In short. An automatic transfer switch (ATS) is the switching device that decides which source, DISCOM mains or diesel generator, feeds the load, and moves the load between them under interlock. It does not generate power and it is not the same as AMF logic. A correct ATS is specified by rated current, number of poles, short-circuit withstand, neutral arrangement, transition type and control interface, all matched to the actual 415 V system it serves.
What an ATS does and what it does not do
Starting the generator is only half of a standby sequence. The load still has to reach the generator, and later return to the mains, without ever connecting the two sources together uncontrolled. That is the job of the transfer switch. It connects the load to the healthy source, disconnects it from the failed one, and interlocks the two so that a backfeed into a dead DISCOM line, or a dead short between two unsynchronised sources, cannot happen.
An ATS does not sense mains failure on its own in most Indian installations; that sensing and the start command usually sit in the generator controller or a separate AMF panel. The common error we meet on site is the statement “the set has AMF, so we already have an ATS”. AMF is the decision and coordination logic. The ATS is the power path. Some panels combine both in one enclosure, but when you specify, test or fault-find, treat them as two functions.
Selection inputs that must come from the real system
A transfer switch is not a catalogue item chosen by kVA alone. Each of the inputs below changes the device or its settings.
| Input | Why it matters | Where to get it |
|---|---|---|
| Rated voltage and frequency | 415 V, 50 Hz three-phase for most sites; 230 V single-phase for small premises | Single-line diagram, nameplates |
| Rated current | Must carry the full load continuously at site ambient, not just generator output current | Load study or maximum demand record |
| Number of poles | Three-pole keeps a solid neutral; four-pole switches the neutral, needed where sources are separately earthed or RCD protection is upstream | Earthing and neutral design |
| Short-circuit withstand | The switch must survive the prospective fault current from the mains until the upstream breaker clears | Fault-level calculation at the ATS location |
| Transition type | Open, delayed, closed or bypass-isolation, decided by what the load tolerates | Load classification |
| Control interface | Volt-free start signal, source-available feedback, BMS or SCADA points | Generator controller and site control philosophy |
| Environment | Dust, humidity, monsoon ingress and rooftop temperatures decide IP rating and derating | Site survey |
Rated current is the one most often underestimated. If the generator is 250 kVA but the building maximum demand on mains is 400 A, the ATS carries 400 A all year on the mains side and must be rated for it.
The transfer sequence, step by step
The sequence below is the general form. Each timer is set per project, and the values quoted are typical ranges rather than defaults.
- Mains is healthy and the load is on mains. The controller monitors voltage on all three phases, frequency and phase sequence.
- Mains goes outside limits. A confirmation delay, commonly 2 to 5 seconds, filters momentary dips so the set does not start for every flicker.
- The generator receives a start command, cranks, and builds voltage and frequency. On most sets in the 50 to 1,000 kVA range this takes 5 to 15 seconds.
- Once generator voltage and frequency are within limits for a short settling time, the ATS is permitted to transfer. The mains contact opens, and after the transition logic completes, the generator contact closes.
- Mains returns. A retransfer delay, commonly 2 to 10 minutes, confirms that the supply is stable before the load is moved back.
- The generator runs unloaded for a cooldown of roughly 3 to 5 minutes and stops, and the system returns to standby.
Every one of these steps should be visible on the controller or panel as a state, and every one should be checked during commissioning, including the abnormal cases: mains returning while the set is still cranking, generator tripping while on load, and failure of the ATS to reach its commanded position.
Choosing the transition type
The transition type is how the load physically moves between the sources, and it follows from the load, not from the price list.
Open transition
Break-before-make. The load sees a short interruption on every transfer, which most standby loads accept and UPS-backed loads ride through. It is the default for the majority of commercial and industrial sites. See open-transition ATS.
Delayed transition
Open transition with a deliberate pause in the neutral position so that motor residual voltage decays and drives reset before the new source is applied. See delayed-transition ATS.
Closed transition
Make-before-break with a momentary parallel of the two sources, which removes the retransfer interruption but requires synchronising, protection and DISCOM consent. See closed-transition ATS.
Bypass isolation
Any of the above with a second, manual path that keeps the load fed while the automatic switch is isolated for maintenance. See bypass-isolation ATS.
Neutral, earthing and protection details that decide safety
In India the generator neutral is normally earthed separately at the generator, and the DISCOM supply is earthed at the distribution transformer. When both neutrals are solidly connected through a three-pole switch, two earth references exist and earth-fault current can divide between them, which can defeat earth-leakage protection and cause nuisance tripping or, worse, non-tripping. A four-pole switch that transfers the neutral with the phases removes that path, at the cost of needing an overlapping or early-make neutral so that the load is never left with phases but no neutral. This is a design decision, not an option to tick, and it is treated in detail under earthing and neutral architecture.
Short-circuit withstand is the other item that is often ignored on small projects. A switch rated for 10 kA installed where the mains prospective fault current is 25 kA will not survive a fault downstream of it. Ask the DISCOM or calculate the fault level at the point of installation, and match the switch, or protect it with a current-limiting breaker whose let-through has been checked against the switch rating.
Related photographs and videos
Common mistakes
- Rating the ATS to the generator current rather than the mains maximum demand; the switch overheats on normal supply, which is where it spends most of its life.
- Installing a three-pole switch on a system with separately earthed generator and mains neutrals; earth-fault protection becomes unreliable and circulating neutral currents appear.
- Testing only the generator start and never the transfer under load; the first real outage then reveals a stuck mechanism or a wrong phase sequence.
- Assuming the ATS supports closed transition and that this makes it permitted; utility paralleling needs approval and protection regardless of what the switch can do.
What to send us
For an ATS review or a new transfer panel, send a photo of the generator nameplate and controller, the DISCOM sanctioned load or the latest maximum demand reading, a single-line diagram or a photo of the main switchboard, and a short note on which loads must stay on and whether any UPS is installed. If there is an existing changeover, photograph its nameplate and the cable terminations. We respond with the pole count, rating, transition type and interface we would specify, and why. Start with the application engineering enquiry.
Questions engineers ask
Is an ATS compulsory with a standby generator?
No regulation forces automatic transfer, but every generator connected to a building that also has a DISCOM supply must have an interlocked changeover so that the two cannot be paralleled accidentally. Whether that changeover is manual or automatic depends on how long the site can be without power and whether anyone is present to operate it.
What is the difference between an ATS and an AMF panel?
The AMF panel senses the mains, decides when to start and stop the generator, and issues the transfer command. The ATS is the switching device that actually moves the load. Many small Indian installations put both in one cubicle, but they remain distinct functions with distinct failure modes.
How long is the interruption during transfer?
With an open-transition ATS the break itself is typically tens to a few hundred milliseconds for a contactor or motorised switch, and longer for breaker-based schemes. The total outage seen by the load on a mains failure includes the confirmation delay and generator start time, usually 10 to 25 seconds in all.
Do I need a four-pole ATS?
You need to switch the neutral when the generator and mains have separate earth references, when residual-current protection sits upstream of the switch, or when the site earthing design calls for a single point of neutral-earth connection per source. Most Indian LT installations with a separately earthed generator fall into this category, so four-pole is the safe default unless an engineer has justified three-pole.
Technical content reviewed by Srinivas Reddy, Managing Partner, Reddy Generators. Last updated 19 October 2026.