When you select the wrong transformer enclosure rating, moisture will get into the transformer enclosure and cause faults. NEMA 3R transformers are effective at keeping out rain, but sometimes no snow drifts.
This was a NEMA 3R transformer (some of the rain shields are removed in the photo to gain access) and supposed to be rated for outdoor locations and rain, but snow drifts were able to push up and under the shields and into the enclosure where it cause a significant fault. The system outage lasted for months while the insurance companies fought it out.