An ATS (automatic transfer switch) is the device that watches your mains supply, starts the generator when it fails, moves the load across, and hands it back when the grid returns — with nobody on site. Without one, somebody has to walk to the machine, start it, and throw a changeover switch by hand. That is the whole difference, and it decides whether a standby set is worth owning at an unmanned site.
What actually happens, second by second
The sequence is fixed, and knowing it prevents the most common disappointment:
- Mains fails. The ATS confirms it is a real outage, not a flicker — typically a 1–3 second confirmation delay.
- Start signal. The controller cranks the engine. A healthy set fires in a few seconds.
- Warm-up. The set runs unloaded until voltage and frequency are stable — usually 5–15 seconds.
- Transfer. The load moves from mains to generator.
- Grid returns. The ATS waits to be sure it is stable (often 1–3 minutes), transfers back, then lets the engine run unloaded to cool before stopping.
Total time from outage to power restored is typically 10–30 seconds. That is fast enough for lighting, pumps, refrigeration, workshops and most commercial loads.
The thing an ATS is not
An ATS is not a UPS. Those 10–30 seconds are a genuine outage at the socket. Anything that cannot tolerate a break — servers, network gear, medical equipment, PLCs mid-cycle, point-of-sale — needs a UPS to bridge the gap, with the generator behind it to carry the long haul. The two are complements, not alternatives, and buyers who assume the ATS covers it discover otherwise during the first real outage.
The other limit is mechanical: an ATS needs an electric-start engine and a charged battery. A recoil-start set cannot be automated at all — there is nothing for the controller to command. This is why automatic operation is offered on electric-start models and not across the board.
Do you actually need one?
You need an ATS when:
- The site is unmanned or overnight — telecoms huts, pumping stations, warehouses, remote installations.
- The load is temperature-sensitive: cold rooms, hatcheries, dairy, vaccine storage. A four-hour outage nobody noticed is a spoiled stock write-off.
- Outages are frequent and short, and manual starting has become a daily chore.
- A contract or tender requires automatic changeover — common in telecom and healthcare specs.
You can skip it when:
- Staff are on site whenever the load matters, and a manual changeover switch is acceptable.
- The generator is prime power — the mains is not the primary source, so there is nothing to transfer from. See prime versus standby power for which duty you are actually buying.
- The set is portable and moves between sites; an ATS is wired to one installation.
An ATS implies standby duty: the machine sits idle and must start reliably on demand. That changes what matters in the specification — battery health, block heater in cold climates, a monthly test run under load — more than peak output does.
What to specify when you order
An ATS is sized to the load and the supply, not to the generator’s kVA. Give your supplier:
- Current rating (amps) of the incoming supply, not the generator’s rating.
- Phases and poles — single-phase (2-pole) or three-phase (3- or 4-pole). A 4-pole switch also transfers neutral, which some installations and local codes require.
- Voltage and frequency — 220/380 V at 60 Hz in many markets, 230/400 V at 50 Hz in others. Say which, because the controller must match.
- Enclosure and location — indoor panel, outdoor weatherproof, ambient temperature and dust.
- Delay settings you want: start confirmation, warm-up, return delay, cool-down.
- Whether you want remote monitoring — dry contacts, RS485 or GSM alerts, so a failed start is discovered before the outage matters.
Get the amp rating right and most of the rest follows. Get it wrong and the switch either nuisance-trips or sits oversized and overpriced.
Testing: the part everyone skips
A standby set that has never been tested under load is a guess, not a plan. Run it monthly, on load, long enough to reach operating temperature. Light, unloaded test runs cause wet-stacking — unburned fuel fouling the exhaust and injectors — which is exactly what you do not want in a machine expected to work on the day it matters. The same efficiency band applies here as anywhere: see fuel consumption per hour.
Check the battery at every test. A flat battery is the single most common reason a standby generator fails to start, and it is invisible until the moment it is not.
Where HTX fits
Automatic transfer, remote control and one-click start are available on our electric-start generator models — see the diesel generator range for which sets carry electric start, or the emergency power solutions page for standby sizing.
Not sure whether your site needs an ATS or a manual changeover? Tell us your load, your supply (phases, voltage, frequency) and whether the site is manned — or message us on WhatsApp — and our engineers will come back with a recommendation and the switch rating within 24 business hours.