Which component should never have its secondary circuit opened first?

The choice of a current transformer as the component that should never have its secondary circuit opened first is based on the operational principles and safety considerations inherent to current transformers.

Current transformers are designed to produce a secondary current that is proportional to the primary current flowing through them. When the secondary circuit is opened while the primary remains energized, it can lead to a very high voltage developing across the open terminals. This occurs because, with the load removed, the transformer will try to maintain the current flow, potentially resulting in dangerous conditions that can damage equipment and pose serious safety hazards to personnel.

In contrast, although power transformers, voltage transformers, and isolation transformers have their own operational considerations, they do not present the same level of risk when their secondary circuits are opened under load. Power transformers can typically be disconnected safely without immediate risk, while voltage transformers are designed to handle open circuits. Isolation transformers are similarly built to ensure safety during disconnection.

Thus, for safety and operational integrity, it is critical to always open the secondary circuit of a current transformer last, preventing any dangerous voltage buildup and protecting both equipment and personnel from potential harm.