The International Electrotechnical Commission (IEC) has published a series of standards for instrument transformers, including IEC 61869-5. This standard specifically deals with the requirements for low-power instrument transformers, which are used to measure electrical quantities such as current and voltage in power systems.
Unfortunately, it's unlikely that you'll find a completely free PDF download of IEC 61869-5. However, you may be able to access a free preview or a draft version of the standard through various online sources.