Current transformer installed inside the cable tray

A current transformer (CT) can be installed inside a cable tray if properly secured, oriented, and grounded, with secondary leads separated from power conductors to ensure safety and measurement accuracy.

Physical Installation

When installing a CT inside a cable tray, the primary conductor must pass through the CT toroid opening, and the CT should be securely mounted to the tray to prevent vibration, which can affect measurement accuracy (Optim-Elec) . For retrofit applications, split-core CTs can be used, allowing the existing conductor to remain connected while the CT is installed, though they may have slightly lower accuracy (±1–2%) compared to solid-core CTs .

Orientation and Polarity

CTs have marked terminals: P1/P2 for primary and S1/S2 for secondary. The current direction through the primary determines the secondary polarity. Incorrect orientation can cause negative meter readings or relay malfunctions, so verify the CT orientation before energizing the circuit .

Secondary Wiring and Grounding

• Secondary leads should be routed separately from power conductors, maintaining a minimum distance of 200 mm to reduce electromagnetic interference (EMI) .
• Keep secondary runs short (<10 m) to minimize voltage burden and prevent measurement errors .
• Ground the CT secondary at one point only, typically at the meter ground terminal. Double grounding can create circulating currents and measurement errors. If the CT is remote from the meter, a separate earth conductor can connect the CT secondary to the main earth bar .
• Always insert a shorting link before disconnecting meter wires to prevent the CT secondary from being open-circuited, which can generate dangerously high voltages .

Cable Tray Considerations

Cable trays can serve as equipment grounding conductors (EGC) if they are metallic and electrically continuous, following NEC guidelines. Ensure that the tray is properly bonded and continuous to provide a safe path for fault currents . The CT should be mounted in a way that does not compromise the tray's grounding function.

Safety Precautions

• Never open-circuit the CT secondary while the primary is energized .
• Verify that the load is within the CT's VA rating to avoid saturation and overheating .
• Ensure all connections are tight and secure to prevent heat buildup and potential fire hazards .

Summary

Installing a CT inside a cable tray is feasible and safe if the CT is securely mounted, correctly oriented, and properly grounded, with secondary leads separated from power conductors. Following these practices ensures accurate measurement, electrical safety, and compliance with NEC grounding requirements.