In short. An AMF (automatic mains failure) panel watches the DISCOM supply, decides when it has genuinely failed, starts the diesel generator, checks that the set has built proper voltage and frequency, permits the transfer, and later brings the load back to mains and stops the set after cooldown. It is the decision-making layer of a standby system, and a healthy generator with a badly set or wired AMF panel is still a backup-power failure.
What the AMF panel is responsible for
The generator engine, alternator and fuel system produce power. The transfer switch moves the load. The AMF panel is everything in between: sensing, judgement, sequencing, protection and reporting. In small and medium Indian installations, from 15 kVA to about 500 kVA, the AMF function lives in a microprocessor controller mounted on the generator or in a wall panel next to it, with the changeover contactors or MCCBs in the same cubicle. On larger sets the controller sits in a dedicated panel and drives separate switchgear. In either case the logic is the same, and the detail of that logic is what separates a system that works at 2 a.m. in a monsoon from one that merely passed a demonstration.
What the panel monitors
A properly configured AMF controller watches more than “is there voltage”.
- Mains voltage on all three phases, against under and over limits. Single-phasing, where one phase of the DISCOM supply is lost, is common on rural and industrial feeders and must be detected as a failure.
- Mains frequency, against limits that distinguish a real fault from a sagging grid.
- Phase sequence of the mains, so that a reversed supply after DISCOM maintenance is flagged rather than transferred to.
- Generator voltage and frequency, to confirm the set is ready before transfer is allowed.
- Engine parameters, including oil pressure, coolant temperature, speed, battery voltage and fuel level, for protection and alarms.
- Position feedback from the transfer devices, to confirm commands were obeyed.
The controller functions and signal lists are covered in detail under AMF control logic; what follows is about applying them.
The sequence and its timers
Each step in an AMF sequence has a timer, and each timer is a decision about the trade-off between fast response and false operation. The values below are typical ranges that we find work on Indian mains; the final figures belong to the site.
| Timer | Purpose | Typical range | Set too short | Set too long |
|---|---|---|---|---|
| Mains failure confirmation | Ignore momentary dips and auto-reclose blips | 2 to 5 s | Set starts for every flicker | Outage extended needlessly |
| Start delay and crank attempts | Allow preheat; retry a failed crank | 3 attempts, 5 to 10 s crank, 10 s rest | Battery drained, starter overheated | Slow response on a hard start |
| Warm-up and generator stabilisation | Let voltage and frequency settle before load | 3 to 10 s | Transfer onto unstable supply | Longer outage |
| Mains return confirmation | Avoid retransfer to a supply that fails again | 2 to 10 min | Repeated transfers during restoration work | Unnecessary diesel burned |
| Cooldown | Let turbocharger and coolant temperatures fall off load | 3 to 5 min | Turbocharger bearing wear | Fuel and hours wasted |
Two settings deserve particular attention in Telangana and Andhra Pradesh. The mains failure confirmation delay should be long enough to ride through DISCOM auto-reclose operations, which clear most transient faults within a second or two, but not so long that a UPS runs out of battery before the generator is called. And the mains return confirmation should be long, because supply restored after a storm often comes back several times before it stays.
Interfaces beyond the generator
A modern AMF controller is also where the standby system talks to the rest of the building: remote start and stop from the BMS, a common alarm and a running status to the fire alarm or security desk, a Modbus or similar link to SCADA, and on unattended sites a GSM or IP gateway that sends a message when the set starts, fails to start or trips. Each should be listed in the specification as an input or output with its voltage and contact type, and each proven during commissioning, because an alarm nobody receives is not an alarm. Larger sites extend this into SCADA, BMS and DCIM integration.
Where automatic standby actually fails
When a site reports that the generator did not come on during an outage, the engine is rarely the cause. In our experience the failure is in one of the following places, in roughly this order of frequency.
Battery and charger
The starting battery is flat because the trickle charger failed months ago or was switched off at the socket. A battery voltage alarm wired to somewhere that is watched, and a weekly test start, prevent most of these.
Mode selector left in manual or off
After a service or a test the selector was not returned to auto, so a healthy controller does nothing. A “not in auto” alarm to the BMS or a daily visual check is the fix.
Sensing wiring and fuses
A blown sensing fuse or a disturbed wire makes the controller see a permanent failure, so the set runs continuously, or never see the failure at all. Sensing circuits should be fused, labelled and checked at every service.
Transfer device
The set runs and the contactor or MCCB does not close, because the coil has failed or the mechanism is stiff from years without operation. The controller may still show “on load” because it only looked at its own command. Position feedback and a loaded transfer test under the AMC catch this.
Protection trips on load
The set takes the load and trips on overload, coolant temperature or oil pressure, because the load has grown, the radiator is choked with dust or the oil was never changed. The AMF panel reports these accurately but cannot prevent them.
Specifying an AMF panel
A specification should state the generator rating and voltage, the mains sensing arrangement, the number, type and rating of transfer devices and their pole count, every remote input and output with its purpose, the communication protocol if any, the enclosure rating and panel-room ambient, the intended value of each timer in the table above, and who will commission the system with a full loaded transfer test. Which controller family is used matters less than whether these items are defined.
Related photographs and videos
Common mistakes
- Leaving the mains failure confirmation at the factory default of under a second; the generator starts dozens of times a month for dips that would have passed.
- Not wiring a “not in auto” or low battery alarm to anywhere watched; the first real outage finds a flat battery or a selector left in manual.
- Sensing mains on one phase only; a single-phasing fault leaves two-thirds of the building without supply while the controller sees healthy mains.
- Commissioning by watching the set start and stopping there; the contactor that will not close is discovered during the first outage.
What to send us
Send a photo of the generator nameplate and of the existing controller or panel front, a note of the fault or behaviour you are seeing (does the set start, does it take load, does it return to mains), the hours run, how many phases the mains sensing is connected to if you know, and whether the site is attended. For a new panel, add the single-line diagram and the list of remote signals you need. We will diagnose the sequence or specify the panel. Use the service and AMC enquiry for an existing installation.
Questions engineers ask
What does AMF stand for and what does the panel do?
AMF is automatic mains failure. The panel monitors the DISCOM supply, starts the generator when the supply fails, permits transfer of the load once the set is stable, retransfers when mains returns and stays healthy, and stops the set after a cooldown. It also protects the engine and reports alarms.
Is an AMF panel the same as an ATS?
No. The AMF panel decides and sequences; the ATS physically switches the load. Many small installations combine both in one cubicle, with the controller driving two interlocked contactors, but they are separate functions and should be specified and tested separately.
Why does my generator start and run but the load stays off?
The usual causes are a transfer contactor or breaker that has not closed, a generator voltage or frequency that sits just outside the controller’s acceptance window, or a stabilisation timer that never completes because of fluctuating speed. Check the controller’s event log first; it normally records exactly where the sequence stopped.
How often should an AMF system be tested?
A no-load test start once a week or fortnight keeps the battery, starter and fuel system exercised. A full test with the load actually transferred should be done at least quarterly, and some sites do it monthly, because it is the only test that proves the transfer devices and the complete sequence.
Technical content reviewed by Srinivas Reddy, Managing Partner, Reddy Generators. Last updated 19 October 2026.