--- title: "Emergency vs back-up supply, online/offline UPS" subject: "Industrial Power Distribution and Illumination" chapter: 9 key: "semester_6/industrial_power_distribution_and_illumination:emergency-vs-backup-ups" source: https://www.bctengineer.com/q/semester_6/industrial_power_distribution_and_illumination/emergency-vs-backup-ups site: BCT Engineering --- # Emergency vs back-up supply, online/offline UPS Industrial Power Distribution and Illumination · Chapter 9: Emergency and Backup Supply Systems - Asked in: 2077 Chaitra, 2+6 marks; 2076 Bhadra, 8 marks; 2069 Bhadra, 8 marks (asked 3 times) - Video: [Importance of emergency supply system || On-line UPS || Offline UPS || Schematic Diagram || IOE](https://www.youtube.com/watch?v=LiWaf3Z00h8) — House Of Learners - 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/emergency-vs-backup-ups - Whole chapter: https://www.bctengineer.com/md/semester_6/industrial_power_distribution_and_illumination/chapter-9 ## Question State the difference between Emergency power Supply and Back-up power supply. Explain on-line UPS and off-line UPS. ## Short answer Emergency supply powers life-safety loads (escape lighting, alarms) with very fast transfer (≤15 s) for limited duration; back‑up supply maintains production/business with longer runtime and larger capacity. On‑line UPS: double conversion (AC→DC→AC) with zero transfer time, isolates load. Off‑line UPS: load fed directly from mains, transfers to battery inverter within 4–10 ms. ## Answer #### Emergency power supply vs back-up power supply An **emergency power supply** keeps only the **life-safety and essential loads** working when normal supply fails, usually within seconds and for a limited time. A **back-up power supply** keeps **normal production or business loads** running during an outage so that work continues. | Point | Emergency supply | Back-up supply | |---|---|---| | Purpose | Safety of people, safe shutdown | Continuity of production/service | | Loads | Escape lighting, fire alarm, fire pumps, lifts, control | Machines, computers, process loads | | Required by | Safety codes/law | Owner's choice (economic) | | Changeover time | Very short (≤ 0.5–15 s) | Few seconds to minutes acceptable | | Duration | Usually 1–3 h | As long as needed | | Capacity | Small | Large, may equal full load | | Source | Batteries, central battery, small DG | Large DG sets, UPS, second feeder | #### On-line UPS (double conversion) - The load is **always fed through the inverter**. Mains AC → rectifier/charger → DC bus (battery floats on it) → inverter → load. - When mains fails, the battery supplies the DC bus with **zero transfer time**. - A static bypass switch transfers load directly to mains if the inverter fails or is overloaded. - **Advantages:** no break, isolates load from spikes, sags, frequency variations; pure sine output. - **Disadvantages:** higher cost, lower efficiency (≈ 90–94%), more heat. - Used for data centres, process control, hospitals. ```text ┌────── static bypass ──────┐ AC ──►[Rectifier]──►DC──►[Inverter]─┴─► Load │ [Battery] ``` #### Off-line UPS (standby) - Normally the load is fed **directly from mains** through a transfer switch; the charger keeps the battery charged. - When mains fails, the **transfer switch** shifts the load to the inverter, which runs from the battery. Transfer time is about **4–10 ms**. - **Advantages:** cheap, simple, high efficiency in normal mode. - **Disadvantages:** no protection against sags/spikes in normal mode, short break at changeover, often square/quasi-sine output. - Used for PCs and small office loads. ```text AC ──────────────────────┐ │ [Transfer]──► Load └►[Charger]►[Battery]►[Inverter]┘ ``` ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is the difference between emergency and back-up supply? A. Emergency supplies life-safety/essential loads for a limited time; back-up keeps normal production loads running B. Emergency supplies full production load for days; back-up only escape lights C. They are identical D. Emergency is the owner's choice; back-up is required by law Answer: A. Emergency supply is a legal safety requirement for essential loads; back-up is an economic decision for continuity. (Key point: Emergency vs back-up) 2. How does an on-line UPS feed the load? A. Directly from mains through a transfer switch B. Always through the rectifier and inverter, with the battery floating on the DC bus C. Only from the battery D. Through the static bypass at all times Answer: B. Double conversion: mains to DC to inverter to load. (Key point: On-line UPS operation) 3. What are the transfer times of on-line and off-line UPS? A. On-line 4-10 ms; off-line zero B. Both zero C. On-line zero; off-line about 4-10 ms D. Both about 10 s Answer: C. The battery sits on the DC bus of an on-line UPS so no break occurs; off-line needs the transfer switch. (Key point: Transfer time) 4. What is an advantage and disadvantage of an off-line UPS? A. Costly, but complete protection B. Pure sine output and no break C. Needs a rectifier and inverter always running D. Cheap with high efficiency, but little protection from sags/spikes and a short break Answer: D. Off-line is cheap and efficient in normal mode but offers poor conditioning and a brief changeover. (Key point: Off-line trade-offs) ## Related in Industrial Power Distribution and Illumination - [Importance of emergency supply, online vs offline UPS](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Aemergency-supply-ups-importance) — Industrial Power Distribution and Illumination, chapter 9 - [Emergency power supply system for industry](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Aemergency-supply-system-industry) — Industrial Power Distribution and Illumination, chapter 9, asked 2× - [Emergency supply and Ah vs Wh efficiency](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Aemergency-supply-ah-wh-efficiency) — Industrial Power Distribution and Illumination, chapter 9