Correct wiring of a current transformer is essential for accurate measurement and safe operation. The primary winding connections are designated as P1 and P2, while the secondary winding connections are designated as S1 and S2-. The fundamental rule of CT wiring is that the primary side is connected in series with the line being measured, and the secondary side is connected in series with the measuring instrument or protection relay. The polarity must be observed: P1 corresponds to S1, and P2 corresponds to S2-. When the primary current enters the P1 terminal, the secondary current leaves the S1 terminal-this is the standard convention for subtractive polarity-.
In practice, the S1 terminal of the CT should be connected to the terminal marked k on the circuit diagram, and S2 should be connected to the terminal marked l-. The secondary connections should be made using the shortest usable length of a suitably sized cable, and the cable diameter should be selected in accordance with the required length so as not to overburden the CT. Observe the wiring polarities for the specific installation: power coming from the source must go through the P1 side first in order to maintain correct polarity.
A critical safety consideration is that the CT secondary circuit must never be left open while current is flowing in the primary. If the secondary circuit of the CT is open and the primary circuit is live, a high voltage will be induced in the CT which is connected to the open connection cables, creating a risk of serious or fatal injuries. If a CT must be installed without the secondary wiring connections made, a shorting link should be installed between the secondary connections, and this link should only be removed when final secondary connections are made.
For installations where the primary circuit cannot be de-energized, follow this 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. After wiring is complete, verify the connection by checking the power sign displayed on the energy meter. If electricity is consumed, the power must be displayed without a sign; if electricity is supplied, the power must be displayed with a negative sign. If this is not the case, the current transformers are either mounted the wrong way round or connected incorrectly. Always ensure that all external current transformer secondary windings are grounded to the ground bus to prevent capacitive coupling from allowing the secondary voltage to float up to the voltage of the mains, which can pose a serious safety hazard.