2.3 Electronic interlocks

An Electronic Interlock is the digital equivalent of a physical barrier. It is a binary constraint that prevents the manufacturing process from advancing when conditions are unsafe, incorrect, or unknown. Unlike a “Warning” (which operators can ignore), an Interlock kills the “Start” signal. It removes “hope” from the production equation.

Interlocks must be deterministic. If the defined criteria are not met, the machine must not operate. It is helpful to divide interlocks into three tiers of severity for clarity.

Tier 1: data integrity & setup (the “start” gate)

Condition: Wrong Material scanned.
Condition: Wrong Tooling / Fixture ID.
Condition: Operator Certification Expired.
Logic: When the setup does not perfectly match the Bill of Materials (BOM) requirement, the system should automatically disable the Machine Cycle Start. Operating with incorrect materials or tooling is a severe quality risk.

Tier 2: sequence & quality (the “process” gate)

Condition: Unit skipped the previous operation (e.g. trying to Test before Assembly).
Condition: Unit failed previous test and was not repaired.
Logic: When the previous step status is not registered as “PASS”, the system should reject the unit immediately to prevent adding value to a known defect.

Tier 3: equipment health (the “safety” gate)

Condition: Calibration Date expired.
Condition: Recipe Checksum mismatch.
Condition: E-Stop circuit open.
Logic: When the Machine Health status registers as “Critical” (e.g. due to an open safety circuit or an expired calibration), the architecture should enforce an immediate hard stop to protect both the operator and the product.

Pro-Tip: Creating interlocks based on “Soft” parameters like Efficiency or OEE should be avoided. Stopping the line simply because it is running slowly often creates a self-fulfilling prophecy of downtime. Interlocks must focus solely on Safety, Quality, and true Identity verification.

The authority matrix: who holds the keys?

An interlock exists to force an escalation. If an Operator could bypass it, the interlock would be useless. These permission levels must be hard-coded into the MES/SCADA User Groups.

Role	Authority Level	Can Override?	Scope of Control
Operator	Level 0	No	Can acknowledge alarms, but cannot clear a process block.
Line Lead	Level 1	Limited	Can override “Setup Mismatch” (e.g. Alternative Part usage) if validated.
Process Eng	Level 2	Yes	Can override Process/Recipe limits for troubleshooting/NPI.
Quality Mgr	Level 3	Yes	Can override “Failed Previous Step” (e.g. for specialized rework flows).
Plant Mgr	Level 4	Absolute	Emergency Run Mode (The “Break Glass” scenario).

Override governance: the “bypass” lifecycle

A permanent override is a broken process. An override must be treated as a Temporary State, not a transactional event.

Forced logging

Requirement: No override can be executed without a Reason Code and a Comment.
Audit Trail: “User X bypassed Interlock Y on Unit Z at Time T. Reason: ‘Sensor drift, verified manual check’.”

Auto-expiry (the dead Man’s switch)

It is critical to never allow an override to persist indefinitely.

Time-Bound: When an override remains active for more than a defined period (e.g. 4 hours), the system should automatically revoke it.
Quantity-Bound: When the number of units processed under an override exceeds a safe threshold (e.g. 50 units), the system should automatically revoke it.
Shift-Bound: When the current shift ends, all active overrides should be automatically revoked to ensure the incoming shift starts from a known state.

The “red light” indication

Visual Management: When an override is actively in use, the Andon light should flash a distinct color (such as Blue or another defined Maintenance color).
Logic: It is essential that the production floor can visually see that a standard safety or quality net is temporarily down.

Emergency protocols

Sometimes the Manufacturing Execution System (MES) fails while the machine works. A “Business Continuity” mode is required.

The “Emergency Run” Mode

Trigger: MES Server Down or Network Failure.
Authority: Requires Plant Manager Physical Key (or Digital Token).
Action: Disables all Data Interlocks.
Risk: Traceability is lost. Quality is unverified.
Recovery: All units produced in this mode are automatically flagged “Quarantine” in Enterprise Resource Planning (ERP). They must be manually scanned/re-verified when the system restores.

Final Checkout: Electronic interlocks

Category	Metric / Control	Threshold / Rule
Integrity	Sequence Check	100% of units checked for “Previous Pass” status.
Access	Segregation	Operators have 0 capability to bypass interlocks.
Audit	Forced Comment	Override requires min. 10 chars of text explanation.
Safety	Time Limit	All overrides auto-expire after Max 4 Hours (configurable).
Risk	Visual Alert	Active Override triggers visual Andon alert.
Continuity	Quarantine	”Emergency Run” units default to “Hold” status.
Calibration	Lockout	Machine locks out immediately at Calibration_Date + 1 Day.