Generator and UPS working together

For sites that must keep running through long outages, a generator and UPS work as a team. Problems usually appear at the hand-off between them.

The sequence

1. Mains power fails; the UPS instantly carries the load on battery.
2. The generator starts and stabilises.
3. An automatic transfer switch moves the supply to the generator.
4. The UPS returns to mains-type operation, now fed by the generator, and recharges its batteries.
5. When mains returns, the transfer switch moves back and the generator cools down and stops.

Common problems

• **Generator output not accepted by the UPS:** frequency or voltage variations can cause some UPS units to stay on battery. Online UPS units generally cope best; settings may need adjusting.
• **Undersized generator:** it must handle the UPS load plus battery recharging and any other connected loads.
• **Batteries too small:** the UPS must cover the generator's start and transfer time with margin.

Test under load

Test regularly by simulating a mains failure with real load connected, not just by starting the generator. Record the results.

See our backup power and data centre services.