How to wire a current transformer
Jul 13, 2026| I. Basic Wiring Rules
1. Primary Side Wiring: Power is connected from terminal P1 (or L1) of the current transformer and exited from terminal P2 (or L2), directly connected in series in the main circuit being measured.
2. Secondary Side Wiring: Current flows out from terminal S1 (or K1) and connects to the positive terminal of the instrument. It returns from the negative terminal of the instrument and connects to terminal S2 (or K2), forming a complete closed loop.
3. Through-type Current Transformer: The primary side phase wire enters from the end face marked P1 and exits from the end face marked P2. The secondary side S1 and S2 are connected to the instrument according to the above rules.
II. Four Commonly Used Standard Wiring Methods
1. Single-phase Wiring: Only one phase current is connected. Suitable for single-phase current measurement scenarios with three-phase symmetrical loads.
2. Two-phase Incomplete Star Wiring: Current transformers are installed only on phases A and C. The current of phase B is derived through the three-phase vector sum. This is the most commonly used connection method in three-phase three-wire systems, saving one current transformer.
3. Three-phase fully star connection: Three current transformers are connected to phases A, B, and C respectively. The secondary side S2 is uniformly short-circuited and grounded. This is suitable for neutral-point directly grounded systems and can reflect current changes in all phases.
4. Two-phase differential current connection: Only two current transformers (phases A and C) are used. The current difference between the two phases is connected to a relay. This is suitable for phase-to-phase overcurrent protection in 10kV and below distribution networks, significantly reducing equipment costs.
III. Key Wiring Precautions
1. Secondary side must not be open-circuited: Opening the secondary side during operation will generate several kilovolts of high voltage, endangering personal and equipment safety. Before disconnecting or connecting instruments, the secondary side terminals must be reliably short-circuited.
2. Grounding specifications: For 10kV and above high-voltage systems, the non-polar terminal of the secondary side must be reliably grounded at a single point. Grounding is recommended for 400V low-voltage systems. The grounding resistance must be ≤0.5Ω. Multiple grounding points are prohibited.
3. Polarity Must Not Be Reversed: Strictly follow the wiring instructions for the corresponding terminals of P1/S1 and L1/K1. Reversing the polarity will cause metering data deviation, malfunction or failure of protection devices.
4. Wire Specification Requirements: The secondary circuit must use insulated copper wire with a cross-sectional area ≥ 2.5mm². In general scenarios, 1.5-6 square millimeter cables can be used to avoid excessive line impedance leading to measurement errors.
5. Pre-wiring Inspection: Before wiring, confirm that the transformer model and ratio match the system design, that there is no external damage, and that the electrical performance is qualified using a multimeter. The power supply must be disconnected before operation, and operators must wear insulated protective equipment.
