Interlock (I)

Interlock (I) – state-dependent monitoring

The Interlock (I) is the mechanism for state-dependent monitoring of the sequence. It protects the logical consistency of a sequence and prevents a process from entering an invalid state from occurring.

The interlock is therefore not a general safety concept, but a process safeguarding within the model.

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Purpose of the interlock

The interlock answers the question:

May this state be active under the current conditions?

It ensures that:

• mandatory prerequisites are met

• invalid combinations of states do not occur

• the process remains logically consistent

Interlocks are necessary because:

• not all conditions are relevant in every state

• monitoring must be targeted and context-dependent

• safety and process should not be mixed

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Interlock in the system layer

Interlocks are in the system layer via the operands i .

This means:

• each zone can be marked as an interlock in a state

• the meaning is dependent on state

• the evaluation is continuous

In the bit-control cross table, the following applies:

• I → condition must be met

• Deviation is not tolerable

An interlock is a mandatory statement about the validity of a state.

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Behavior on interlock deviation

If an I-condition is violated, the system reacts deterministically:

• the automatic enable is revoked immediately

• the affected sequence stops

• the current state remains

• a clear diagnosis is generated

There are:

• no continuation

• no transition

• no temporal tolerance

An interlock knows no gray area.

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Interlock in automatic operation

In automatic operation an interlock means:

• the process must not continue

• the state is no longer logically valid

• the system stops immediately

The interlock thus protects:

• the logical sequence of the process

• the consistency of the state machine

• the traceability of the behavior

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Interlock in manual operation

Even in manual operation the interlock remains active.

This means:

• States cannot be "forced"

• invalid combinations of states are excluded

• the operator works under the same rules

Important:

Interlocks are not an automatic concept, but part of the model.

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Distinction from CMZ

For clear classification:

• Interlock (I)

  • acts dependent on state

  • protects the process

  • may under certain circumstances allow manual movement

• CMZ

  • acts state-independent

  • protects system integrity

  • Plausibility checks:

Interlock protects the process. CMZ protects people, machine and plant.

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Distinction from MXIC

Equally important:

• Interlock (I)

  • acts in the process context

  • evaluates state validity

• MXIC

  • acts only in manual operation

  • evaluates manual operator actions

Both complement each other, taking on different roles.

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Typical mistakes in dealing with interlocks

Common mistakes are:

• Using interlocks as a substitute for CMZ

• too many interlocks without clear justification

• Using interlocks for permanent conditions

• Misunderstanding interlocks as a comfort feature

Rule of thumb:

Everything that must always apply belongs in a CMZ – not in an interlock.

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Summary

The interlock (I):

• is a state-dependent monitor

• protects the logical consistency of the process

• acts in automatic and manual operation

• generates immediate, unambiguous reactions

• is an integral part of the system layer

Interlocks ensure that a process only takes place where it is logically allowed.