IEC 60947-6-1 Edition 4.0 adds Annex D for ATSE having closed transition capability. What the standard covers, what it does not, and what a specification must name.
Four ideas to carry into the next specification conversation you have.
TSE is the equipment class: transfer switching equipment (IEC 60947-6-1:2021, clause 3.3.1). MTSE is manually operated, RTSE is remotely operated but not self-acting, and ATSE is self-acting, with its controller (clauses 3.3.2 to 3.3.4). Write ATSE in the specification, not ATS alone.
Edition 4.0:2026 adds four annexes: bypass/isolation TSE (Annex C), ATSE having closed transition capability (Annex D), stand-alone ATS controllers (Annex E) and fire pump TSE (Annex F). The 2021 edition has no annex for these.
IEC 62310-1:2005 covers static transfer systems (STS): general and safety requirements. Check which standard matches the equipment type before citing it.
PC (switch-disconnector based) and CB (circuit-breaker based) are classes in IEC 60947-6-1, clauses 3.3.6 and 3.3.7 of the 2021 text. UL 1008 is a separate standard. Name in the specification which standard the ratings follow.
Edition 4.0:2026 is the current edition. It adds four annexes, C to F.
| Term or item | What it is | Reference |
|---|---|---|
| The product term | transfer switching equipment, TSE | IEC 60947-6-1:2021, clause 3.3.1 |
| The automatic variant | ATSE, self-acting, with its controller | IEC 60947-6-1:2021, clause 3.3.4 |
| Manual and remote | MTSE manually operated; RTSE remotely operated, not self-acting | IEC 60947-6-1:2021, clauses 3.3.2 and 3.3.3 |
| Scope | up to 1000 V AC or 1500 V DC | IEC 60947-6-1:2021, clause 1 |
| Closed transition, 2021 edition | no annex for it | IEC 60947-6-1:2026, Annexes C to F |
| Closed transition, Edition 4.0 | Annex D, ATSE having closed transition capability | IEC 60947-6-1:2026, Annex D |
| Open transition | break before make | IEC 60947-6-1:2021, clause 3.4.7 |
| Closed transition, definition | make before break, with a synchronized second source | IEC 60947-6-1:2021, clause 3.4.9 |
| PC and CB classes | PC switch-disconnector based; CB circuit-breaker based | IEC 60947-6-1:2021, clauses 3.3.6 and 3.3.7 |
Five points the consultant must check before signing off.
A specification that cites IEC 60947-6-1 without an edition is ambiguous: the same words now point at a different document. The 2021 edition has no annex for closed transition; Edition 4.0 adds Annex D for ATSE having closed transition capability. If closed transition matters to the project, cite Edition 4.0:2026 and name Annex D, in the specification and in the submittals.
Closed transition is make before break: IEC 60947-6-1:2021, clause 3.4.9 defines it with a synchronized second source. In practice the sources overlap only once they are in synchronism, with voltage, frequency and phase within limits (engineering practice); take the synchronising window from the controller's own documentation (manufacturer data). Because the sources run in parallel for a moment, check whether the utility's connection conditions require approval, and get it in writing if they do. That depends on the utility's regulations, operating arrangements and project requirements, not on IEC 60947-6-1. Where the incomers are interlocked, as QN and QG are on a 2000 A ATS in an NLC panel, the two sources cannot close together: the load transfers by open transition and the sources never run in parallel.
After disconnection, a motor can keep generating a decaying voltage. Closing onto a second source while that residual voltage is still present and out of phase can cause high current and torque. For motor loads, delayed transition (an open transition with a set dead time) may allow the residual voltage to decay before the second source closes. The cost is a longer gap in which the load is not supplied.
Under nominal conditions, ABB's OTM automatic transfer switches have an operating transfer time of 2.0 to 4.0 s (C2D) or 1.5 to 3.0 s (C8D) for OTM160 to 250, 2.0 to 5.0 s (C2D) or 1.5 to 3.0 s (C8D) for OTM315 to 800, and 3.0 to 6.0 s (C2D) or 2.5 to 4.0 s (C8D) for OTM1000 to 1600. OFF time is 0.4 to 1.0 s, or 0.6 to 1.5 s for OTM1000 to 1600. The datasheet does not time a generator start. Where the alternate source is an engine-generator, quote the whole sequence, not the switch alone, and ask for the generator start and the dwell time in writing.
If security matters, name the framework and the level: IEC 62443-3-3:2013 for system security requirements and levels, IEC 62443-4-2:2019 for controller components, IEC 62443-3-2:2020 for the risk-based security level, and the NCA OTCC-1:2022 control level where it is in scope. OTCC-1:2022 applies to ICS in critical facilities run by government bodies, and to critical national infrastructure operators; the control level depends on the risk assessment and that classification. A requirement that cannot be tested is not a requirement.
The edition of IEC 60947-6-1 or UL 1008 the ratings follow, the transfer class (PC or CB) for each unit, and whether closed transition is required and the connection conditions permit it.
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