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Split core CTs and why size matters


Current Transducers
Current Transducers (CTs) ar a essential element of power observance. They come during a style of formats, sizes, current ratings and accuracy levels. The side we'd wish to concentrate on here is size, specifically inside diameter.
The inside diameter (ID) is the width of the opening in the CT that the conductor goes through. It must be large enough to fit around the conductor, however choosing too large of a CT can create other issues. As the graphic above indicates, the overall dimensions of the CT grow substantially as the ID gets bigger.

Current Transducer Types

Current sensors can be either open-loop or closed-loop.
Open-Loop Sensor: This type measures AC and DC currents and provides electrical isolation between the circuit being measured and the output of the sensor. Open-loop current sensors are generally preferred in battery-powered circuits.
Closed-Loop Sensor: The Closed-Loop sensor is sometimes called a ’Zero-Flux’ sensor because it features a Hall-Effect sensor that feeds back an opposing current into a secondary coil, wound on the magnetic core to zero the flux produced in the magnetic core by the primary current. Closed-loop sensors are often used when high accuracy is essential.
Current rating
Be sure to use a CT with a current rating as close to the actual current being monitored as possible to maximize accuracy at the lowest possible load. ACT will be accurate across some portion of its range.
Inside diameter
CTs of the same amperage come in different sizes. For example, RS Components offers 200 amp CTs with 24mm, 36mm and 50mm IDs. The chart below shows that a 200A CT with a 50mm ID is much larger than one with a 24mm ID. Since space inside electrical equipment is often tight, using a CT that is larger than needed can create problems as well.
CT sizing summary
Most customers will try to match the amperage of the Current Transducers to the rating of the over-current protection on the circuit. You need to know both the circumference of the conductor(s) that the CT has to fit around (to determine the minimum acceptable ID) and the amount of available space in the panel where the CTs will be installed. If the conductor is very large or the space is particularly tight, a Rogowski coil ("rope CT") might be the best choice.

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