in clearing a BC fault; 7 and 8 clear it | Not interrupted | | Fig. (b) | The breaker-failure relay of 6 trips the other breaker at that ring node, **5**, and also trips the remote breaker **3** at bus A (by direct transfer trip, or by remote back-up at A), because the line from 3 still feeds the fault through the stuck breaker 6. Breakers 7 and 8 have already tripped. The line from 3 is isolated | Not interrupted; load 1 is fed through 4 from the line of breaker 2 | Conclusion: in a ring bus, a stuck breaker is isolated by opening the breaker on the other side of its ring node, plus the remote end of any line connected at that node. So a breaker failure removes at most one extra circuit, instead of the whole bus as in Fig. (a). To keep load 1 in service for any single breaker failure during a BC fault, load 1 should not be placed next to line BC; it should lie in the ring between the two lines from A (ring order: line 2, load 1, line 3, line BC). #### (ii) Zones of protection ```text [1] Bus A ──┬───┴───┬── Bus A zone: 1, 2, 3 [2] [3] │ AB1 │ AB2 │ │ Ring B: │ │ N45 ●─┴─[5]───┴─● N56 │ │ [4] [6] │ │ N74 ●────[7]────● N67 │ │ Load 1 line BC ──[8]── bus C ──[9]── Load 2 ``` - Bus A zone: 1, 2, 3. - Line AB1 zone: 2 and ring breakers 4, 5 (includes node N45). - Line AB2 zone: 3 and ring breakers 5, 6 (includes node N56). - Line BC zone: ring breakers 6, 7 and breaker 8 (includes node N67). - Load 1 zone: ring breakers 7 and 4 (includes node N74). - Bus C zone: 8, 9; load 2 zone beyond 9. - In a ring bus there is **no separate bus zone**: each ring node between two breakers belongs to the circuit connected to it. - Each zone overlaps its neighbour at the breaker between them. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. In the ring bus, if breaker 7 fails on a BC fault, which extra breaker trips? A. Breaker 5 B. Breaker 4 (neighbour of 7 in the ring) C. Breaker 9 D. None Answer: B. The adjacent ring breaker trips, with 6 and 8. (Key point: Ring bus, 7 fails) 2. Why is load 1 lost when 7 fails in the ring? A. Load 1 lies between 7 and 4 B. It lies between 4 and 5 C. It is fed from line BC D. Breaker 6 supplies it Answer: A. Both 7 and 4 are open, isolating load 1. (Key point: Load 1 in ring) 3. If breaker 6 fails in Fig (b), which breaker trips with 7 and 8? A. Breaker 4 B. Breaker 9 C. Breaker 5 D. Breaker 3 Answer: C. The neighbour of 6 is 5; load 1 stays fed via 4. (Key point: Breaker 6 fails, ring) 4. What is true of zones in a ring bus? A. A single bus zone covers the ring B. Zones do not overlap C. Only line zones exist at A D. There is no separate bus zone; each ring section belongs to the connected circuit Answer: D. Each section between breakers belongs to the circuit attached to it. 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