An ATS (automatic transfer switch) monitors the mains supply. When the mains fails, it starts the generator and moves the load to it. When the grid returns, it moves the load back. No one needs to be on site. Depending on the market, the device is called an automatic transfer switch (ATS), a transfer panel or a changeover panel. Without one, someone has to walk to the machine, start it and operate a changeover switch by hand. That difference decides whether a standby set is worth owning at an unmanned site.
What happens, second by second
The sequence is always the same. Knowing it avoids the most common false expectation:
- The mains fails. The ATS confirms a real outage, not a brief flicker. The confirmation delay is typically 1–3 seconds.
- Start signal. The controller cranks the engine. A healthy set starts within a few seconds.
- Warm-up. The set runs without load until voltage and frequency are stable, usually 5–15 seconds.
- Transfer. The load moves from the mains to the generator.
- The grid returns. The ATS waits until the grid is stable, often 1–3 minutes, and transfers the load back. The engine then runs without load to cool down before it stops.
The total time from outage to restored power is typically 10–30 seconds. That is fast enough for lighting, pumps, refrigeration, workshops and most commercial loads.
What an ATS does not do
An ATS is not a UPS. Those 10–30 seconds are a real outage at the socket. Equipment that cannot tolerate a break needs a UPS to bridge the gap: servers, network equipment, medical equipment, PLCs in mid-cycle and point-of-sale terminals. The generator behind the UPS then carries the load for the long run. A UPS and an ATS work together; one does not replace the other.
The second limit is mechanical: an ATS needs an electric-start engine and a charged battery. A recoil-start set cannot be automated, because the controller has nothing to operate. For this reason, automatic operation is available on electric-start models only.
Do you need one?
You need an ATS when:
- The site is unmanned, or unmanned at night: telecom huts, pumping stations, warehouses, remote installations.
- The load is temperature-sensitive: cold rooms, hatcheries, dairies, vaccine storage. A four-hour outage that nobody noticed means spoiled stock.
- Outages are frequent and short, and starting the generator by hand has become a daily task.
- A contract or tender requires automatic changeover. This is common in telecom and healthcare specifications.
You can do without one when:
- Staff are on site whenever the load matters, and a manual changeover switch is acceptable.
- The generator provides prime power. The mains is not the main source, so there is nothing to transfer from. See prime versus standby power to check which duty you are buying.
- The set is portable and moves between sites. An ATS is wired into one installation.
An ATS means standby duty: the machine stands idle and must start reliably when needed. This shifts the focus of the specification away from peak output and toward battery condition, a block heater in cold climates, and a monthly test run under load.
How it is wired: what connects to what
Buyers ask how an ATS connects more often than what it does. The layout is always the same, and it has four circuits, not one:
Mains / utility ──────┐
├──► [ ATS ] ──► Essential-loads panel
Generator ────────────┘ │
│ control cable (2 wires, dry contact)
▼
Generator controller
(remote-start terminals)- Utility supply → the ATS “normal” input. This is the supply the ATS monitors.
- Generator output → the ATS “emergency” input. Size this cable for the ATS current rating, not for the generator’s kVA.
- ATS output → the essential-loads panel. Here you decide what is backed up. Transferring the whole distribution board usually costs more than a separate sub-panel for the loads that must never lose power.
- Control cable → the controller’s remote-start terminals. This is normally two wires and a dry (potential-free) contact. When the ATS closes the contact, the set starts. When it opens the contact, the set cools down and stops. This circuit makes the generator an automatic system, yet it is often missing from installation drawings.
Three details decide whether the installation works correctly:
- Interlock. Mechanical, electrical or both: the two sources must never be connected at the same time. Without an effective interlock, an ATS can feed power back into the utility line, which endangers anyone repairing it. It can also damage the alternator when the grid returns out of phase. The interlock is a mandatory safety requirement.
- A battery charger fed from the mains side. The set stands idle for weeks. If the battery goes flat, the ATS closes its contact and the engine does not start. A flat battery is the most common reason a standby set fails to start.
- Neutral: 3-pole or 4-pole. A 3-pole ATS switches the phases and leaves the neutral solidly connected. A 4-pole ATS switches the neutral as well, which some local codes and residual-current device (RCD) schemes require. Your national wiring rules decide this, not the generator manufacturer. Check before you buy, because changing from 3 to 4 poles later means replacing the unit.
A licensed electrician must install the ATS, following your local code. This section helps you read the quotation you receive and spot what is missing. It is not a guide to wiring a board connected to your service entrance yourself.
Three kinds of ATS, and which one to ask for
| Type | How it switches | Where it fits | What to watch |
|---|---|---|---|
| Contactor-based | Two interlocked contactors | Small and mid-size installations; the usual choice at this power range | No short-circuit breaking capacity, so it needs upstream protection. It is the fastest and the least expensive |
| Motorised breakers | Circuit breakers with a motor operator | Large installations, high currents | Breaking capacity built in; slower (1–3 s more) and more expensive |
| Closed transition | Overlaps both sources for a few cycles, no break | Loads that cannot tolerate even a brief interruption when the grid returns | Needs synchronisation and almost always the utility’s approval. Small sets usually cannot do it |
For most standby installations of 5–100 kVA, the right choice is a contactor-based ATS with upstream protection. Closed transition for a telecom hut is more than the application needs.
What to specify when you order
Size an ATS for the load and the supply, not for the generator’s kVA. Give your supplier:
- The 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 the 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. State which, because the controller must match.
- Enclosure and location: indoor panel or weatherproof outdoor enclosure, plus the ambient temperature and dust level.
- The delay settings you want: start confirmation, warm-up, return delay and cool-down.
- Whether you need remote monitoring through dry contacts, RS485 or GSM alerts, so that a failed start is reported before an outage causes damage.
If the amp rating is right, most other choices follow from it. If it is wrong, the switch either trips without a fault or is oversized and costs more than necessary.
Testing the system
A standby set that has never been tested under load cannot be relied on. Run it every month, with load, long enough to reach operating temperature. Short, unloaded test runs cause wet-stacking: unburned fuel fouls the exhaust and the injectors, which is exactly the fault you do not want on the day the set is needed. The same efficient load range applies here as in any other duty; see fuel consumption per hour.
Check the battery at every test. A flat battery is the most common reason a standby generator fails to start, and it gives no warning before the moment it is needed.
Where HTX fits
Automatic transfer, remote control and one-button start are available on our electric-start generator models. See the diesel generator range for the sets with 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 staffed, or message us on WhatsApp. Our engineers will reply with a recommendation and the switch rating within 24 business hours.