The use of current transformers involves working with high voltages and currents, and safety must be the paramount concern. A comprehensive set of precautions should be followed during installation, operation, and maintenance.
Before any work begins, de-energize the primary conductor whenever possible. Confirm that the circuit being measured is not energized and that the system is completely powered down. Use lockout/tagout procedures and arc-flash PPE where applicable. Installation and maintenance of CTs should only be performed by qualified, competent professionals who have received training and have experience with high voltage and current devices.
The most critical ongoing safety rule is that the CT secondary must never be left open while primary current is flowing. An open secondary can develop hazardous voltage and may damage equipment or create a shock hazard. Never open or close the secondary circuit if the primary circuit is live. If this cannot be ensured, follow the correct sequence: first connect the current transformers and the energy meters to each other, then position the current transformers based on the position of the phases.
All external current transformer secondary windings must be grounded to the ground bus. If the secondary windings are not grounded, capacitive coupling can allow the secondary voltage to float up to the voltage of the mains, posing a serious safety hazard. Ground connections present on the secondary windings of the current transformers must be connected to ground.
Be aware that CTs can produce high voltages on the secondary terminals if power is applied to the primary while they are in an open circuit condition. This may pose a hazard and can even damage the CT itself. Even current transformers rated as safe for open operation can pose a hazard when touched during operation while open.
Verify that screw terminals for the current transformer connection are adequately tightened. High measurement currents can cause temperatures of up to 80°C on the connections of the current transformers. Use wiring designed for an operating temperature of at least 80°C. The current transformers can be hot even after the power supply has been switched off; allow the connections and connecting cables to cool down before touching them.
Ensure that the CT has basic insulation according to IEC 61010-1:2010 for the nominal voltage of the circuit. Insufficient insulation at the current measurement inputs can lead to life-threatening voltages or damage to your device, module or system. Do not run signal wires in the same conduit or bundle that carries power mains or high-level voltage or currents. Inspect the CT before installation and do not use a unit with any signs of damage or overheating