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 predicable 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).