Using Red Wire for Current Transformers: 3 “Musts” for Wiring and Safety

Want to ensure stable operation of current transformers? These 3 “red lines” must not be touched!

Required 1: The wiring must be “series”, and the secondary side must not be open
There are strict rules for wiring current transformers:

• The primary winding must be connected in series with the circuit under test (equivalent to “replacing” a section of wire in the original circuit, allowing current to flow naturally);
• The secondary winding must be connected in series with all instruments (ammeters, protection devices, etc.) to form a closed loop (the current must “flow” fully through the secondary side and cannot be disconnected).

Why can’t the secondary side be open-circuited? This is the most dangerous “minefield” of CTs! During normal operation, the secondary side is nearly short-circuited, and the excitation current is extremely small (almost negligible). Once the circuit is open, the entire primary current is used for “excitation,” severely saturating the core and inducing high voltages of thousands or even tens of thousands of volts. This can not only break down the secondary insulation (burning the instrument) but can also cause harm to nearby personnel!

Required 2: Single-point grounding on the secondary side to prevent high voltage intrusion
To prevent high voltage on the primary side from leaking to the secondary side through distributed capacitance (for example, due to aging line insulation), the secondary side of the CT must be grounded at a single point (no multiple points). This is the last line of defense for protecting personnel and equipment. Once grounded, even if high voltage is introduced, it will be conducted away by the earth, thus preventing the risk of electric shock.

Required 3: The selection must match, and the transformation ratio should not be randomly selected
When selecting a CT, the key is to match the transformer ratio. The ratio is “primary current / secondary current” (e.g., 500/5 = 100). It must be selected based on the maximum current of the circuit being measured. If the ratio is too large (e.g., using 1000/5 to measure 300A), the secondary current will be too small (1.5A) during normal operation, and the meter may not measure accurately. If the ratio is too small (e.g., using 200/5 to measure 400A), the secondary current will exceed the meter’s range (8A), directly damaging the meter!

Tips: It is best to install a short-circuit switch (such as K0) on the secondary side. In the event of an open circuit due to an operational error, quickly shorting the secondary side can instantly reduce the voltage and prevent the accident from escalating.