What distinction does the manufacturer make?
Eaton explains open transfer as break-before-make and closed transfer as make-before-break. Its guidance also distinguishes transition type from manual, non-automatic and automatic operating modes. An automatic ATS label does not specify which transition sequence has been approved.
Critical Power Drafting is independent and is not affiliated with Eaton. Use the actual equipment variant and approved engineering sequence. This review does not select synchronizing settings or authorize paralleling.
Sources: Eaton: Automatic transfer switch fundamentals: transition types
Which drawing fields need an explicit sequence?
Use source names from the approved one-line rather than assume source 1 always means utility. Identify which connection opens or closes first and where the conditions are documented. A single ATS symbol is too compressed to establish a complete operating sequence.
| Field | Open transition review | Closed transition review |
|---|---|---|
| Sequence | First connection breaks before second makes | Second makes before first breaks |
| Overlap representation | Do not show intended simultaneous source connection | Record approved temporary overlap basis |
| Conditions | Approved availability and timing references | Approved permissive and synchronization references |
| Variant | Actual supported equipment configuration | Actual supported equipment configuration |
| Loss of source | Trace the approved abnormal-event sequence | Do not extrapolate normal transfer to every failure |
| Authority | Responsible engineer and revision | Responsible engineer and revision |
What conflict can a familiar ATS symbol hide?
Illustrative example. A fictional equipment schedule states open transition, while a control narrative describes connecting the alternate source before disconnecting the preferred source. The same ATS tag appears on both documents. Matching tags have not resolved the sequence conflict.
Log the two exact source references, revisions and conflicting statements. Ask the responsible engineer to identify the approved requirement and obtain the correct vendor configuration. Do not redraw the symbol as closed transition merely to match the narrative. The drawing correction follows an approved decision.
How should the revised issue be checked?
Reconcile the equipment schedule, one-line, control sequence and vendor submission against the same decision. Retain superseded references so the reviewer can see why the representation changed. Keep transition type separate from bypass arrangement, pole count and service classification; none substitutes for this sequence check.
This documentation worksheet provides no switching procedure, safe isolation instruction or universal overlap time. Technical approvals and engineering sign-off are performed only by the registered engineer on the job.
Common questions
What else should the bid team know?
Does automatic mean closed transition?
No. Operating mode and transition type are different attributes.
Can source labels establish switching order?
No. Use the approved sequence and equipment configuration.
Should an unapproved timing value be added?
No. Refer missing timing and conditions to the responsible engineer.