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Interlock (engineering)
Interlock (engineering)
Interlock (engineering)
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Trapped key interlock switchgear door.

An interlock is a feature that makes the state of two mechanisms or functions mutually dependent. It may consist of any electrical or mechanical devices, or systems. In most applications, an interlock is used to help prevent any damage to the machine or to the operator handling the machine. For example, elevators are equipped with an interlock that prevents the moving elevator from opening its doors and prevents the stationary elevator (with open doors) from moving.

Interlocks may include sophisticated elements such as curtains of infrared beams, photodetectors, simple switches, and locks. It can also be a computer containing an interlocking computer program with digital or analogue electronics.

Trapped-key interlocking

[edit]

Trapped-key interlocking is a method of ensuring safety in industrial environments by forcing the operator through a predetermined sequence using a defined selection of keys, locks and switches.

It is called trapped key as it works by releasing and trapping keys in a predetermined sequence. After the control or power has been isolated, a key is released that can be used to grant access to individual or multiple doors. Below is an example of what a trapped key interlock transfer block would look like. This is a part of a trapped key interlocking system.

In order to obtain the keys in this system, a key must be inserted and turned (like the key at the bottom of the system of the picture). Once the key is turned, the operator may retrieve the remaining keys that will be used to open other doors. Once all keys are returned, then the operator will be allowed to take out the original key from the beginning. The key will not turn unless the remaining keys are put back in place.

Trapped key interlock transfer block.


Another example is an electric kiln. To prevent access to the inside of an electric kiln, a trapped key system may be used to interlock a disconnecting switch and the kiln door. While the switch is turned on, the key is held by the interlock attached to the disconnecting switch. To open the kiln door, the switch is first opened, which releases the key. The key can then be used to unlock the kiln door. While the key is removed from the switch interlock, a plunger from the interlock mechanically prevents the switch from closing. Power cannot be re-applied to the kiln until the kiln door is locked, releasing the key, and the key is then returned to the disconnecting switch interlock.[1] A similar two-part interlock system can be used anywhere it is necessary to ensure the energy supply to a machine is interrupted before the machine is entered for adjustment or maintenance.

Mechanical

[edit]
In this photo, the key is the mechanical interlock that allows the steering wheel to move the direction of the front wheels. Without the key, the car cannot move.

Interlocks may be strictly mechanical. An example of a mechanical interlock is a steering wheel of a car. In modern days, most cars have an anti-theft feature that restricts the turning of the steering wheel if the key is not inserted in the ignition. This prevents an individual from pushing the car since the mechanical interlock restricts the directional motion of the front wheels of the car.[2]

In the operation of a device such as a press or cutter that is hand fed or the workpiece hand removed, the use of two buttons to actuate the device, one for each hand, greatly reduces the possibility of operation endangering the operator. No such system is fool-proof, and such systems are often augmented by the use of cable–pulled gloves worn by the operator; these are retracted away from the danger area by the stroke of the machine. A major problem in engineering operator safety is the tendency of operators to ignore safety precautions or even outright disabling forced interlocks due to work pressure and other factors. Therefore, such safeties require and perhaps must facilitate operator cooperation.

Electrical

[edit]

Many people use generators to supplement power to a home or business in the event that main (municipal) power has gone offline. In order to safely transfer the power source from a generator (and back to the main), a safety interlock is often employed. The interlock consists of one or more switches that prevent both main power and generator power from powering the dwelling simultaneously. Without this safeguard, both power sources running at once could cause an overload condition, or generator power back-feed onto the main could cause the dangerous voltage to reach a lineman repairing the main feed far outside the building.

Electrical interlock on wire mesh

An interlock device is designed to allow a generator to provide backup power in such a way that it (a) prevents main and generator power to be connected at the same time, and (b) allows circuit breakers to operate normally without interference in the event of an overload condition. Most interlock devices for electrical systems employ a mechanical device to manage the movement of circuit breakers. Some also allow for the use of padlocks to prevent someone from accidentally activating the main power system without authorization.[3]

Defeatable

[edit]

Interlocks prevent injuries by preventing direct contact with energized parts of electrical equipment. Only qualified personnel, who must use a tool (such as a screwdriver), are allowed to bypass the interlock. Such interlocks are called defeatable interlocks, and are specified by Underwriters Laboratory (UL) standard UL508a, and National Electrical Code (NEC) Article 409.2. Defeatable interlocks are allowed on electrical equipment up to 600 volts.[4]

Security

[edit]
Different kinds of security interlocks can range from doors to electronic systems such as face or fingerprint recognitions.

In high-security buildings, access control systems are sometimes set up so that ability to open one door requires another one to be closed first. Such setups are called a mantrap.

