What country and region ?
What I know this is different, and depends from region and country. I supose you mention UL for US.
UL 506 Special Transformers
UL 544 Medical and Dental Equipment
UL 131 Direct Plug-in Transformers
UL 1585 Class 2 and 3 Transformers
What are the UL standards that users of transformers should be interested in? The significant UL
standards for users of transformers include:
UL506 Specialty Transformers -- covers the requirements for particular construction methods,
dielectric strengths and a maximum operating temperature rise for power transformers rated 10.0
KVA or less.
UL544 Medical and Dental Equipment -- covers equipment in which transformers may be used.
Requirements for transformers recognized to UL544 include a limit on leakage current, some means
of internal thermal or current fusing, or a manufacturer-specified external current fuse, and either
double insulation or an electrostatic shield between primary and secondary.
UL1411 Transformers and Motors for Use in Audio, Radio and Television Type Appliances -- like UL544, this is an equipment standard to which a transformer can be recognized. Requirements for
transformers recognized to UL 1411 are very similar to UL506. The major difference is a
requirement for the use of UL94VO listed material (flame retardant) for terminal anchoring points
and the abnormal performance testing required per end product specifications.
UL1585 Class 2 and Class 3 Transformers -- covers current-limited devices designed to protect
against the consequences of an overload. They can be either inherently or non- inherently currentlimited.
Inherently current limited transformers are constructed so that during a long-term
overload, or short-circuit, they will not produce a fire. Non-inherent current-limited transformers
contain either a current or a thermal fuse to