When a mains failure occurs, the gap between “utility lost” and “generator carrying the load” is set by the
automatic transfer switch (ATS), not by the generator. For buyers, the two numbers that decide whether sensitive
equipment keeps running are the transfer time and the transition type. This guide
covers what to specify and what to verify before the switchgear is built.
What buyers should know first
The generator engine needs a few seconds to reach rated speed and voltage after a start command. The ATS then
moves the load. Two distinct intervals matter:
- Sensing + start time — how long the controller waits before deciding the mains is gone, then cranks the set.
- Transfer time — the time the ATS actually takes to move the load from one source to the other once the set is ready.
For most commercial loads a short break of under a second is acceptable. For hospitals, data centres and process
lines, the break must be near zero — which points to a closed-transition switch.
Open transition vs closed transition
| Transition type | Load break | Typical use | Note |
|---|---|---|---|
| Open (break-before-make) | Brief break, often 1–5 s on standard switches | General commercial, industrial | Simplest and lowest cost; sources never connect |
| Closed (make-before-break) | Near-zero interruption | Hospitals, data centres, critical process | Two sources momentarily paralleled; requires synchronising |
| Delayed / in-phase | Break minimised by matching voltage | Motor loads, soft transition | Reduces inrush when re-closing to utility |
Switching equipment is built to IEC 60947-6-1 (transfer switching) and listed to UL 1008
in North America. Ask the panel builder which standard the ATS is certified to.
Pole count: 2, 3 or 4 poles
Pole count decides which conductors are switched:
- 2-pole — switches line and neutral; used on single-phase or where only the live conductor is interrupted.
- 3-pole — switches all three phases; the neutral is bonded and not switched.
- 4-pole — switches the three phases and the neutral. Needed when the two sources have different
neutral treatments, or where isolation of the neutral is required for safety/earthing on changeover.
For a three-phase generator feeding a building with its own neutral-earth arrangement, 4-pole switching is the
safe default. If in doubt, raise it during the RFQ rather than after commissioning.
Common configuration mistakes
- Specifying a standard open-transition switch for a load that cannot tolerate a break, then discovering trips
after the first test. - Forgetting that closed transition needs the generator controller to synchronise to the bus — not every set is
configured for it. - Mixing pole counts between the generator breaker and the ATS, which complicates neutral bonding.
Procurement checklist
| Item | Confirm |
|---|---|
| Transition type | Open or closed, matched to load sensitivity |
| Transfer time | Stated in the datasheet (seconds, or “near zero”) |
| Pole count | 2 / 3 / 4, consistent with neutral bonding |
| Certification | IEC 60947-6-1 or UL 1008 |
| Controller role | Who commands the switch — genset controller or separate ATS logic |
Related equipment to compare
Pair the ATS with the right set: review the generator range,
Cummins-powered units for critical sites, and
medical application sets where closed transition is common. See the full
product list or contact us for a switch sizing review.
Frequently asked questions
How fast is “fast enough” for an ATS?
For general loads, a sub-second to few-second break is fine. For IT and life-safety loads, specify closed transition
with near-zero break and confirm synchronising is enabled.
Does the generator brand affect transfer time?
Not directly. The engine start time is set by the set; the transfer time is set by the ATS and controller. Both must
be specified together.
When do I need 4-pole switching?
When the neutral must be isolated on changeover, or when the two sources have different earthing/neutral arrangements.
Is closed transition more expensive?
Yes — it adds synchronising logic and a more capable switch, but it is the only option for loads that cannot break.
Can I retrofit closed transition later?
Usually not without replacing the ATS and re-configuring the controller, so decide it at specification stage.
What standard should the ATS meet?
IEC 60947-6-1 for most markets; UL 1008 where the project calls for North American listing.