Interlocks can be used as a high level entrance security. There are two kinds of interlocking systems for security. The first form of interlocking security is more mechanical. For example, if an individual is entering a building, there may be two sets of doors to enter from. As the individual enters the first door, that door will close before they enter through the second door. This type of interlocking security can prevent piggybacking or tailgating. The second form of interlocking security is electronic. This is in the form of detection and identification systems. Examples of such systems can be PIN codes, face recognition, and/or fingerprint recognition.[5]

Microprocessors

[edit]

In microprocessor architecture, an interlock is digital electronic circuitry that stalls a pipeline (inserts bubbles) when a hazard is detected until the hazard is cleared. One example of a hazard is if a software program loads data from the system bus and calls for use of that data in the following cycle in a system in which loads take multiple cycles (a load-to-use hazard).

An interlock may be used to prevent undesired states in a finite-state machine.

See also

[edit]
Look up interlock in Wiktionary, the free dictionary.

References

[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Interlock (engineering)

View on Grokipedia
from Grokipedia
In engineering, an interlock is a mechanical, electrical, or electromechanical device designed to prevent the operation of hazardous machine functions or systems when a guard is open or conditions are unsafe, ensuring that the states of multiple components or processes are mutually dependent to maintain . These devices monitor positions such as gates or doors and interrupt power or control signals to stop operations, thereby minimizing risks like unexpected startups or exposure to . Interlocks are integral to and are commonly applied in industrial settings, including manufacturing equipment, conveyor systems, and process controls, to comply with regulations and protect operators. Interlocks function by integrating sensors, switches, or actuators that detect unsafe states and trigger responses, such as halting motion or locking guards until conditions are resolved. Key types include contact-based mechanical interlocks, such as hinged or rotary switches for direct guard monitoring, and non-contact variants like magnetic or RFID-coded sensors that offer resistance to tampering and suitability for harsh environments. Guard locking interlocks, often solenoid-activated, provide additional security for machines with longer stopping times by preventing guard access until hazards cease. These categories are classified by defeat resistance levels according to ISO 14119 (2024), from easily bypassable Type 1 designs to highly secure Type 5 systems (including trapped-key interlocks), balancing cost, reliability, and security needs. The design and implementation of interlocks are governed by international standards to ensure performance levels and risk reduction. ISO 14119 (2024) specifies principles for interlocking devices associated with guards, defining them as mechanisms that inhibit hazardous operations based on guard positions and emphasizing anti-defeat measures like tool-resistant actuators. Complementary standards such as ISO 13849-1 (2023) outline categories (B, 1, 2, 3, 4) and performance levels (a to e), requiring like dual switches for higher-risk applications, while ISO 12100 (2010) provides broader -of-machinery guidelines incorporating interlocks. In the , compliance with the 2006/42/EC mandates interlock integration for , and in the U.S., OSHA's machine guarding requirements (29 CFR 1910.212) incorporate interlocks, alongside procedures (29 CFR 1910.147), though interlocks do not replace them. Overall, interlocks enhance system reliability by preventing accidents, with ongoing advancements focusing on smart sensors and integration with safety PLCs for complex .

Overview

Definition and Principles

An interlock in is a mechanism or control device that prevents a , , or system from operating in an unsafe or unintended manner by enforcing sequential or conditional actions, such as requiring one condition to be met before another can proceed. This feature ensures mutual dependence between mechanisms, where the state of one directly influences the functionality of another to mitigate potential hazards. At their core, interlocks operate on permissive logic, where an action is only allowed if predefined conditions are satisfied—for instance, a motor cannot start unless a guard is closed and levels are within limits. They also incorporate fail- design principles, which default the system to a state during failures, such as power loss or component malfunction, by using normally closed contacts that open to interrupt operation. These principles prioritize prevention over convenience, ensuring that unsafe states are inherently blocked rather than merely warned against. Key terminology includes hardwired interlocks, which use physical wiring and relays for direct, reliable control without reliance on software, and programmable interlocks, implemented via software in systems like PLCs for greater flexibility in complex sequences. Interlocking sequences refer to the ordered steps enforced by these devices, such as starting a conveyor only after a feeder is positioned, while guard interlocking specifically monitors protective barriers, halting operations if access is gained to hazardous areas. Mechanical and electrical interlocks exemplify these concepts in practice, with the former using physical linkages and the latter circuit-based controls. Interlocks originated in the mid-19th century, with the first mechanical systems developed for signaling to prevent collisions by switches and signals, as pioneered in in 1855 and Britain in 1856.

