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Emergency vs back-up supply, online/offline 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.
State the difference between Emergency power Supply and Back-up power supply. Explain on-line UPS and off-line UPS.
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.
┌────── 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.
AC ──────────────────────┐
│ [Transfer]──► Load
└►[Charger]►[Battery]►[Inverter]┘
✓ Answer checked by Claude Opus 5.5 (high effort)
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