Common Mistakes in Current Transformer Installation

I. Secondary Side Open Circuit (Most Dangerous)

Incorrect Manifestations: Installing switches or fuses on the secondary circuit, or loose wiring leading to an open circuit during operation.

Consequences:
 Generates thousands of volts of high voltage, endangering personnel and equipment safety;
 Severely saturates the iron core, causing overheating and burning out the coil;
 Measuring instruments show no indication, and protection devices fail to operate.

Correct Practice: Switches are strictly prohibited from being installed on the secondary circuit. Wiring must be secure. Before maintenance, short-circuit terminals S1-S2.

II. Reversed Polarity and Direction

Incorrect Manifestations:
Primary side P1 is not facing the power supply side; P2 is reversed;
Secondary side S1 and S2 wiring is reversed.

Consequences:
Energy meter reverses direction or undercounts by 2/3;
Differential protection trips falsely due to abnormal differential current;
Power factor display is abnormal.

Correct Procedure: P1 must face the power supply side, S1 connected to the instrument's positive terminal, S2 grounded. Polarity must be verified using a DC method or testing instrument.

III. Incorrect Grounding Method

Incorrect Manifestations: S2 terminal not grounded or unreliable grounding; Multiple grounding points create circulating current, interfering with protection devices.

Consequences: High voltage enters the secondary circuit, causing equipment damage; Protection devices malfunction or fail to operate, affecting system stability.

Correct Procedure: S2 terminal should be reliably grounded at one point, connected to a dedicated grounding copper busbar, not connected to the cabinet bolts.

IV. Improper Primary Side Installation

Incorrect Manifestations: Through-core conductor eccentric, tilted, or not centered; Three-phase asymmetry when multiple cables pass through the core; Open-type CT not securely closed.

Consequences: Magnetic circuit asymmetry, increased measurement error; Inaccurate zero-sequence current detection; Degraded performance, increased heat generation.

Correct Procedure: Conductors should pass vertically and centered, with symmetrical three-phase arrangement. Open-type CTs must be securely fastened after installation.

V. Turns Ratio Mismatch with Load

Incorrect Symptoms: Turns ratio selected too high or too low; operating current outside the 60%~120% rated range; too many instruments connected to the secondary side, total impedance exceeding limits.

Consequences: Decreased metering accuracy; CT saturation, output distortion.

Correct Practice: Select an appropriate turns ratio based on the actual load, ensuring the secondary load is within its rated capacity.