Why is the secondary side of a current transformer not allowed to be open‑circuited?

A current transformer (CT) converts a large primary current into a proportional small secondary current for measurement and relay protection purposes. When the secondary side is open‑circuited, I₂=0, and the demagnetizing effect of the secondary current disappears. The magnetic flux generated by the primary current cannot be counterbalanced, causing a sharp rise in core magnetic flux and severe saturation of the iron core. An extremely high voltage will be induced across the secondary winding, which may break down insulation and endanger personal safety as well as equipment. It will also result in inaccurate metering and abnormal protection operations.

Proper Operation and Precautions

The secondary side of an operating CT must remain closed by connecting to meters, relays or short‑circuit terminals. Before meter removal, maintenance or equipment replacement, reliably short‑circuit the CT secondary side in advance. It is strictly prohibited to disconnect or break the secondary circuit while the CT is in service. Wiring shall be firm and reliable, with proper earthing and inspections carried out in accordance with relevant regulations.