Identify the transformer type: which transformer normally operates with its secondary nearly short-circuited under standard conditions?

Introduction / Context:
Instrument transformers scale primary quantities to safe measurable levels. Current transformers (CTs) provide a secondary current proportional to primary current and are designed to work with very low secondary burden (nearly short circuit). Recognizing this operating requirement is vital for safety and accuracy.

Given Data / Assumptions:

• Normal operation of a CT: secondary connected to an ammeter or burden rated in VA, with very low impedance.
• Open-circuiting a CT secondary is dangerous due to high induced voltage.

Concept / Approach:

A CT behaves like a current source; to maintain magnetizing current small, its secondary must be close to short circuit so that the exciting voltage is low. In contrast, potential transformers and power/distribution transformers are voltage sources intended to feed specified loads at rated voltage, not a short circuit.

Step-by-Step Solution:

Verification / Alternative check:

Practical rule: never open-circuit a CT secondary; keeping it shorted (or on rated burden) is mandatory for safety and accuracy—corroborating the selection.

Why Other Options Are Wrong:

• PT and power/distribution transformers must not be short-circuited in service; that would cause large fault currents.
• Isolation transformer behaves like a standard power transformer; not short-circuited.

Common Pitfalls:

• Confusing CTs with PTs; remember CT ≈ current source, PT ≈ voltage source.

Final Answer:

Current transformer (CT)