IEC 61869-1 is a standard published by the International Electrotechnical Commission (IEC) that specifies the general requirements for instrument transformers, including their design, testing, and use. The standard is part of the IEC 61869 series, which replaces the older IEC 60044 series.
Unfortunately, I couldn't find a free and legitimate source for downloading the IEC 61869-1 PDF. The IEC website and other authorized distributors typically require a purchase or a subscription to access the standard. iec 61869-1 pdf free download