Correctly identifying the primary and secondary terminals of a CT is fundamental to proper wiring and safe operation. CT terminals are marked according to established standards, and understanding these markings is essential for any installation or maintenance work.
The primary winding connections are typically designated as P1 and P2, or alternatively as K and L. The secondary winding connections are designated as S1 and S2, or alternatively as k and l-. On many CTs, the primary side is also marked with the letter "H" (H1 and H2), while the secondary side is marked with the letter "X" (X1 and X2). A white dot or polarity mark is often placed on the casing to indicate the polarity terminal-.
The physical appearance of the terminals varies by CT type. On a window-type or through-core CT, the primary is not a wound winding but rather the conductor that passes through the central aperture. The aperture itself may be marked with "K-P1" or "L-P2" to indicate which side faces the source and which faces the load-. The secondary terminals are screw terminals or lead wires extending from the CT body, marked with S1 and S2 or X1 and X2.
The polarity convention is that when primary current enters the P1 (or H1) terminal, secondary current leaves the S1 (or X1) terminal. This is known as subtractive polarity and is the standard configuration for metering and most protection applications. If the primary current enters P1 and secondary current leaves S2 instead of S1, the CT is said to have additive polarity, which is generally considered a wiring error that must be corrected-.
In practice, the S1 terminal should be connected to the terminal marked k on the circuit diagram, and S2 to the terminal marked l. The secondary connections S1 and S2 should be connected to k and l respectively as before. For multi-ratio CTs, there may be additional secondary terminals such as S3 and S4. The appropriate ratio is selected by connecting either S2, S3, or S4 to the terminal marked I, and the CT ratio should be selected as close as possible to the required specification. Unused secondary taps on multi-ratio CTs must be left open circuit during normal operation.
When identifying terminals, always refer to the CT's nameplate or datasheet for the specific model. Different manufacturers may use slightly different marking conventions, and relying on assumptions can lead to wiring errors. If terminal markings are unclear or illegible, use a continuity tester or consult the manufacturer's documentation before proceeding with installation.