FM Approvals 3990 PDF

This standard sets performance requirements for liquid-insulated distribution and power transformers, both naturally cooled and transformers utilizing forcibly circulated cooling medium. The standard limits certification for the naturally cooled (KNAN) transformers rated from 5 to 10,000 kVA and 35 kV class or lower. Certification of larger transformers will require the use of forcibly circulated cooling mediums (forced air or forced liquid (water or oil) cooling).

This standard applies to any component intended for use as a protective device and that is normally provided as part of the transformer assembly. Use of components previously certified as individual protective devices shall not automatically result in a certification transformer assembly.