Cooling classes of dry-type transformers are covered by ANSI/IEEE standard C57.12.01 Section 5.1 . A short explanation of each class is given below.
1. Class AA transformers are ventilated and self-cooled. This means there are ventilation ports located in outside walls of the transformer enclosure. There are no fans to force air into and out of the enclosure, with typically no external fins or radiators. Cooler air enters the lower ports, is heated as it rises past windings, and exits the upper ventilation ports. (Although it is not repeated below; it is obvious that, in every cooling class, some heat is also removed by natural circulation of air around the outside of the enclosure.)
2. Class AFA transformers are self-cooled (A) and additionally cooled by forced circulation of air (FA). This means that there are ventilation ports for fan inlets and outlets only. (Inlets are
usually filtered.) Normally, there are no additional ventilation ports for natural air circulation.
3. Class AA/FA transformers are ventilated and self-cooled (same as Class AA in item 1). In addition, they have a fan or fans providing additional forced-air cooling. Fans may be wired to
start automatically when the temperature reaches a pre-set value. These transformers generally have a dual load rating—one for AA (self-cooling natural air flow) and a larger load rating for FA (forced air flow).
4. Class ANV transformers are self-cooled (A), non-ventilated (NV) units. The enclosure has no ventilation ports or fans and is not sealed to exclude migration of outside air, but there are no provisions to intentionally allow outside air to enter and exit. Cooling is by natural circulation of air around the enclosure. This transformer may have some type of fins attached outside the enclosure to increase surface area for additional cooling.
5. Class GA transformers are sealed with a gas inside (G) and are self-cooled (A). The enclosure is hermetically sealed to prevent leakage. These transformers typically have a gas, such as nitrogen or freon, to provide high dielectric and good heat removal. Cooling occurs by natural circulation of air around the outside of the enclosure. There are no fans to circulate cooling air; however, there may be fins attached to the outside to aid in cooling.
3.3.1 Potential Problems and Remedial Actions for Dry-Type Transformer Cooling Systems 
It is important to keep transformer enclosures reasonably clean. It is also important to keep the area around them clear. Any items near or against the transformer impede heat transfer to cooling air around the enclosure. As dirt accumulates on cooling surfaces, it becomes more and more difficult for air around the transformer to remove heat. As a result, over time, the transformer temperature slowly rises unnoticed, reducing service life.
Transformer rooms and vaults should be ventilated. Portable fans (never water) may be used for additional cooling if necessary. A fan rated at about 100 cubic feet per minute (cfm) per kilowatt (kW) of transformer loss , located near the top of the room to remove hot air, will suffice. These rooms/vaults should not be used as storage. When the transformer is new, check the fans and all controls for proper operation. After it has been energized and the loading and temperature are stable, check the temperature with an infrared (IR) camera and compare loading with the nameplate. Repeat the temperature checks after 1 week of operation. Once each year under normal load, check transformer temperatures with an IR camera [4, 8]. If the temperature rise (above ambient) is near or above nameplate rating, check for overloading. Check the temperature alarm for proper operation. Check enclosures and vaults/rooms for dirt accumulation on transformer surfaces and debris near or against enclosures. Remove all items near enough to affect air circulation. To avoid dust clouds, a vacuum should first be used to remove excess dirt. Low-pressure (20 to 25 psi), dry compressed air may be used for cleaning after most dirt has been removed by vacuum.
The transformer must be de-energized before this procedure, unless it is totally enclosed and there are no exposed energized conductors. Portable generators may be used for lighting. After de-energizing the transformer, remove access panels and inspect windings for dirt- and heat-discolored insulation and structure problems . It is important that dirt not be allowed to accumulate on windings because it impedes heat removal and reduces winding life. A vacuum should be used for the initial winding cleaning, followed by compressed air . Care must be taken to ensure the compressed air is dry to avoid blowing moisture into windings. Air pressure should not be greater than 20 to 25 psi to avoid imbedding small particles into insulation. After cleaning, look for discolored copper and insulation, which indicates overheating. If discoloration is found, check for loose connections. If there are no loose connections, check the cooling paths very carefully and check for overloading after the transformer has been re-energized. Look for carbon tracking and cracked, chipped, or loose insulators. Look for and repair loose clamps, coil spacers, deteriorated barriers, and corroded or loose connections.
Check fans for proper operation, including controls, temperature switches, and alarms. Clean fan blades and filters if needed. A dirty fan blade or filter reduces cooling air flow over the windings and reduces service life. If ventilation ports do not have filters, they may be fabricated from home furnace filter material. Adding filters is only necessary if the windings are dirty upon yearly inspections.