The secondary of a current transformer (CT) should never be left open when current is flowing through the primary because doing so can create dangerous high voltages. Here’s why:
1. Induced High Voltage:
- A current transformer operates on the principle of electromagnetic induction. The current in the primary winding induces a current in the secondary winding. If the secondary is left open, there is no current flowing through it, which prevents the magnetic field from opposing the primary current.
- As a result, the magnetic flux in the core increases dramatically, which leads to a very high voltage being induced in the secondary winding. This high voltage can damage the CT insulation or create a serious safety hazard for anyone nearby.
2. Saturation and Core Damage:
- With no load (open secondary), the core of the CT may become saturated due to the high magnetic flux. Prolonged exposure to this condition can permanently damage the CT core by altering its magnetic properties, reducing its accuracy and performance over time.
3. Safety Hazards:
- The high voltage produced across the open secondary can be lethal and may damage connected equipment. In some cases, the voltage can even cause arcing, which presents a significant fire or electrical shock risk.
4. Inaccuracy in Measurements:
- If the CT secondary is open, it cannot perform its primary function, which is to accurately measure current. With the secondary circuit open, there is no reliable way to measure the current in the primary circuit, which can lead to incorrect readings and relay malfunctions in protection systems.
