Installation and Maintenance of Current Transformers (CTs)

Safety and proper handling of CT secondaries
Key idea: A CT secondary with 5 A or 1 A outputs must never be left open when current is flowing through the primary.

Short-circuiting the secondary: Yes, the secondary must never remain open under load. If the measuring device is disconnected, always short the CT secondary first.
Risks of an open secondary: An open secondary can induce dangerous high voltages, potentially damaging both the CT and the connected instrument.
Use of shorting terminals: To ensure safe disconnection and shorting, it is recommended to use terminals with short-circuiting/test blocks.
Split-core and clamp-on CTs: The same rules apply to 5 A / 1 A outputs of split-core or clamp-on CTs as to conventional CTs: never leave the secondary open under current, short before disconnecting, and provide single-point grounding.

Grounding of CT secondaries
Key idea: A CT secondary must always be grounded at one single point to ensure safety and correct operation.

Grounding rule: Ground one side of the secondary (usually S2 or terminal l) at a single point in the secondary circuit.
Purpose of grounding: This removes capacitive coupling effects and reduces dangerous touch voltages.
Emphasis on one point: It is critical that there is only one grounding point along the entire secondary path.

Recommended procedure for installation and maintenance
Key idea: A standardized procedure minimizes risk and optimizes performance.

Location of shorting terminals: Install shorting terminals near the measuring instrument.
Disconnecting procedure: When disconnecting an instrument, first close the shorting link and only then disconnect the leads.
Grounding location: Ground one side of the secondary (S2/l) at a single point in the switchboard.
Route optimization: For shorter runs (up to 2 m using 2.5 mm² conductors), the benefit is clear: line losses (VA) are reduced, and it is easier to remain within the CT’s rated burden.

Calculations and parameters of copper conductors
Key idea: Correct resistance and loss calculations are essential for proper system dimensioning.

Specific resistance of copper:
• At 0 °C: 0.0178 Ω·mm²/m
• At 25 °C: 0.020826 Ω·mm²/m
Specific conductivity of copper:
• At 0 °C: 56.18 m/(Ω·mm²)
• At 25 °C: 48.02 m/(Ω·mm²)
Temperature coefficient of copper: 0.0068 K⁻¹

Example (20 m run, 4 mm² cross-section, 5 A current):

These guidelines are essential to ensure safe, reliable, and efficient operation of current transformers.