When mains power fails at 2 a.m., who starts the generator? If the answer is "the watchman", your building is running on hope. An automatic transfer switch (ATS) panel removes the human from the loop.
What an ATS actually does
An ATS panel sits between the utility supply, the generator and your main distribution board. Its controller continuously monitors mains voltage and frequency. When it detects a failure it:
- waits a few seconds to ignore short dips
- sends a start signal to the generator
- waits until generator voltage and frequency are stable
- switches the load from mains to generator
- when mains returns and stays healthy, switches back and lets the engine cool down before stopping it
The whole sequence normally takes 5–15 seconds, most of it engine start and warm-up.
Types of ATS panel
- Motorised changeover switch — compact and cost-effective, common from 63 A to about 1600 A.
- Contactor-based ATS — simple and fast, suited to smaller ratings.
- ACB-based changeover — two interlocked air circuit breakers, used from roughly 630 A to 4000 A where protection and metering are needed too.
How to size an ATS
Match the ATS rating to the generator's full-load current, or the incoming mains breaker if that is larger. At 400 V three-phase:
Current (A) ≈ kVA × 1.443
So a 250 kVA generator produces about 361 A, and needs a 400 A ATS. A 500 kVA set needs about 722 A — so an 800 A panel.
3-pole or 4-pole?
A 4-pole ATS switches the neutral as well as the phases. It is recommended when the generator's neutral is earthed separately from the utility neutral, and where earth-leakage protection must work correctly on both sources. In many UAE installations 4-pole is the safer default — we confirm against your earthing arrangement.
Mistakes we see on site
- ATS sized to the load, not the generator. When the site grows the ATS becomes the bottleneck.
- No mechanical interlock. Both sources must never be connected at the same time.
- No cool-down timer. Stopping a hot turbocharged engine immediately shortens its life.
- Panels without IP protection installed in dusty plant rooms or outdoors.
Where to go next
Browse our ATS panel range from 63 A to 4000 A, or use the kVA calculator, which also recommends the ATS rating for the generator it selects. If you already have a generator, send us its nameplate photo on WhatsApp and we will propose the matching panel.
