Interlocks convert the safe operating philosophy into physical constraints. A robust design prevents prohibited transitions even when an operator issues the wrong command or control power is unavailable.

Learning objectives

Create a complete state model, assign each prohibited transition to an interlock and verify manual, motorised and emergency operating modes.

Core engineering principles

States must describe actual primary connections

Service, test and disconnected positions refer to primary and secondary connection status. Labels alone are insufficient; contact engagement, shutters and earthing must match the indication.

Mechanical interlocks provide direct prevention

Racking a closed breaker, closing onto an earthed cable or opening an unsafe compartment should be blocked by validated mechanical relationships where the hazard demands it.

Electrical interlocks coordinate system conditions

Voltage, synchronism, source selection and remote permissives often require electrical logic. Loss of supply and broken wires must have a defined fail-safe response.

Key interlocks span separate equipment

Trapped-key systems can enforce sequence across panels and rooms, but key exchange, duplicates and maintenance bypasses must be governed.

Bypass is a controlled exceptional state

Defeat mechanisms used for maintenance should require tools, authorization and clear indication. An undocumented bypass becomes a latent hazard.

Engineering application method

  1. Step 1: List every valid equipment state and transition.
  2. Step 2: Mark prohibited transitions and their hazard.
  3. Step 3: Allocate mechanical, key or electrical prevention with fail-safe behaviour.
  4. Step 4: Test positive and negative sequences in local, remote and manual modes.
  5. Step 5: Control bypasses and record as-left status.

Practical example

A breaker should not be rackable from test to service while closed, even if the electrical position switch has failed. Mechanical prevention protects against the unsafe movement; electrical contacts provide indication and command permissives.

Common mistakes

  • Relying only on PLC logic for a direct mechanical hazard.
  • Testing permitted operations but not prohibited attempts.
  • Ignoring manual emergency operation.
  • Allowing duplicate trapped keys.
  • Leaving bypass tools permanently installed.

Design and review checklist

  • Are all states and transitions documented?
  • Which interlock prevents each unsafe action?
  • What happens with DC power lost?
  • Are local, remote and manual paths covered?
  • Are bypasses controlled and indicated?

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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