IEEE C57.109:2018

Revision Standard - Active.
Recommendations believed essential for the application of overcurrent protective devices applied to limit the exposure time of transformers to short circuit current are set forth. Transformer coordination curves are presented for four categories of transformers. There is no intent to imply overload capability.

This guide applies to transformers referenced in IEEE Std C57.12.00 as Categories I, II, III, and IV.1 It sets forth recommendations essential for the application of overcurrent protective devices applied to limit the exposure time of transformers to short-circuit currents (see IEEE Std C37.91™ [B2]).2 This guide is not intended to imply overload capability.
Protective devices such as relays and fuses have well-defined operating characteristics that relate fault magnitude to operating time. These characteristic curves should be coordinated with a comparable curve(s) applicable to transformers that relate duration and fault magnitude to withstand capability.