What is a current transformer and why is it needed?

Current transformer definition and purpose:

• Current transformer (CT) : Instrument transformer that converts high alternating current (primary circuit) to proportionally lower current (secondary circuit) suitable for meters, relays, and protective devices.
• Transformation ratio : Defined as primary current ÷ secondary current. Examples: - 100/5A CT: 100A primary → 5A secondary (ratio 20:1). - 200/5A CT: 200A primary → 5A secondary (ratio 40:1). - 400/1A CT: 400A primary → 1A secondary (ratio 400:1).
• Why CT needed : - Safety: Protects operators from high-current exposure. Meter secondary isolated from high-current circuits. - Practicality: Meters/relays typically rated 5A maximum. Direct connection to 100A+ circuit impossible; meter would overheat, be damaged. - Conductivity: Measuring 200A directly requires heavy copper conductors (>10mm²) and robust relay contacts. CT secondary only 5A; uses thin wire (2.5mm²), standard relay contacts. - Cost: Instrument-grade CT (€200–500) cheaper alternative to directly-rated 200A meter (€5000+).
• Measurement accuracy : Standard CTs certified per IEC 60044-8 (class 0.5, 0.2, or 0.6 accuracy per metering application). Secondary current proportional to primary within ±0.5% accuracy across rated range (20%–120% of rated current).
• CT burden (impedance) : Connected meter/relay load (measured in volt-amperes). Insufficient burden causes CT saturation (secondary current non-linear, measurement errors). Typical meter burden: 2–5 VA; CT specification states maximum burden (e.g., "burden <10VA at rated secondary current").
• Saturation risk : If secondary load impedance too high or primary current exceeds rated capacity, CT magnetic core saturates. Secondary current no longer proportional to primary. Causes meter reading error or relay misoperation. Avoided by proper CT sizing and burden verification.

Socomec CTs (DCTWRM range) rated per IEC 60044-1 (class 0.2S for metering, class 3P for protection relays). Socomec meters (DIRIS) specify maximum CT burden; Optim-Elec verifies compatibility. Contact optim-elec.com for CT selection and burden calculation.