What distinction does the manufacturer make?
Schneider Electric distinguishes a two-winding transformer whose primary and secondary are not electrically connected from an autotransformer with an electrical connection between them. Its explanation is a useful reason to check the winding arrangement rather than rely on the voltage ratio.
Critical Power Drafting is independent and is not affiliated with Schneider Electric. The selected equipment documents and engineer-approved design govern the actual installation. This article does not establish safe touch conditions, a grounding scheme or permission to work on an electrical system.
What should the drawing reference identify?
Trace the exact transformer tag to the approved manufacturer model and connection diagram. Keep source and load system references with the drawing revision. A label saying transformer does not resolve which construction or function was selected.
| Drawing field | Required source | Separate review question |
|---|---|---|
| Equipment identity | Exact model and approved submittal | Does the supplied variant match? |
| Voltage ratio | Approved input and output values | Which system quantities are identified? |
| Winding arrangement | Manufacturer connection diagram | Is the construction actually isolated? |
| System interface | Engineer-approved design reference | What does the full system require? |
| Drawing change | Authorised revision and disposition | Which downstream documents change? |
How can identical voltage labels conceal different constructions?
Illustrative example. Two fictional drawing proposals both show 480 V to 208 V. Proposal A is backed by an approved separate-winding connection diagram. Proposal B is backed by an approved autotransformer diagram. The shared pair of voltage labels does not make their isolation arrangements equivalent.
These invented values are not an equipment recommendation or a design for a real supply. The drafter records the exact approved construction and asks the engineer to resolve a substitution instead of changing a symbol to make the proposals look the same.
How should a substitution be documented?
Keep the original approved model, proposed alternative, manufacturer connection information and the engineer's disposition together. Update only the authorized drawing references, then check the linked equipment schedule and revision trail.
Do not infer fault performance, neutral treatment, earthing, protection or installation compliance from this limited comparison. Engineering sign-off is performed only by the registered engineer on the job.
Common questions
What else should the bid team know?
Does a changed voltage prove electrical isolation?
No. Confirm the approved winding arrangement.
Are two matching voltage ratios enough to approve a substitution?
No. The responsible engineer must assess the actual system interfaces.
Is galvanic isolation a statement that equipment is safe to touch?
No. Follow the authorized electrical safety process.