Standby and emergency power, UPS and generators
Domain 6: Standby and emergency power, UPS and generators
This domain tests how critical loads remain supplied during mains failure. You must distinguish standby, emergency and uninterruptible supplies, and size sources for starting, step loads and battery autonomy.
Core rules
- Define the load category and transfer time: generator standby, UPS, or both; emergency loads may require automatic transfer and fire-rated routes.
- Generator ratings follow ISO 8528 definitions (e.g., prime, limited-time prime, emergency standby); state the rating used.
- Size generators for block load, motor starting (DOL, star-delta, VFD), harmonic distortion and site altitude/temperature derating.
- UPS sizing uses kW and kVA at the load power factor; battery autonomy depends on load, inverter efficiency, ageing and end-voltage.
- Specify neutral switching and earthing for transfer schemes to avoid parallel neutral paths or lost protection.
- Test on load, including black-start, transfer, cooling, fuel and exhaust systems.
Worked example
An 80 kW critical load at 0.90 power factor needs 80/0.90 = 88.9 kVA, so a 100 kVA UPS is adequate. A 100 kWh battery with 80% usable capacity and 0.95 inverter efficiency gives 100 × 0.80 × 0.95 = 76 kWh delivered. At 80 kW, runtime is 76/80 = 0.95 h, about 57 minutes before ageing and temperature derating. Confirm the specified end-voltage and ageing margin.
Traps
- Adding kW and kVA directly.
- Ignoring motor-starting kVA.
- Assuming full battery capacity is usable.
- Forgetting neutral/earth arrangement in transfer switches.