Short Answer:

A Current Transformer (CT) should never be operated with an open-circuited secondary because it can generate very high voltage, which is dangerous to people and equipment. When the secondary is open, the magnetic flux in the core increases sharply, leading to excessive voltage across the secondary terminals.

This can result in electric shock, insulation damage, core saturation, or even explosion. To avoid these risks, CT secondary windings should always be short-circuited or connected to a burden (load) during operation.

Detailed Explanation:

Why CT should not be open-circuited

A Current Transformer (CT) is a special type of transformer used to step down high current values to safe, measurable levels for instruments and protective devices. It operates on the principle of electromagnetic induction, and the secondary winding plays a crucial role in maintaining proper flux levels in the magnetic core.

When the secondary is left open, while the primary carries current, it causes dangerous operating conditions that can harm the transformer, damage connected equipment, and even pose serious safety hazards.

What Happens When a CT is Open-Circuited:

1. Core Saturation:
   • With the secondary open, the primary current continues to flow.
   • No opposing current flows in the secondary to balance the core’s magnetic field.
   • This causes core saturation, where the transformer loses its ability to operate linearly.
2. High Voltage Induction:
   • The saturated core produces a rapidly increasing magnetic flux.
   • This results in a very high voltage across the open secondary terminals, often several kilovolts.
   • This voltage is not limited and depends on the core material, flux, and primary current.
3. Dangerous to Personnel:
   • Touching the open secondary terminals may result in severe electric shock.
   • Maintenance personnel are at risk if unaware of the CT’s status.
4. Insulation Breakdown:
   • The excessive voltage can break down insulation within the CT or connected devices.
   • May lead to fire, explosions, or system failures.
5. Heating and Damage:
   • High core loss due to saturation generates excessive heat, which may permanently damage the CT core.
   • Also reduces accuracy and lifespan of the CT.

Preventive Measures:

1. Always Keep Secondary Loaded:
   • The secondary of a CT should always be connected to a burden (load like a meter or relay).
2. Short-Circuit if Unused:
   • If the CT is not in use, its secondary terminals should be shorted with a wire link to avoid voltage buildup.
3. Use of Shorting Terminal Blocks:
   • CT circuits often have shorting links or switches in panels for safe testing and disconnection.
4. Safety Signage and Training:
   • Proper labels and safety training should be provided to ensure no one mistakenly opens the secondary circuit.

Applications Where This is Critical:

• Switchgear panels
• Power plants and substations
• Generator and transformer protection systems
• Energy metering systems

Conclusion:

A Current Transformer must never be operated with an open secondary because it causes core saturation, generates dangerously high voltage, and can lead to equipment damage and personal injury. To ensure safety and proper function, CT secondaries should always be kept shorted or connected to a proper load. This is a fundamental rule in electrical systems involving CTs and should always be followed strictly.