Identify which instrument type is used both for system protection and for energy/metering applications in electrical power systems.

Introduction / Context:
Power systems require devices that simultaneously support accurate metering and reliable protection. Instrument transformers scale high currents and voltages to safe, measurable levels and provide galvanic isolation. They are indispensable at the interface of protection relays and revenue-grade meters.

Given Data / Assumptions:

• Current transformers (CTs) and potential/voltage transformers (PTs or VTs) are passive, ratio-defined devices.
• Protective relays and meters connect to their secondary windings.
• Accuracy and burden ratings are specified to suit both protection and metering duties.

Concept / Approach:
Protection requires reliable reproduction of system quantities during faults (high primary values, transient behavior), while metering needs accuracy near nominal conditions. Special classes of instrument transformers (e.g., metering-class CTs, protection-class CTs) address each need; in practice, both protection relays and meters are fed by instrument transformers, sometimes dedicated, sometimes shared via separate cores.

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