Current transformer cores: accuracy classes, burden and core choice
A current transformer core must deliver the required secondary current with low ratio and phase error at the rated burden. For metering (class 0.2S/0.5S) choose high-permeability nanocrystalline; for protection CTs choose cores with predictable saturation, often silicon steel.
Accuracy classes
Class 0.2S / 0.5S meters small currents accurately; class 1 / 3 for general measurement; class 5P / 10P for protection. Higher accuracy needs higher µi and tighter core tolerance.
Burden and saturation
Burden (VA) sets the secondary voltage; a high burden pushes the core toward saturation. Check the saturation factor and choose core area accordingly.
Core material choice
Nanocrystalline toroids: highest µi, smallest size at class 0.2S.
Silicon steel toroids or C-cores: robust and cheaper for protection and larger ratios.
Epoxy coating provides the insulation between core and secondary winding.
What to send
Rated primary/secondary current, accuracy class, burden, dimensions, insulation requirement and annual quantity.
FAQ
Why does my CT fail at low current?
Low-current accuracy depends on initial permeability and core losses at very low flux. Nanocrystalline or annealed silicon steel with tight tolerance is usually required.
