--- title: "Isolator in SLD; application of contactors" subject: "Switchgear and Protection" chapter: 4 key: "semester_6/switchgear_and_protection:isolator-sld-contactor-application" source: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/isolator-sld-contactor-application site: BCT Engineering --- # Isolator in SLD; application of contactors Switchgear and Protection · Chapter 4: Isolators and Contactors - Asked in: 2070 Magh, 6 marks - Video: [Difference between Circuit breaker and Isolator | TheElectricalGuy](https://www.youtube.com/watch?v=8QLVvyNfEgc) — Gaurav J - TheElectricalGuy - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/isolator-sld-contactor-application - Whole chapter: https://www.bctengineer.com/md/semester_6/switchgear_and_protection/chapter-4 ## Question With reference to a single line diagram of a power system, discuss the function of isolator. Also mention the application of contactors in power system. ## Short answer In a substation single line diagram, isolators disconnect circuit breakers, lines, and transformers for maintenance, providing a visible break after the CB opens. Contactor applications include frequent motor starting, capacitor switching, lighting control, and remote automation, requiring fuse backup. ## Answer An **isolator** is an off-load switch that provides a visible break so that equipment can be safely separated from live parts after the circuit breaker has interrupted the current. #### Function of isolator in a single line diagram ```text Incoming 132 kV line | [LI]--ES LI = line isolator + earth switch | [CB1] | [BI] BI = bus isolator | =======+========[BS]========= 132 kV bus | sectionaliser [BI] | [CB2] | [TI] | ( T/F ) 132/33 kV | ... ``` 1. **Isolation of CB:** isolators BI and LI on both sides of CB1 allow the breaker to be taken out for maintenance. Sequence: open CB1 → open LI and BI → close ES. 2. **Line isolation and earthing:** the line isolator with earth switch earths the incoming line during line work and discharges trapped charge. 3. **Bus sectionalising:** isolator BS divides the bus into two sections, so one half can be maintained while supply continues from the other. 4. **Transformer isolation:** TI isolates the transformer after its breaker opens. 5. **Bus selection:** in double-bus arrangements, isolators transfer a feeder from main to reserve bus. 6. **Visible open gap** for confirming safe working conditions. Isolators are interlocked so they can be operated only when the related CB is open. #### Application of contactors in power system A **contactor** is an electromagnetically operated switch for frequent on-load switching of normal currents. Applications: 1. **Motor starters:** DOL, star-delta, reversing and auto-transformer starters for pumps, fans, compressors and conveyors. 2. **Capacitor bank switching** in automatic power factor correction (APFC) panels. 3. **Lighting control:** street lighting with timers or photocells, large building lighting. 4. **Heating loads:** furnaces, ovens and heaters controlled by thermostats. 5. **Automatic transfer switches (ATS)** between mains and diesel generator. 6. **Remote and automatic control** with PLCs, level switches and pressure switches (e.g. water pumping stations). 7. **Auxiliary supply in substations** and battery charger switching. 8. **No-volt and overload protection** of motors together with overload relays and back-up fuses. Unlike isolators, contactors switch load current many times a day; unlike CBs, they cannot interrupt short-circuit currents and need fuse or MCCB back-up. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. In the SLD, why are isolators placed on both sides of CB1? A. To let the breaker be taken out for maintenance B. To increase its breaking capacity C. To limit fault current D. To quench the arc Answer: A. Isolators on both sides separate the CB from all live parts. (Key point: Isolating a CB) 2. What is the sequence to isolate CB1 for maintenance? A. Open LI and BI, close ES, then open CB1 B. Open CB1, open LI and BI, close ES C. Close ES, open CB1, open BI D. Open BI only Answer: B. CB breaks current first, isolators then open, then earth switch closes. (Key point: Sequence) 3. Which is an application of contactors in power systems? A. Interrupting transmission-line faults B. Switching capacitor banks in APFC panels and motor starters C. Visible isolation of 132 kV lines D. Lightning protection Answer: B. Contactors switch load currents frequently, e.g. motor starters and APFC capacitor banks. (Key point: Contactor applications) 4. How does a contactor differ from a CB and an isolator? A. It switches load current often but cannot interrupt short-circuit currents B. It breaks short circuits like a CB C. It operates only at no load D. It gives visible isolation Answer: A. Unlike isolators it switches load many times a day; unlike CBs it needs fuse or MCCB back-up. (Key point: Contactor vs CB/isolator) ## Related in Switchgear and Protection - [Isolator function and application](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aisolator-function-application) — Switchgear and Protection, chapter 4 - [Isolator construction, principle and application](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aisolator-construction-principle) — Switchgear and Protection, chapter 4 - [Isolator operation with CB in HV installation](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aisolator-sld-cb-operation) — Switchgear and Protection, chapter 4 - [Isolator vs CB and why both used](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aisolator-vs-cb-simultaneous) — Switchgear and Protection, chapter 4, asked 4× ## Related in other subjects - [Single line diagram and its significance](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Asingle-line-diagram-significance) — Power System Analysis I, chapter 3 - [Interconnected system SLD with bus voltages](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Ainterconnected-sld-bus-voltages) — Power System Analysis I, chapter 1 - [Single line diagram from 11 kV to end use](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Asld-11kv-to-end-use) — Industrial Power Distribution and Illumination, chapter 4, asked 3× - [Interconnected system SLD and functions of each part](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Ainterconnected-sld-functions) — Power System Analysis I, chapter 1