Cable-type CT

A cable type CT is a current transformer designed to measure current in a cable by passing the cable through a toroidal core, commonly used for protection, metering, and zero-sequence current detection.

Overview

A cable type current transformer is a specialized CT where the primary conductor is the cable itself, which passes through a toroidal or ring-shaped magnetic core. The secondary winding is wound around the core and provides a scaled-down current proportional to the primary current for measurement or protection devices . Unlike conventional CTs with a dedicated primary winding, the cable acts as a single-turn primary, simplifying installation in existing cable systems .

Applications

• Protection systems: Cable type CTs are used to detect overcurrent, earth faults, and unbalanced currents in medium-voltage and low-voltage networks .
• Zero-sequence current measurement: They are often combined with zero-sequence CTs to detect ground faults and residual currents .
• Metering: They can be used for revenue metering or monitoring branch circuits where the cable is the primary conductor .

Installation Considerations

• Cable shield handling: The cable shield should be returned through the CT before grounding to avoid false tripping and ensure accurate measurement .
• Grounding: Only one point of grounding is recommended for the CT secondary to prevent circulating currents and measurement errors .
• Core and winding design: The toroidal core is often split into two halves for easy installation around existing cables, with the secondary winding distributed over both halves .
• Elimination of metallic interference: Any metal armor or protective conductor in the cable must be routed properly to avoid affecting the CT measurement .

Advantages

• Non-intrusive: No need to disconnect the cable; the CT can be installed around it.
• Compact: Suitable for retrofitting in switchgear or cable trays.
• Versatile: Can measure phase currents, residual currents, and zero-sequence currents simultaneously when combined with other CTs .

Summary

Cable type CTs are essential for accurate current measurement and protection in cable-based electrical systems. They provide a practical solution for monitoring currents without interrupting the primary circuit, while careful attention to grounding, shielding, and installation ensures reliable operation .