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 virtually every building with three-phase service and single-phase services greater than 100 amps.
The CT is typically described by its current ratio from primary to secondary. 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 metering and protection circuits, and allows current transformers with different characteristics (accuracy, overload performance) to be used for the devices.
Current transformers, together with voltage transformers (VT) (potential transformers (PT)), are known as instrument transformers. When current in a circuit is too high to apply directly to measuring instruments, a current transformer produces a reduced current accurately proportional to the current in the circuit, which can be conveniently connected to measuring and recording instruments. A current transformer also isolates the measuring instruments from what may be very high voltage in the monitored circuit.
Current transformers are often constructed by passing a single primary turn (either an insulated cable or an uninsulated bus bar) through a well-insulated toroidal core wrapped with many turns of wire.
A common method for temporary or difficult retro fit energy monitoring projects uses current transformers with a split cores and therefore makes it easy to wrap around conductors within a circuit. Permanent installations generally use more economical types of solid state current transformer.
Care must be taken that the secondary of a current transformer is not disconnected from its load while current is in the primary, as the transformer secondary will attempt to continue driving current across the effectively infinite impedance up to its core saturation voltage. This may produce a high voltage across the open secondary into the range of several kilovolts, causing arcing, compromising operator and equipment safety, or permanently affecting the accuracy of the transformer.
To avoid personal injury or equipment damage, the secondary must always be short-circuited or connected to a burden.
A solution for this problem is to install a Test Terminal Block (TTB) that allows for safe testing, disconnection of the equipment leaving CT’s securly short-circuited.
Using our test/disconnect terminals also reduces possibility of damage to Current Transformers.