--- title: "Automation with a contactor" subject: "Switchgear and Protection" chapter: 4 key: "semester_6/switchgear_and_protection:contactor-automation" source: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/contactor-automation site: BCT Engineering --- # Automation with a contactor Switchgear and Protection · Chapter 4: Isolators and Contactors - Asked in: 2076 Bhadra, 6 marks - Video: [What is a Contactor? | Working Principles](https://www.youtube.com/watch?v=08ozhRb7HEU) — RealPars - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/contactor-automation - Whole chapter: https://www.bctengineer.com/md/semester_6/switchgear_and_protection/chapter-4 ## Question Explain with a neat sketch how automation of a system is done with the help of a contactor. ## Short answer A contactor is an electromagnetic switch energized by a small control signal to switch a high-power load. In an automatic pump system, tank-low and sump-level contacts control the coil, an auxiliary contact provides sealing, and high-level/overload contacts stop it. Thus sensors and logic decide while the contactor executes switching. ## Answer A **contactor** is an electromagnetic switch whose coil can be energised by any small control signal (push-button, timer, sensor, PLC). Because a low-power signal switches a high-power load, contactors are the basic building block of **automation** of electrical systems. #### Example: automatic water pumping system Assumption: a pump fills an overhead tank. A float/level switch in the tank and a dry-run sensor in the sump control the pump through a contactor. ```text Power: L1 L2 L3 --[MCB/fuse]--[K]--[OL]--( Pump M ) Control circuit: L --[F]--[OL NC]--[Sump level OK]--+---------------------+ | | [Tank LOW float NO] [K aux NO] | | | [Tank HIGH float NC] | | +----------+----------+ | ( K coil ) | N --------------------------------------------+ ``` #### Working 1. When the tank level falls below the LOW float, its contact closes; the **contactor coil K is energised** and the pump starts. 2. The auxiliary contact of K, in series with the HIGH float (NC), holds the coil, so the pump keeps running as the water rises above the LOW point. 3. When the water reaches the HIGH level, the HIGH float contact opens; the coil drops out and the pump stops. 4. If the sump runs dry, the sump-level contact opens and stops the pump (**dry-run protection**). 5. The **overload relay** stops the motor on overload; the contactor gives **no-volt protection** on supply failure. 6. No operator is needed; the system runs automatically. #### Other automation done with contactors - **Star-delta starter:** a timer switches the star contactor off and the delta contactor on after the motor speeds up. - **APFC panels:** a power factor controller switches capacitor steps through capacitor-duty contactors. - **Street lighting:** a photocell or timer energises the lighting contactor at dusk. - **Automatic transfer switch:** two interlocked contactors change load from mains to generator on supply failure. - **Conveyors and lifts:** limit switches and PLC outputs drive contactors for start, stop and reversal. - **Temperature control:** a thermostat switches heater contactors in furnaces. The contactor thus acts as the **power output stage** of the control system: sensors and logic decide, and the contactor carries out the switching safely and repeatedly. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why is a contactor the basic building block of automation? A. It limits short-circuit current by itself B. A small control signal energising its coil can switch a high-power load C. It provides visible isolation D. It stores energy to trip faster Answer: B. The coil takes only low power from a push-button, timer, sensor or PLC while the main contacts carry the load current. (Key point: Contactor principle) 2. In the water pump circuit, what keeps the contactor coil energised after the water rises above the LOW float? A. The HIGH float contact closing B. The overload relay C. The auxiliary NO contact of K, in series with the HIGH float (NC) D. The sump level switch Answer: C. The auxiliary contact latches the coil until the HIGH float opens at full tank, which drops the coil out. (Key point: Holding action in pump circuit) 3. What happens if the sump runs dry? A. The sump-level contact opens and the pump stops B. The tank HIGH float closes C. The contactor welds shut D. The pump speeds up Answer: A. The sump level contact is in series with the coil, so opening it de-energises the contactor and protects the pump. (Key point: Dry-run protection) ## Related in Switchgear and Protection - [Basic requirements of a protection scheme](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Abasic-requirements-protection-scheme) — Switchgear and Protection, chapter 1, asked 3× - [Knee point voltage and CT selection](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aknee-point-ct-selection) — Switchgear and Protection, chapter 2 ## Related in other subjects - [Manufacturing execution system and automation](https://www.bctengineer.com/question.md?key=semester_4%2Fcomputer_aided_design%3Amanufacturing-execution-system-automation) — Computer Aided Design, chapter 6 - [How symmetrical components help analysis](https://www.bctengineer.com/question.md?key=semester_5%2Fpower_system_analysis_ii%3Asymmetrical-components-unbalance-help) — Power System Analysis II, chapter 4 - [Case study: new system vs existing (cost, manpower)](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Acs-cost-manpower-automation) — Instrumentation II, chapter 10, asked 4× - [Instrumentation system with block diagram](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Ainstrumentation-system) — Basic Electronics Engineering, chapter 7, asked 3×