--- title: "Feeder switching guideline; contactor in place of CB" subject: "Switchgear and Protection" chapter: 4 key: "semester_6/switchgear_and_protection:feeder-isolator-cb-guideline" source: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/feeder-isolator-cb-guideline site: BCT Engineering --- # Feeder switching guideline; contactor in place of CB Switchgear and Protection · Chapter 4: Isolators and Contactors - Asked in: 2071 Magh, 3+3 marks - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/feeder-isolator-cb-guideline - Whole chapter: https://www.bctengineer.com/md/semester_6/switchgear_and_protection/chapter-4 ## Question A [high?] voltage feeder has a circuit breaker and isolator. Develop a guideline for connecting and disconnecting the feeder through isolator and circuit breaker. Can you replace circuit breaker with contactor in the feeder? Justify your answer. ## Short answer To disconnect a feeder: open CB first, then line and bus isolators, then earth it. To connect: remove earths, close isolators, then CB last. A contactor cannot replace the CB because it cannot interrupt fault currents and lacks relay tripping; it is only for frequent load switching with fuse backup. ## Answer Taking the question as a **high-voltage feeder** (e.g. 33 kV) fed from a bus through isolators and a circuit breaker. ```text Bus ==+== | [I1] bus isolator | [CB] | [I2] line isolator --[ES] earth switch | Feeder ``` #### Guideline for disconnecting the feeder 1. Inform the control room and load end; reduce load if possible. 2. **Open the circuit breaker** by the control switch; confirm "OFF" on the indicator and ammeter reading zero. 3. **Open the line isolator I2**, then the **bus isolator I1** (no-load operation). 4. Check visibly that all three blades are open. 5. Test for absence of voltage, then **close the earth switch ES** (and apply portable earths at the work site). 6. Lock the isolators and CB, and attach "Do not operate – men at work" tags (permit-to-work). #### Guideline for connecting the feeder 1. Cancel the work permit; make sure all persons and portable earths are removed. 2. **Open the earth switch ES.** 3. Check that the CB is open; **close the bus isolator I1**, then the **line isolator I2**. 4. **Close the circuit breaker** last; watch the ammeter and relay flags. If a fault exists, the CB trips safely. 5. Record the operation in the log book. Rule to remember: **CB opens first and closes last; isolators move only when the CB is open.** Interlocking enforces this. #### Can a contactor replace the circuit breaker? **No**, not on an HV feeder. Reasons: - A contactor is built to make and break **normal load currents** (and motor starting currents), with breaking capacity of only about 8–10 times rated current. A feeder short circuit can be tens of kA; the contactor would fail to interrupt it and could weld or explode. - A contactor has **no tripping by protective relays** in the usual sense; it opens only when the coil is de-energised, and it also drops out on voltage dips, causing unwanted outages. - Contactors are mostly made for LV and up to about 11 kV (vacuum contactors); they do not have the voltage rating, insulation level or making capacity of a 33 kV CB. - A contactor needs back-up fuses for short circuits; HV fuses large enough for a feeder are not practical and must be replaced after each fault. A contactor is suitable only for frequent switching of a motor or capacitor bank; the feeder needs a circuit breaker with relays to clear faults. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. When taking a HV feeder out of service, which sequence is correct? A. Open the bus isolator, then the line isolator, then the CB B. Open the CB, then the line isolator, then the bus isolator C. Open the line isolator first, then the CB D. Close the earth switch first, then open the CB Answer: B. The CB must open first to break the load current; the isolators are opened afterwards because they are off-load switches. (Key point: Disconnect order) 2. When energising the feeder after work, what is the correct order? A. Close the CB, then close the isolators B. Close the line isolator, then the CB, then the bus isolator C. Open the earth switch, close the isolators with the CB open, then close the CB last D. Close the isolators and the earth switch together, then the CB Answer: C. Isolators must be moved only with the CB open, so the CB closes last and any existing fault is cleared by its tripping. (Key point: Connect order) 3. Why can a contactor not replace the circuit breaker on a 33 kV feeder? A. It is too heavy for outdoor use B. Its breaking capacity is only a few times rated current, far below feeder fault currents of tens of kA C. It cannot be operated remotely D. It has a visible air gap that is too large Answer: B. A contactor is designed for normal load and motor-starting currents and would fail or weld on a heavy short circuit. (Key point: Contactor cannot replace CB) 4. What must be done before the earth switch is closed for maintenance? A. Test for absence of voltage after the isolators are open and visibly checked B. Close the CB to discharge the line C. Reduce the load to 50% D. Remove the interlocks Answer: A. The line is first proved dead by a voltage test, then earthed and locked with permit-to-work tags. 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