An interlock is proven by attempting the unsafe action under the exact prohibited condition. Positive tests show equipment can operate; negative tests show it cannot enter a hazardous state.

Learning objectives

Build a prohibited-transition matrix and test mechanical, electrical and key interlocks without damaging equipment.

Core engineering principles

Every hazard maps to a blocked transition

Examples include closing onto earth, racking closed, opening a live compartment or paralleling unsynchronised sources.

Mechanical and electrical layers are tested separately

Removing control power can reveal whether a mechanical hazard is truly prevented.

Manual operation belongs in the matrix

Emergency handles and local levers can bypass electrical logic if not mechanically controlled.

Bypass tools require authorization

Tests that defeat an interlock should follow manufacturer procedure and restore the normal state.

No excessive force is allowed

The tester confirms prevention without damaging locks or linkages.

Engineering application method

  1. Step 1: Create states and prohibited transitions.
  2. Step 2: Set each unsafe precondition deliberately.
  3. Step 3: Attempt normal electrical and manual operation.
  4. Step 4: Record which interlock prevents the action.
  5. Step 5: Restore and verify the permitted sequence.

Practical example

With the earthing switch closed, attempt breaker closing from local, remote and manual mechanisms as applicable. All paths should remain blocked according to design.

Common mistakes

  • Testing only allowed operations.
  • Forcing a blocked handle.
  • Checking PLC logic but not mechanical lock.
  • Ignoring manual emergency paths.
  • Leaving a defeated interlock unrestored.

Design and review checklist

  • Is every prohibited transition listed?
  • Are all command sources tested?
  • Does prevention remain with DC lost?
  • Are bypasses controlled?
  • Is restoration verified?

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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