In short. Utility, ATS or AMF panel, and a generator form the basic backup chain. The AMF senses mains failure, starts the engine, waits for stable output, transfers the load through an interlocked changeover, and returns when mains is healthy.
Chain
Utility → ATS/changeover ← generator → load. The ATS makes sure the two sources never feed the load together unless designed to.
Sequence
T0 mains fails → T1 delay and start signal → T2 engine cranks and runs → T3 voltage and frequency stable → T4 transfer → T5 retransfer after mains returns with a timer, cool-down and stop.
See ATS and AMF control logic and ATS service.
Design points
| Item | Why it matters |
|---|---|
| Mechanical and electrical interlock | Prevents backfeed and paralleling |
| Transfer timing | Settings avoid nuisance start and unstable transfers |
| Neutral switching | Affects earth-fault protection. See earthing |
| Load shedding | If the generator is smaller than the load. See load shedding |
| Breaker ratings | Fault level and withstand |
| Testing | Regular on-load tests |
Open versus closed transition
Open transition has a brief dead time. Closed transition parallels briefly, needing synchronising equipment and utility approval. See synchronisation.
Protection
Coordinate generator, ATS and downstream breakers. See coordination and generator protection.
Illustrative example
A commercial building tests its ATS monthly on load. One test reveals that the ATS retransfer timer is too short and the generator stops before the mains is stable. Adjusting the timers prevents repeated switching during unstable mains.
Timing parameters and what they do
| Parameter | Purpose | Risk if wrong |
|---|---|---|
| Mains failure delay | Ignore very short dips | Too short: nuisance starts. Too long: longer outage |
| Engine start and warm-up | Allow stable output | Short: transfer onto an unstable set |
| Generator availability check | Voltage and frequency in range | Missing check allows poor supply |
| Transfer time | Mechanical switching | Long break for sensitive loads |
| Mains return delay | Confirm stable mains | Short: repeated transfers |
| Cooling run | Cool the engine before stop | Skipping shortens engine life |
See ATS and AMF logic.
Neutral and earthing arrangements
With a three-pole switch, the neutral is common to both sources, and the earth fault protection works the same on both. With a four-pole switch, the generator neutral is separate and bonded at the generator. This affects where earth fault protection is placed and how stray currents flow. The design must be consistent with the earthing system. See earthing.
Testing programme
- Monthly: run the generator on load or load bank.
- Quarterly: simulated mains failure test on load, where safe.
- Annual: full test including protection and alarms.
- After any modification: repeat the relevant test.
Common problems found in tests
- Flat starting batteries. See starting system.
- Fuel issues: low level or contaminated fuel.
- Controller settings changed and never restored.
- ATS contacts worn or sticking.
- Wrong phase rotation after maintenance.
Records
Log every test, with date, result, defects and fixes. These logs help show diligence and identify trends.
The master engineering question
What does the load see at each moment from T0 to T5, and what could go wrong at each?
What to send us
Send single-line and generator details. Send your application details.
Questions engineers ask
AMF or ATS?
AMF adds start and stop control; ATS is the switching device.
Why does the transfer take time?
Start, warm-up and stabilise delays.
Is neutral switching needed?
Depends on the earthing design.
How often to test?
Regularly, on load.
Values are illustrative. Follow code and maker guidance.