- 2075 Baisakh · 8 marks
Importance of emergency supply, online vs offline UPS
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.
Explain the importance of emergency supply system in industrial plant? Differentiate between on-line and off-line UPS with proper schematic diagrams.
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:
- Safety of people: escape lighting, exit signs, fire alarm and fire pumps must work during a power cut or fire.
- Safe shutdown of processes: furnaces, boilers, chemical reactors and kilns need cooling, stirring or control to avoid explosion, fire or damaged products.
- Protection of equipment: lubrication pumps, cooling fans, control and switchgear tripping supplies must continue.
- Avoid production and data loss: PLCs, SCADA and computers need no-break power; restarting a continuous process can take hours.
- Communication and security: telephones, CCTV, access control.
- 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
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 |
✓ Answer checked by Claude Opus 5.5 (high effort)
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