ATS Explained: The Ten Seconds After a Power Cut

The generator is the visible half of a backup system. The transfer switch is the half that decides whether the building actually sees power — and most post-incident reports point at the switch, not the engine.

The sequence, second by second

An ATS is a state machine with a timer for each transition. Typical settings, which vary by installation:

Those delays exist for good reasons. A start delay prevents the generator from chasing every two-second voltage sag; a return delay stops it from cycling when the utility recloses and trips again. Setting them too short produces nuisance starts; setting them too long leaves the load on batteries longer than necessary.

Poles, neutrals and the earthing question

A three-pole switch transfers the phases and leaves the neutral solid. A four-pole switch transfers the neutral as well, which matters where the generator is a separately derived source and the installation uses earth-fault protection that would otherwise see two neutral paths. Getting this wrong is not a nuisance — it is a safety and discrimination problem. The decision belongs to the electrical designer, and the ATS must match it.

Open or closed transition

Open transition breaks before it makes: a short interruption, typically 100–300 ms, which most loads ride through and some — older UPS systems, certain medical equipment — do not. Closed transition briefly parallels mains and generator, making the transfer nearly seamless, at the cost of synchronising equipment and a more complex switching sequence. For data centres and hospitals the choice is usually made at design stage; for a workshop it is rarely justified.

The maintenance nobody schedules

• Exercise the switch monthly, not only the generator — a stuck mechanism is invisible until needed.
• Torque the power terminals annually; loose lugs are a common cause of transfer failure.
• Test the failure path: simulate a mains loss and confirm the transfer, not just the engine start.
• Log the settings. A switch that was commissioned at 3 s start delay and later found at 30 s has been changed by someone.

FAQ

Can I use a manual changeover instead?

For a site where someone is always present and a few minutes of downtime is acceptable, yes. For anything unmanned — telecom sites, remote pumping, cold storage — the economics favour automatic transfer.

Does the ATS need to match the generator brand?

No, but it must match the system: current rating, pole configuration, transition type and control voltage. Mixing brands is normal; mixing incompatible control logic is not.

What size ATS do I need?

At least the generator's rated current, with the enclosure and fault rating suited to the installation. Undersizing the switch to save cost is a false economy — it becomes the weakest link in a system built for reliability.

Browse ATS and switchgear