IEC 60185: Current Transformers — When the CT Saturates, Your Protection Goes Blind

CT Saturation: The Protection Engineer Worst Nightmare

IEC 60185 specifies current transformer (CT) technical requirements. CTs are the “eyes” of protection relays — if a CT saturates under fault current, the secondary waveform distorts, and the relay cannot “see” the true primary fault current.

Three selection parameters: (1) Rated primary current — not sized for normal load, but for maximum fault current where the CT must remain unsaturated (error ≤5–10%). (2) Accuracy Limit Factor (ALF) — the key protection CT parameter. ALF=10 means the CT maintains accuracy (Class 5P composite error ≤5%) at 10× rated current. (3) Secondary burden — relay input impedance + connecting cable resistance. Actual burden must not exceed rated burden, or the CT saturates prematurely.

Quick CT sizing:
System max fault current Is = 40 kA
Rated primary Ipn = 2,000 A
Required ALF = Is/Ipn = 40/2 = 20
→ Need ALF ≥ 20: specify 5P20 or higher

Most dangerous mistake: Open-circuiting a CT secondary under load — the secondary winding generates extreme induced voltage (exceeding several kV), sufficient to destroy insulation and injure personnel. A CT secondary circuit must never, under any circumstances, be open-circuited while energized.

TN Lab — CT selection goes far beyond ratio. ALF and burden are what determine protection reliability.