By the end of this chapter, you will be able to apply the principles of setting up the basic equivalent circuits of a voltage transformer (VT) and a current transformer (CT) in real-world protection relay measurements. This knowledge will enable you to clearly distinguish between the two transformers’ primary differences in design criteria and set up an equivalent circuit of a capacitor voltage transformer (CVT) as used in protection relays. You will also be equipped to define the steady-state and transient errors (including the effect of saturation) and the accuracy requirements of the measuring transformer in practical scenarios.