---
title: "Importance of emergency supply, online vs offline UPS"
subject: "Industrial Power Distribution and Illumination"
chapter: 9
key: "semester_6/industrial_power_distribution_and_illumination:emergency-supply-ups-importance"
source: https://www.bctengineer.com/q/semester_6/industrial_power_distribution_and_illumination/emergency-supply-ups-importance
site: BCT Engineering
---

# Importance of emergency supply, online vs offline UPS

Industrial Power Distribution and Illumination · Chapter 9: Emergency and Backup Supply Systems

- Asked in: 2075 Baisakh, 8 marks
- 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-supply-ups-importance
- Whole chapter: https://www.bctengineer.com/md/semester_6/industrial_power_distribution_and_illumination/chapter-9

## Question

Explain the importance of emergency supply system in industrial plant? Differentiate between on-line and off-line UPS with proper schematic diagrams.

## Short answer

Emergency supply keeps essential loads safe during failure: safety, safe shutdown, equipment protection, and legal compliance. On-line UPS always feeds the load through inverter with zero transfer; off-line UPS normally passes mains and transfers in 4-10 ms.

## Answer

#### Importance of emergency supply in an industrial plant
An emergency supply system automatically supplies essential loads when the utility supply fails. It is important because:
1. **Safety of people:** escape lighting, exit signs, fire alarm and fire pumps must work during a power cut or fire.
2. **Safe shutdown of processes:** furnaces, boilers, chemical reactors and kilns need cooling, stirring or control to avoid explosion, fire or damaged products.
3. **Protection of equipment:** lubrication pumps, cooling fans, control and switchgear tripping supplies must continue.
4. **Avoid production and data loss:** PLCs, SCADA and computers need no-break power; restarting a continuous process can take hours.
5. **Communication and security:** telephones, CCTV, access control.
6. **Legal requirement:** fire and factory regulations require emergency lighting and supply.

Sources: batteries (central/self-contained), UPS, DG sets with AMF panel, and a second utility feeder.

#### On-line UPS vs off-line UPS
```text
ON-LINE (double conversion)
        ┌───────── static bypass ────────┐
 AC ─►[Rectifier/]─► DC bus ─►[Inverter]─┴─► Load
      [ charger  ]     │
                   [Battery]

OFF-LINE (standby)
 AC ─────────────────────────────┐
  │                          [Transfer ]─► Load
  └►[Charger]─►[Battery]─►[Inverter]┘ switch
```

| Point | On-line UPS | Off-line UPS |
|---|---|---|
| Normal path of load | Always through rectifier and inverter | Directly from mains |
| Transfer time | Zero (no break) | 4–10 ms |
| Protection from sags, spikes, noise | Complete | Very little in normal mode |
| Output waveform | Pure sine, stable V and f | Mains in normal; often quasi-sine on battery |
| Efficiency | Lower (≈ 90–94%) | Higher (≈ 97–99%) |
| Cost and size | High | Low |
| Inverter duty | Continuous | Only during outage |
| Typical use | Data centres, process control, hospitals | Home PCs, small offices |

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Why is emergency supply important in an industrial plant?
   A. For safety of people, safe shutdown of processes and protecting equipment and data
   B. To lower tariffs
   C. To increase production capacity
   D. To replace the utility
   Answer: A. It supports escape lighting, fire pumps, cooling/control and no-break supplies for PLCs. (Key point: Importance)

2. What is the normal load path in an on-line UPS?
   A. Rectifier, DC bus, inverter, load
   B. Mains direct to load
   C. Battery direct to load
   D. Charger direct to load
   Answer: A. Static bypass is used only if the inverter fails. (Key point: Online path)

3. Which has higher efficiency in normal mode?
   A. On-line
   B. Off-line (about 97-99%) over on-line (about 90-94%)
   C. They are equal
   D. Neither
   Answer: B. On-line double conversion wastes more power as heat. (Key point: Efficiency comparison)

4. Which UPS gives complete protection from sags, spikes and noise?
   A. Off-line UPS
   B. On-line UPS
   C. Neither
   D. Both equally
   Answer: B. The load is isolated from mains by double conversion. (Key point: Protection)

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