Install current transformers in the distribution box

Current transformers (CTs) should be installed inside the distribution box near the power inlet, with correct orientation, secure mounting, and proper secondary wiring to ensure accurate measurement and safe operation.

Selecting the Location

Install the CT close to the power inlet side of the distribution box, where it is convenient for installation and maintenance. Ensure there is adequate safety distance from other electrical components to prevent interference and allow safe access for servicing ( ). The CT should be mounted in a well-ventilated area to avoid overheating.

Orientation and Primary Connections

• Connect the L1 terminal of the primary side to the incoming power conductor and L2 to the load side.
• For CTs with a wound primary, connect the primary directly in series with the conductor carrying the current to be measured.
• If multiple turns of the primary conductor are required, follow the label instructions on the CT to achieve the correct transformation ratio ( ).

Secondary Connections

• Connect the K1 terminal to one end of the current coil of the measuring instrument or protection device, and K2 to the other end and grounding terminal to maintain correct current direction ( ).
• Use the shortest possible cable length of appropriate size to avoid overloading the CT.
• Never leave the secondary circuit open while the primary is energized, as this can generate dangerously high voltages and damage the CT. If secondary connections are not yet made, install a shorting link between secondary terminals until final wiring is completed ( ).

Grounding

• For low-voltage systems (≤1 kV), the secondary circuit should generally not be grounded, unless required for metering.
• For medium- and high-voltage systems (>1 kV), the secondary circuit must be grounded at a single point, typically at S1 or the designated grounding terminal, to comply with EN IEC 61936-1 standards ( ).
• Avoid multiple grounding points to prevent circulating currents and measurement errors.

Mounting and Safety

• Secure the CT using a suitable bracket or fixture to prevent loosening due to vibration or movement ( ).
• Only competent personnel should install CTs, as they can produce high voltages if mishandled.
• Apply common-sense precautions, including wearing appropriate personal protective equipment and ensuring the system is de-energized when possible ( ).

Additional Considerations

• If using multiple CTs, ensure the total transformation ratio is correctly programmed in the measurement device ( ).
• For Earth leakage protection, pass all live conductors and the neutral through the CT aperture, but do not pass the Earth connection through the CT ( ).
• Verify the CT ratio and orientation before energizing the system to ensure accurate readings and protection functionality. By following these steps, CTs can be safely and accurately installed in a distribution box, ensuring reliable measurement and protection of the electrical system.