--- title: "Emergency supply and Ah vs Wh efficiency" subject: "Industrial Power Distribution and Illumination" chapter: 9 key: "semester_6/industrial_power_distribution_and_illumination:emergency-supply-ah-wh-efficiency" source: https://www.bctengineer.com/q/semester_6/industrial_power_distribution_and_illumination/emergency-supply-ah-wh-efficiency site: BCT Engineering --- # Emergency supply and Ah vs Wh efficiency Industrial Power Distribution and Illumination · Chapter 9: Emergency and Backup Supply Systems - Asked in: 2070 Magh, 8 marks - 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-ah-wh-efficiency - Whole chapter: https://www.bctengineer.com/md/semester_6/industrial_power_distribution_and_illumination/chapter-9 ## Question What is emergency supply system for industrial plant? Differentiate between Ah efficiency and Wh efficiency of battery. ## Short answer Emergency supply independently powers essential loads when normal supply fails, using batteries, UPS, DG with AMF, or a second feeder. Ah efficiency is Ah out divided by Ah in; Wh efficiency is Wh out divided by Wh in, and is lower because charge voltage exceeds discharge voltage; example gives 80% and 72.13%. ## Answer #### Emergency supply system for an industrial plant An **emergency supply system** is an independent, automatically started power source that supplies the **essential loads** of a plant when the normal supply fails, to protect life, equipment and critical processes. **Essential loads:** escape and exit lighting, fire alarm, fire pumps, lifts, control and protection supplies, process loads needing safe shutdown (cooling, stirring, lubrication), communication and security. **Sources:** 1. **Battery systems** – self-contained or central battery; supply is instant. 2. **UPS** – no-break supply for computers, PLCs and instruments. 3. **Diesel generator with AMF panel** – starts automatically in about 10–15 s; feeds large loads for long periods. 4. **Second utility feeder** with automatic transfer switch (ATS). ```text Utility ─►[ATS]─► Essential bus ─► fire pump, lights ▲ DG set ─────┘ Battery/UPS ─────► PLC, alarm, control (no break) ``` Essential loads are kept on a separate bus, batteries are on float/trickle charge, and the system is tested regularly. #### Ah efficiency vs Wh efficiency ```text η_Ah = Ah delivered on discharge / Ah supplied on charge η_Wh = Wh delivered on discharge / Wh supplied on charge = η_Ah × (V_avg discharge / V_avg charge) ``` | Point | Ah efficiency | Wh efficiency | |---|---|---| | Based on | Quantity of charge only | Energy (charge × voltage) | | Considers voltage | No | Yes | | Losses included | Gassing, side reactions | Also I²R and polarisation losses | | Value (lead-acid) | 80–90% | 70–80% | | Relation | Always higher | Lower, since V_charge > V_discharge | | Use | Deciding charging time/Ah needed | Energy cost and economics | **Example:** discharge 4 A × 12 h at 2.2 V, charge 3 A × 20 h at 2.44 V → η_Ah = 48/60 = 80%, η_Wh = 105.6/146.4 = 72.13%. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is an emergency supply system? A. An independent, automatically started source supplying essential loads when the normal supply fails B. A transformer that reduces voltage C. A manual change-over to the utility D. A system that supplies all loads of the plant equally Answer: A. It protects life, equipment and critical processes by automatically feeding essential loads. (Key point: Emergency supply definition) 2. What is a typical start-up time of a diesel generator with an AMF panel, and what does this imply? A. Instant, so no battery is needed B. About 10-15 s, so critical loads need a UPS or battery to bridge it C. About 10 minutes, so it is useless D. About 1 hour Answer: B. The DG takes about 10-15 s, so no-break loads like PLCs are put on a UPS or battery. (Key point: DG with AMF) 3. How is Ah efficiency defined? A. Wh delivered / Wh supplied B. Ah supplied / Ah delivered C. Ah delivered on discharge / Ah supplied on charge D. Average discharge voltage / average charge voltage Answer: C. Ah efficiency is the ratio of charge delivered on discharge to charge supplied on charge. (Key point: Ah efficiency formula) 4. Why is the Wh efficiency lower than the Ah efficiency? A. Because charging voltage is higher than discharge voltage B. Because discharge voltage is higher than charging voltage C. Because Ah is measured at higher current D. Because Wh efficiency ignores losses Answer: A. Wh efficiency = Ah efficiency x (V discharge / V charge), and V charge > V discharge, so it is smaller. 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