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Current Transformers

The Digital Brain's Analogue World

💡 Key Concept

Microprocessors - the brains of modern electronics - only understand voltage as their analogue input. Even when measuring current, we need to convert it to voltage first!

Current Transformer (CT) Operation

10A10 Secondary Turns1 Primary Turn1ACT Ratio 10:1

💡 Key Points

  • 🔄 Electromagnetic Principles

    • Current flowing through a conductor creates circular magnetic fields
    • Field lines emerge perpendicular (90°) to current flow direction
    • Core material specially designed for high permeability
    • Creates efficient path for magnetic flux
    • Secondary winding converts magnetic flux back to current

  • 🔧 Installation Guidelines

    • CT Orientation
      • Slight tilt is acceptable
      • Current strength remains constant along cable length
      • Magnetic flux measurement stays consistent
      B₁BStraightB₁B₁15° TiltMagnetic field lines are concentricaround the conductorB₁ = B₁ (Equal field strength)
    • Multiple Conductors
      • Two phase cables through CT window will sum their currents
      • Useful for measuring total current in parallel conductors
      • Direction matters: opposing currents will subtract
      • Common application: measuring differential current in RCD/GFCI protection
      100A100ATotal: 200A
    • Cable Position
      • Always center the cable in CT window
      • Off-center positioning affects measurement accuracy
      • Magnetic field strength varies with distance
      d₁d₁Centered (Correct)d₁d₂Off-Center (Incorrect)
    • Shorting Terminals
      • Essential safety feature for CT installations
      • Allows safe disconnection of measuring equipment
      • Must be closed before disconnecting meter

      💡 Always follow the sequence: Close shorting bar → Disconnect meter → Connect new meter → Open shorting bar

    • 🤔 Brain Teaser

    What happens if you put all three phases (L1, L2, L3) through a CT window?

    L1L2L3?
    Click to reveal the answer!

    Answer: You're measuring the neutral current (or system imbalance)!

    In a balanced 3-phase system: L1 + L2 + L3 = 0 (they cancel out)

    Therefore, any reading you get is either:

    • Current that would flow in the neutral conductor
    • System imbalance between phases
    • Harmonic currents (which don't cancel like fundamental frequencies)

    💡 This is actually a common technique for measuring neutral current without directly accessing the neutral conductor!

  • 📊 Secondary Circuit Operation

    • Number of secondary winding turns determines CT ratio
    • S1/S2 Terminal Output
      • Current scales linearly with primary current
      • Ratio examples: 100:1, 1000:1, etc.
      • Output maintains phase alignment with primary
      • Phase alignment critical for accurate power calculations
    ⚡DANGER: Never Open-Circuit Secondary
    • Removing burden resistor causes voltage to spike dangerously
    • Can produce visible arcing between terminals
    • May damage CT and create safety hazard
    • Always maintain closed circuit through burden or shorting block
    Safe CT Disconnection Using Shorting Block
    S1S2S1S2From CTTo MeterShorting Pin
    Safe Disconnection Sequence:
    1. Insert shorting pin to connect S1 and S2
    2. Disconnect meter leads
    3. Connect new meter leads
    4. Remove shorting pin

    ⚠️ Never remove shorting pin while meter is disconnected

🛠️ Troubleshooting Tip

Understanding CT ratios is crucial for system verification:

  • If CT ratio is 1000:1, a primary current of 1000A produces 1A secondary current
  • You can verify meter readings by comparing:
    • CT secondary current × CT ratio = Primary current
    • Example: 0.5A × 1000 = 500A primary current
  • This helps identify issues between CT readings, meter displays, and EPM values