A standby generator without an automatic transfer switch is just an engine waiting for someone to throw a lever in the dark. The ATS detects mains failure, starts the genset, transfers the load, and reverses the whole sequence when the grid returns — and choosing the wrong type or rating is one of the most common (and expensive) mistakes in backup power projects.
What an ATS actually does
The transfer switch continuously monitors the utility supply. When voltage or frequency drifts outside limits for longer than a set delay (typically 3–5 seconds, to ride through flickers), it signals the generator to start, waits for stable genset output, then transfers the load. When mains power returns and holds stable (typically 5–30 minutes), it transfers back and lets the engine cool down before shutdown. Total time from blackout to generator power is normally 10–15 seconds for a standby-class installation.
The four transition types
| Type | How it works | Break in supply | Typical use |
|---|---|---|---|
| Open transition (break-before-make) | Disconnects one source before connecting the other | Yes — milliseconds to seconds | Most standby installations; simplest and cheapest |
| Delayed transition | Open transition with a deliberate pause in neutral position | Yes — 1–3 s programmed | Motor-heavy loads; lets residual voltage decay before re-energising |
| Closed transition (make-before-break) | Momentarily parallels genset with mains (≤100 ms overlap) | No | Hospitals, data centres, process loads; requires utility approval |
| Bypass-isolation | Adds a manual bypass path around the ATS | Depends on base type | Critical facilities that must maintain power during ATS servicing |
Sizing the ATS: amperage
The ATS must be rated for the full load current of the circuit it protects — not the generator’s output. The calculation for a three-phase 400 V system:
I = kW × 1000 / (√3 × 400 × PF)
Example: a 200 kVA genset backing a 160 kW load at 0.8 PF draws about 289 A. The next standard ATS frame size up is 400 A — never size down, and allow 25% headroom for motor starting and future load growth. Common frame sizes run 63 / 100 / 160 / 250 / 400 / 630 / 800 / 1000 / 1250 / 1600 / 2000 / 3200 A.
3-pole or 4-pole?
- 3-pole switches the three phases and leaves neutral solidly connected. Correct for most TN-S systems where genset and mains share an earthing arrangement.
- 4-pole also switches the neutral. Required where the generator is a separately-derived source with its own neutral-earth bond, where earth-fault protection would otherwise mis-operate, and by many utility codes for larger installations. When in doubt, 4-pole avoids circulating-current problems — at a modest cost premium.
Standards that matter
- UL 1008 — the North American transfer switch standard, including withstand and closing ratings (WCR) coordinated with upstream breakers.
- IEC 60947-6-1 — the international standard for transfer switching equipment; look for AC-33A utilisation class for motor-mixed loads.
- Short-circuit withstand — the ATS must survive the prospective fault current at its installation point until the upstream protection clears; verify the kA rating against your fault study.
Installation notes from the field
- Mount the ATS at the service entrance or main distribution board — upstream of everything it must protect.
- Set mains-return delay long enough (≥5 min) to avoid cycling on an unstable grid.
- Exercise weekly: a no-load or loaded test run keeps the engine and the transfer mechanism proven.
- Specify controller communications (Modbus/RS485) if the site has a BMS or remote monitoring.
Every Econo Solar genset from 5.5 to 4,000 kVA can ship with a factory-matched ATS panel and controller. Send us your load schedule and we’ll spec the switch with the genset.