The transformer is one of the important electrical appliances in the electric system. It's similar to a power transformer and is used to transform voltage or current in the circuit. However, transforming voltage and current by a power transformer aims at transferring electric energy, while transforming voltage and current by a transformer aims at measuring and monitoring voltage or current in the circuit. Generally, a power transformer owns large capacity as well as large change difference of voltage or current.

While a transformer owns small capacity as well as a smaller change difference of voltage or current. In a measuring circuit, the transformer that transforms current and supplies electricity for a current meter and relay is called a current transformer. The transformer that transforms voltage and supplies electricity for a voltage meter and relay is called a voltage transformer.