A dry type transformer is air-cooled, which means dust and moisture can reach parts of it that an oil tank would simply seal away. The IP rating on the enclosure is what controls that exposure, and it is one of the most often under-specified lines on an enquiry.
What the two digits mean
The first digit is protection against solid objects, the second against water. IP20 keeps fingers and larger objects out but lets dust in. IP54 keeps most dust out and protects against splashing water. A higher second digit means more water protection, not more dust protection.
What a dry type transformer actually needs
In a clean, dry, air-conditioned electrical room, an IP20 or IP21 enclosure is often enough — the room is doing the protecting. Add filters and the rating rises without sealing the transformer in.
In a dusty plant, a coastal site or an area with wash-down, a higher rating earns its cost. Dust on a winding insulates it and blocks cooling; moisture on an insulation surface invites tracking. Sealing against both is cheaper than replacing a failed unit.
The trap with high IP ratings
A sealed enclosure also traps heat. Pushing an IP rating up without re-checking cooling can overheat a transformer that was fine when it was open. Every step up in sealing should be matched against the temperature rise and, if needed, forced ventilation or a larger frame.
Indoor and outdoor cases
An indoor dry type unit in a controlled room can stay at a low rating. An outdoor dry type unit needs a weatherproof enclosure, and that usually means IP54 or better with attention to condensation and drainage.
How to specify it
State the installation environment, not just a number. Tell us whether the room is clean or dusty, humid or dry, washed down or not, indoor or outdoor. From that we will recommend an enclosure rating and confirm the cooling still works.
Hebei Youge builds dry type and oil immersed transformers and matches the enclosure to the site. Send the environment description with the electrical data.