to carry current through the core. Window-type current transformers are also common, which can have circuit cables run through the middle of an opening in the core to provide a single-turn primary winding. To assist accuracy, the primary conductor should be centered in the aperture. CTs are specified by their current ratio from primary to secondary. The rated secondary current is normally standardized at 1 or 5 amperes. For example, a 4000:5 CT secondary winding will supply an output current of 5 amperes when the primary winding current is 4000 amperes. This ratio can also be used to find the impedance or voltage on one side of the transformer, given the appropriate value at the other side. For the 4000:5 CT, the secondary impedance can be found as , and the secondary voltage can be found as . In some cases, the secondary impedance is referred to the primary side, and is found as . Referring the impedance is done simply by multiplying initial secondary impedance value by the current ratio. The secondary winding of a CT can have taps to provide a range of ratios, five taps being common. Current transformer shapes and sizes vary depending on the end-user or switch gear manufacturer. Low-voltage single ratio metering current transformers are either a ring type or plastic molded case. Split-core current transformers either have a two-part core or a core with a removable section. This allows the transformer to be placed around a conductor without disconnecting it first. Split-core current transformers are typically used in low current measuring instruments, often portable, battery-operated, and hand-held (see illustration lower right). Use Current transformers are used extensively for measuring current and monitoring the operation of the power grid. Along with voltage leads, revenue-grade CTs drive the electrical utility's watt-hour meter on many larger commercial and industrial supplies. High-voltage current transformers are mounted on porcelain or polymer insulators to isolate them from ground. Some CT configurations slip around the bushing of a high-voltage transformer or circuit breaker, which automatically centers the conductor inside the CT window. Current transformers can be mounted on the low voltage or high voltage leads of a power transformer. Sometimes a section of a bus bar can be removed to replace a current transformer. Often, multiple CTs are installed as a "stack" for various uses. For example, protection devices and revenue metering may use separate CTs to provide isolation between