Try it

Explore how a relay sees the power system

Move the slider and watch the measurement scale – then notice what changes beyond the accurate operating range.

0 2,400 A · 4× rated
Secondary current = Primary current ÷ 600
Ideal proportional input
0.50 A
Relay actually receives
0.50 A
Primary system 300 A What is happening in the power system
CT 600:1 Ratio factor 600
Protection relay 0.50 A Accurate proportional measurement
Accuracy operating range
Accurate
Accurate0–2× rated Reducing2–3× rated Outside3–4× rated
Nice – the relay is receiving a faithful proportional signal.

Simplified learning model – actual accuracy depends on transformer class, burden and operating conditions.