Current Transformer Secondary Winding Always Closed

The question asks which transformer type requires its secondary winding to be always kept closed.

Understanding Current Transformers (CTs)

• A Current Transformer (CT) is used to measure high AC currents.
• It works by stepping down the high primary current to a lower, measurable secondary current.
• Crucially, the CT is connected in series with the circuit through which the current is being measured.

Reasoning for Closed Secondary

The secondary winding of a CT must always be connected to a closed circuit (a burden, like an ammeter or relay coil, or simply short-circuited) for the following reasons:

• Safety: If the secondary circuit is opened while the primary circuit has current flowing, the primary winding's magnetic field collapses rapidly. This induces a very high, potentially lethal voltage across the open secondary terminals.
• Operation: A CT is designed to operate with its secondary terminals having a very low impedance. Keeping the secondary closed maintains this low impedance, ensuring the secondary current is proportional to the primary current according to the transformer's turns ratio ( \(I_s \approx (N_p / N_s) \times I_p\) ). An open circuit presents a high impedance, disrupting this relationship and potentially damaging the transformer core due to over-excitation.

Comparison with Other Transformers

• Potential Transformers (PTs): Used for measuring voltage, connected in parallel. Their secondaries are typically connected to high-impedance voltmeters and can be left open without significant danger.
• Power/Distribution Transformers: Used for stepping voltage levels for power transfer. Their secondaries supply power to loads and are not required to be continuously closed in the same manner as CTs for operational safety.

Conclusion

Based on its operational principle and safety requirements, the Current transformer is the type where the secondary winding must always be kept closed.