A method and a protection IED for supervising a secondary circuit of an instrument transformer in a power system are disclosed. The instrument transformer is connected to a power system component. The method includes: detecting, by a protection IED which is connected to the secondary circuit of the instrument transformer and protects the power system component, disturbances in a signal from the instrument transformer; and determining whether a failure occurs in the secondary circuit of the instrument transformer, according to both a disturbance detection result from the protection IED and another disturbance detection result from at least one of other protection IEDs. The other protection IEDs include protection IEDs which are connected to other secondary circuits of the instrument transformer or to secondary circuits of another instrument transformer connected to the power system component, and protection IEDs which are connected to other instrument transformers connected to other power system components connected to the same busbar as the power system component is connected to. The secondary circuit supervision solutions provide more reliable and sensitive performance and need very small amount of information to be collected.