Functional testing demonstrates that the implemented control scheme behaves correctly in allowed and prohibited states. It must test logic outcomes, not merely lamp operation.

Learning objectives

Create state-based scripts, verify local and remote commands and test failure conditions and restoration.

Core engineering principles

Start from operating philosophy

Normal, emergency and maintenance modes define expected behaviour.

Every command needs permissive and block tests

A successful close proves less than a close correctly blocked by earthing or missing synchronism.

Indications must follow actual state

Breaker contacts and mechanism position should drive feedback rather than command echo.

Supply loss is a functional state

Loss of AC, DC, relay or communication power should produce defined alarms and fail-safe outputs.

End-to-end paths matter

Customer terminals, SCADA and breaker coil should be included where FAT scope permits.

Engineering application method

  1. Step 1: Build a state and transition test matrix.
  2. Step 2: Test local, remote and automatic commands.
  3. Step 3: Remove each permissive and attempt the prohibited action.
  4. Step 4: Simulate supply and communication failures.
  5. Step 5: Record observed indications, outputs and restoration.

Practical example

Closing the breaker once from the local switch does not prove the scheme. The FAT should also attempt closing with the earthing switch closed, spring discharged and selector in remote mode.

Common mistakes

  • Testing positive sequences only.
  • Forcing relay bits without field outputs.
  • Accepting command echo as position.
  • Ignoring loss and restoration states.
  • Using drawings of the wrong revision.

Design and review checklist

  • Are all modes covered?
  • Are negative tests included?
  • Does feedback use actual state?
  • Are supply failures tested?
  • Is evidence recorded per step?

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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