A ring type current transformer is a specific configuration of a window or toroidal CT characterized by a circular or ring-shaped magnetic core. Like other window-type CTs, it has no integral primary winding. Instead, the primary conductor-typically a cable or busbar-passes through the central opening of the ring-shaped core, effectively acting as a single-turn primary winding. The secondary winding is wound around the ring core and delivers a reduced current proportional to the primary current for measurement or protection purposes.
The defining feature of a ring type CT is its continuous, unbroken magnetic core in a circular or toroidal form. This geometry provides several important advantages. First, the closed magnetic path minimizes air gaps, which reduces leakage inductance and ensures efficient magnetic coupling between the primary and secondary circuits. This results in highly accurate measurement performance and stable operation across a wide range of primary currents. Second, the circular core shape distributes magnetic flux evenly, minimizing harmonic distortion and improving overall measurement fidelity.
Ring type CTs are widely used in both new-build installations and OEM equipment designs. They are commonly found in motor control centers, switchboards, power distribution panels, and industrial control systems. Their compact construction and relatively low cost make them a popular choice for general-purpose current measurement. In high-voltage applications, ring-type CTs are often installed in gas-insulated switchgear (GIS), where the ring cores are mounted concentrically around the primary conductor tube, providing reliable measurement and protection in compact, high-voltage environments.
One of the primary limitations of the ring type CT is installation. Because the core is a continuous, unbroken ring, the primary conductor must be disconnected and threaded through the central opening during installation. This requires de-energizing the circuit and can be highly disruptive in live facility retrofits or continuous processes where downtime is costly. For applications where the circuit cannot be de-energized, a split-core CT is the preferred alternative.
Another consideration is the physical size of the ring opening. The window must be large enough to accommodate the primary conductor or busbar, which can be a limiting factor in installations with large cables or multiple parallel conductors. Engineers must select a ring type CT whose window diameter and overall dimensions match the installation requirements while still providing the required accuracy class and burden capacity.
From a performance standpoint, ring type CTs are available in a wide range of accuracy classes. Metering-grade ring CTs typically achieve accuracy classes of 0.2, 0.5, or 1, while protection-grade versions are specified with classes such as 5P or 10P to ensure reliable operation during fault conditions. The continuous core construction of ring type CTs makes them particularly well suited for high-precision metering applications where stable, linear performance across the normal operating current range is essential.
In summary, the ring type current transformer represents a reliable, accurate, and widely used solution for current measurement and protection. Its continuous toroidal core delivers excellent magnetic performance, while its simple design keeps costs low and reliability high. The main trade-off is installation inconvenience, which limits its use in retrofit scenarios where circuit interruption is not feasible.