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Overview

Current transformers (CTs) convert high alternating currents into a scaled secondary current for meters, relays and protection systems. Available as window, wound or bar types, CTs vary by ratio, accuracy class and saturation behavior. Proper selection focuses on rated primary current, burden, insulation, knee-point for protection use, and condition when buying used for substations, switchgear, or industrial panels.

FAQ

What should I check when buying a used CT?

Verify the nameplate ratio and accuracy class, inspect for physical damage or oil leaks, request recent ratio and insulation test reports, and confirm the CT type (wound/window/bar) and its knee-point if used for protection.

How should a CT be prepared and shipped safely?

Drain and seal oil-filled CTs if required, protect bushings and cores, short and ground the secondary terminals, secure the unit to prevent shock, and use a carrier experienced with electrical equipment plus insurance.

What routine maintenance and tests are essential for CTs?

Perform visual inspections, insulation resistance tests, ratio and polarity checks, check for oil contamination on liquid-filled units, and schedule periodic accuracy and dielectric tests per manufacturer guidelines.