This guide describes the effects of geomagnetic disturbances (GMD) on power transformers when there is the presence of geomagnetically induced current (GIC) in a power transformer. It establishes specification parameters and performance characteristics for power transformers to help minimize the risk and impact when GIC is present in the power system. The intent is to provide a background that can help evaluate the effect of GIC on a power transformer design and its GIC capability. This includes the evaluation techniques to determine the performance characteristics while under the influence of GIC. It does not include the effect of GIC on other power system devices beyond power transformers and accessories. It does not discuss mitigation techniques and mitigation devices such as neutral-blocking devices on equipment beyond power transformers and accessories. New IEEE Standard - Superseded. The effects of geomagnetic disturbances (GMD) on power transformers when there is the presence of geomagnetically induced current (GIC) in a power transformer are described. Specification parameters and performance characteristics for power transformers to help minimize the risk and impact when GIC is present in the power system are established. The intent is to provide a background that can help evaluate the effect of GIC on a power transformer design and its GIC capability. This includes the evaluation techniques to determine the performance characteristics while under the influence of GIC.