In short. An automatic transfer switch is only proven by running the complete sequence: mains failure detected, generator started, generator source accepted, load transferred, mains restored, retransfer after the return delay, and generator cooled down and returned to standby. ATS service inspects the switching mechanism, interlocks, contacts and control supply, verifies source sensing against a meter, checks the start signal to the generator and the timers, and records a witnessed functional test agreed with the site.
Why the transfer path deserves its own service
A standby generator that starts perfectly but never receives the load has failed, as far as the facility is concerned. The transfer switch sits between the DISCOM supply and the generator on one side and the essential load on the other, carrying full load current daily on the mains side while its generator side may not operate for months. Mechanisms stiffen, relays stick, sensing transformers drift and the start-signal wire corrodes, all without visible symptoms until the night the supply fails. The guide to automatic transfer switches explains the types; service is about keeping whichever type you have ready.
What an ATS service visit covers
A proper visit splits into inspection with the panel isolated, electrical checks and a live functional test. The isolated inspection covers the switching device, whether a motorised changeover switch, a pair of contactors with mechanical and electrical interlock, or a pair of air circuit breakers with a mechanical interlock. We check contact wear, termination tightness with a torque wrench, signs of heating, free movement of the interlock and mechanism, and the control fuses, relays and controller. Thermal imaging under load before isolation finds loose terminations a visual check misses.
The electrical checks cover insulation resistance of the main circuit, operation of the motor or coils on manual command, and proof that the mechanical interlock prevents both sources closing together under every combination of manual and electrical command. On contactor panels the electrical interlock is checked separately, since a welded auxiliary contact can defeat it. The ATS and switchgear parts page lists what is normally replaced at these visits.
Source sensing is where most faults hide
The controller decides that the mains has failed, or that the generator is acceptable, from voltage and frequency measurements through sensing circuits. Service verifies these against a calibrated meter on all three phases. The table lists the settings that need checking against the site’s requirements rather than being left at default. The ranges are indicative; the right values depend on the load, the quality of the local supply and the engine manufacturer’s recommendations.
| Setting | What it does | Typical range, 415 V 50 Hz site | What goes wrong if set incorrectly |
|---|---|---|---|
| Mains under-voltage dropout and pickup | Defines when the mains is failed and when restored | Dropout 80 to 85 percent, pickup 90 to 95 percent, with hysteresis | Too tight causes nuisance transfers on DISCOM dips; too loose leaves the load on a brown-out |
| Phase loss and phase sequence | Detects single phasing and wrong rotation | Enabled on all three phases | Single-phase sensing misses loss of one phase and three-phase motors burn |
| Mains failure delay | Rides through short dips before starting the generator | 2 to 10 seconds | Zero delay starts the set on every flicker; a long delay extends the outage |
| Generator warm-up or transfer delay | Lets voltage and frequency stabilise before transfer | 5 to 15 seconds | Transfer onto an unstable source trips UPS and motor protection |
| Mains return delay | Confirms the mains is stable before retransfer | 1 to 5 minutes | A short delay causes repeated transfers on a flickering supply |
The start signal and the generator interface
The link between the ATS and the generator is a pair of wires carrying a volt-free contact or a DC signal, and it fails more often than either device. Service confirms that the contact closes on mains failure, that the generator controller receives it and starts, and that the generator’s ready-to-load signal, where used, returns to the ATS. Where the ATS and generator come from different suppliers, we confirm which device is the master. Either the ATS commands the start and the generator controller obeys, or the generator’s AMF controller senses the mains itself and the ATS is a slave switching device. Two devices both sensing the mains with different settings produce transfers nobody can explain. The ATS and AMF control logic guide describes the arrangements.
Running the functional test
A visual inspection cannot prove automatic transfer. The functional test is agreed in writing with the site because it interrupts the essential load, so the test window, the loads affected and the abort criteria are recorded first. The sequence we witness and time is as follows.
- Simulate mains failure by opening the incoming mains breaker to the ATS rather than by using the controller’s test button, so the sensing circuit is exercised.
- Record the time to generator start, to voltage and frequency within limits, and to transfer. Note voltage and frequency at the instant of transfer.
- Observe the load on generator for a few minutes. Motor protection trips, UPS alarms or lighting flicker indicate an unstable source.
- Restore the mains and time the return delay, the retransfer and the cool-down. Confirm the generator stops and returns to automatic mode.
- Confirm from the event log, where available, that the mains device had fully opened before the generator device closed.
The record from this test, kept with the panel, is the evidence that the transfer system works and the baseline for the next test.
Faults we see repeatedly
Transfer never happens though the generator runs
Usually the generator-available signal, a failed transfer relay, a generator-side mechanism seized through disuse, or a generator voltage marginally outside the acceptance window. Compare the controller’s acceptance readings with a meter at the generator terminals.
Retransfer fails or cycles
Usually a mains return delay too short for a flickering supply, a drifted mains sensing transformer, or loss of one phase that single-phase sensing cannot detect.
Both sources closed, or a burnt panel
The mechanical interlock has been defeated, often during a previous repair where someone bypassed it to get the site running. This is the most dangerous condition a transfer switch can be in, because it back-feeds the generator into the DISCOM network and endangers line workers. Any bypassed interlock is restored before the panel is left.
Related photographs and videos
Common mistakes
- Testing only from the controller’s test button, so the mains sensing circuit is never exercised and a drifted sensing transformer goes undetected until a real failure.
- Leaving a bypassed interlock in place after an emergency repair, which exposes the site and DISCOM line workers to a back-feed.
- Setting a zero mains-failure delay, which starts the generator on every momentary dip and wears out the starter and battery.
- Ignoring the generator-side contacts because they rarely operate, so they seize or pit and fail to carry load on the first real transfer.
- Letting the ATS controller and the generator AMF controller both sense the mains with different settings, producing transfers and retransfers that appear random.
What to send us
Send photographs of the ATS panel with the door open, the nameplates of the switching device and the controller, the generator nameplate, and a description of what happens during a mains failure now, including whether the generator starts, whether the load transfers and whether retransfer works. Include a single-line diagram or the previous test record if you have one. Submit through the service and AMC enquiry form; transfer switch service can be included in a generator AMC.
Questions engineers ask
How often should an ATS be tested?
A full mains-failure simulation with load transfer monthly is good practice for critical sites and quarterly for most commercial sites, with an inspection and torque check annually. Many sites run the generator weekly on no load, which exercises the engine but not the transfer switch, so make sure some tests include an actual transfer.
Can the ATS be tested without interrupting the load?
The logic can be tested from the controller’s test mode without transferring, and a bypass-isolation ATS allows the switch to be maintained while the load stays on the mains. A genuine transfer test always interrupts the load briefly, which is why it is scheduled and agreed with the site in advance.
How long is the supply interrupted during a transfer?
An open-transition transfer breaks one source before making the other, typically for a few hundred milliseconds to a second. Loads that cannot tolerate this need a UPS. Closed-transition transfer avoids the break but needs synchronised sources and is a different class of equipment.
Why does the generator start but the load stays on the mains?
The mains sensing saw a dip the controller treated as a failure, but the mains recovered before the transfer delay expired, or the generator voltage or frequency did not enter the acceptance window. Check the event log, compare the acceptance settings with the generator output, and review the mains failure delay.
Technical content reviewed by Srinivas Reddy, Managing Partner, Reddy Generators. Last updated 19 October 2026.