CT FAT tests confirm that the installed core, ratio and polarity match the protection design and that the secondary circuit can be tested safely without an open circuit.

Learning objectives

Create a core-by-core test matrix and verify the complete route from CT terminals through test facilities to relay input.

Core engineering principles

Nameplate and core identity come first

Each core has ratio, class and application that must match drawings.

Polarity is tested end to end

Correct marks at the CT are insufficient if wiring reverses downstream.

Continuity must include test blocks

Links, shorts and relay terminals can introduce open circuits.

Single-point earthing is verified physically

A schematic symbol does not prove the absence of a second earth.

Unused cores remain safe

They are shorted and earthed as required by the approved design.

Engineering application method

  1. Step 1: List every CT, core and destination.
  2. Step 2: Perform ratio and polarity tests.
  3. Step 3: Check continuity through all links.
  4. Step 4: Verify short-before-open test sequence.
  5. Step 5: Measure or inspect the single intended earth.

Practical example

A differential core with one reversed phase can trip on load even though all ratios are correct. End-to-end polarity testing detects the error.

Common mistakes

  • Testing only at CT terminals.
  • Ignoring multi-ratio tap selection.
  • Leaving unused cores open.
  • Assuming earth from drawing.
  • Operating a disconnect before shorting.

Design and review checklist

  • Is every core identified?
  • Are ratio and polarity correct?
  • Is the circuit continuous?
  • Is shorting sequence safe?
  • Is earthing single-point?

Standards basis and official sources

Engineering note: Verify the contracted standard edition, amendments, manufacturer evidence and project-specific studies before applying these principles to a supplied assembly.

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