CT 300/5-40

CT 300/5-40 is a Current Transformer (CT) used for measuring and monitoring high AC currents in electrical power systems.

Meaning of the rating

300/5 – The CT converts 300 A flowing in the primary conductor to 5 A at the secondary output (a 60:1 ratio). This allows meters and protection relays to safely measure large currents.

40 – This commonly refers to the window size (40 mm) on many ring-type CTs, indicating the approximate maximum diameter of the cable or busbar that can pass through the CT. The exact meaning can vary slightly by manufacturer.


Common uses

Measuring current with digital energy meters.

Monitoring electrical load in distribution panels.

Providing input to protection relays for overload and fault detection.

Used in generator panels, MCC panels, switchboards, and industrial power distribution systems.


Important: Never leave the secondary terminals of a CT open while the primary conductor is carrying current, as dangerous high voltages can develop across the secondary winding.