As an electrical technician for Rozel I have had hundreds of conversations with Plant managers, safety directors, and maintenance personnel about the labeling process once the evaluation is complete. One question that seems to always be asked is "why are transformers not labeled"? Especially when we know that the secondary side of most transformers (30kVA and above) usually have a higher arc flash rating. Just to be clear, we are discussing dry type indoor distribution type transformers, not service transformers with doors. This is due to the internal impedance of a transformers causing a long trip time on the primary side protection.
Here is why we choose not to label these transformers:
NFPA 70E 2018 130.5(H) States: “Electrical equipment such as switchboards, panelboards, industrial control panels, meter socket enclosures, and motor control centers that are in other than dwelling units and that are likely to require examination, adjustment, servicing, or maintenance while energized shall be marked with a label...”
From a practical consideration, these transformers should not generally be accessed or worked on while energized due to the risky methods of exposing the actual transformer components. I.e. we should restrict tasks of performing examination, adjustment, servicing or maintenance while energized on transformers without doors on them. Also, an issue with labeling transformers is that it can become confusing for a worker because we have to apply two different labels. Historically, our engineering software has not allowed us to create one label that indicates both the highest voltage and highest arc flash so we have to make one for higher voltage (lower arc flash) and another for lower voltage (higher arc flash).
We do label primary side disconnects and secondary sides disconnects and electrical panels connected directly to transformers.
Larger transformers such as service type with doors on them, that are owned by the customer do receive labels. we place a high voltage label on the primary side door (typically lower arc flash) and a low voltage (typically high arc flash) label on the secondary side door.