A conventional CT secondary can develop dangerous voltage if opened while primary current flows. LVC design must make the safe sequence—short, isolate, inject and restore—obvious and difficult to perform incorrectly.

Learning objectives

Allocate CT cores, preserve polarity, provide controlled shorting and testing and prevent multiple secondary earths.

Core engineering principles

Each core has one declared purpose

Protection, metering and differential circuits have different accuracy and burden requirements. Sharing or reassigning cores requires calculation.

Polarity defines protection direction and zone

P1/P2 and S1/S2 orientation must match the schematic and relay convention. A reversed phase can defeat differential or directional logic.

Short before disconnecting

Test blocks should make the CT shorting connection before opening the relay path and prevent accidental access to an open live secondary.

Secondary earthing is normally single-point

Multiple earths can create circulating current and measurement error; no earth can allow the circuit to float dangerously. The selected point must be documented.

Burden includes test facilities

Terminal resistance, test switches and long leads add to the CT loop and can worsen saturation.

Engineering application method

  1. Step 1: Assign cores and ratios from the protection design.
  2. Step 2: Draw polarity and single-point earth explicitly.
  3. Step 3: Select make-before-break CT test facilities.
  4. Step 4: Calculate complete secondary burden.
  5. Step 5: FAT-test ratio, polarity, continuity, shorting and earthing.

Practical example

Opening a normal knife terminal in a loaded CT circuit can generate hazardous voltage. A dedicated test block first shorts S1-S2 and then isolates the relay safely.

Common mistakes

  • Using ordinary disconnect terminals.
  • Earthing both at CT and relay.
  • Mixing protection cores.
  • Omitting test-switch burden.
  • Leaving unused CT secondaries open.

Design and review checklist

  • Is every core allocated?
  • Is polarity consistent end-to-end?
  • Does shorting occur before opening?
  • Is there exactly one intended earth?
  • Is burden within CT capability?

Standards basis and official sources

  • IEC 61869-1:2023 — General requirements for instrument transformers and low-power instrument transformers.
  • IEC 60255-1:2022 — Common requirements for measuring relays and protection equipment.
  • IEC 60947-7-1:2025 — Terminal blocks for copper conductors.

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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