Instrument transformers are used to step-down current or voltage to measurable values. They provide standardized, useable levels of current or voltage in a variety of power monitoring and measurement applications.

There are two basic types of instrument transformers:

Current transformers: are used to step-down current in a predictable fashion with regard to both current and phase. They are often used as inputs to current-powered instruments.

Voltage transformers: produce a secondary voltage that is substantially proportional to the primary voltage, but that differs in phase by an angle that is approximately zero for an appropriate direction of the connections. Often, voltage transformers are used as potential transformers or in combination with current transformers.

Some instrument transformers have a waterproof case or are rated for outdoor use. Others meet U.S. military specifications (MIL-SPEC) or standards for electric metering from the American National Standards Institute (ANSI) or the International Electrotechnical Commission (IEC).

Instrument transformers are used mainly in measurement and protective equipment. They are also used to provide energy to protective relays.

Accuracy, the degree to which instrument transformers produce a current or voltage that is proportional to monitored value, varies among devices. Highly accurate current transformers are used to measure turn-on, turn-off and conducting performance in power semiconductor switches.

Current transformers are also used in the design of switch-mode power supplies, the development of power electronics, and lightning simulation tests for surge arrestors.

In addition, current transformers are used in capacitor discharge surge testing, the development of motor controllers and electronic variable speed drives, the monitoring of laser tube and diode pulse currents, current injection, the measurement of harmonics in main power cables, and in electromagnetic compatibility (EMC) testing.