Purpose and Safety Benefits

Interlocks in engineering serve primarily to protect human operators from injury by preventing access to hazardous areas or operations during unsafe conditions, such as when protective guards are opened or required sequences are not followed. They also safeguard equipment from damage by halting processes that could lead to mechanical failure or overload, while maintaining process integrity to avoid unintended releases or contaminations in industrial settings. Compliance with safety regulations is a core purpose, including OSHA's standards under 29 CFR 1910.212, which mandate interlocks as part of barriers that disengage power sources to eliminate hazards, and , which specifies requirements for safety-related control systems to achieve defined performance levels. The safety benefits of interlocks include a significant reduction in through enforced operational sequences, ensuring that machinery only activates when all preconditions are met, thereby minimizing risks like amputations or crushing injuries. For instance, interlocks on guarded machines prevent startup if access panels are breached, directly addressing common scenarios in . According to OSHA , machinery-related incidents accounted for approximately 5,080 nonfatal amputations in 2005 alone, representing 60% of all such workplace injuries, underscoring the protective role of interlocks in averting these outcomes when properly implemented. In June 2025, OSHA reissued its National Emphasis Program on Amputations in Industries, targeting workplaces with machinery hazards and promoting interlocks as key safeguards to further reduce such incidents. Interlocks integrate seamlessly into broader frameworks, such as HAZOP studies, where they are evaluated as safeguards to mitigate identified deviations like or flow imbalances in process plants. Under , interlocks contribute to determining performance levels (PL a to e), quantifying the reliability needed to reduce to tolerable levels based on hazard severity, frequency, and avoidance possibilities. This structured approach ensures interlocks align with overall safety integrity levels in . Beyond safety, interlocks offer economic and operational advantages by minimizing from unsafe startups or accidents, which can otherwise result in costly repairs and production halts. They also facilitate by monitoring system states, allowing early detection of faults to prevent failures, thereby enhancing overall equipment reliability and reducing long-term operational expenses in industrial environments.

Types of Interlocks

Mechanical Interlocks

Mechanical interlocks are physical safety devices that utilize non-electrical components to prevent hazardous operations by enforcing sequential or conditional actions through direct mechanical constraints. These devices ensure that certain functions cannot occur unless specific safety conditions, such as the closure of a guard, are met, thereby minimizing risks to operators. According to ISO 14119:2024, mechanical interlocks form a subset of interlocking devices associated with guards, relying on tangible barriers rather than electrical signals to achieve safety objectives. The design of mechanical interlocks typically incorporates components such as levers, cams, rods, and linkages to create physical blocks or releases. For instance, a cam mechanism may engage a rod to lock a guard door in place, preventing access to moving parts until the machine cycle completes. These elements operate through positive mechanical actuation, where the geometry of the components ensures that motion in one part directly influences or restricts another, as outlined in ISO 14119:2024 Annex F. Spring-loaded pins serve as a common example, providing automatic locking upon guard closure via stored . Operation principles center on force transmission and to reliably enforce constraints. Levers and linkages amplify input forces, allowing a small operator action—such as closing a —to generate sufficient locking force against potential hazards. In practice, these systems transmit force directly without intermediaries, ensuring that any attempt to bypass the interlock requires overcoming the designed mechanical resistance, such as disengaging a bolted linkage. This approach guarantees behavior, where opening a guard immediately halts motion via a blocking rod or cam. Mechanical interlocks offer high reliability in harsh environments, such as those with dust, vibrations, or extreme temperatures, due to their absence of electrical dependencies and robust construction. They provide a straightforward physical barrier that is difficult to defeat without tools, enhancing overall system integrity. However, limitations include susceptibility to wear from repeated cycling, which can lead to jamming or reduced effectiveness over time, and the need for regular like to preserve smooth operation of . Dynamic loads may also accelerate in components like rods or springs, necessitating periodic inspections. Common applications include machine tools, where interlocks secure enclosures during cutting operations; conveyor systems, preventing startup until guards are positioned; and valves in fluid handling, ensuring sequential opening to avoid pressure surges. In these settings, mechanical interlocks integrate with guards to comply with safety standards, such as those in OSHA guidelines for point-of-operation safeguarding. Hybrid systems may occasionally interface mechanical elements with electrical controls for monitoring, but the core enforcement remains physical.

Electrical Interlocks

Electrical interlocks are implemented using electrical circuits and sensors to monitor system states and prevent hazardous operations by interrupting power or signal paths when unsafe conditions are detected. Key components include limit switches, which detect the position of machine guards or moving parts; proximity sensors, such as inductive or capacitive types, that sense the presence or absence of objects without physical contact; relays, which use electromechanical or solid-state mechanisms to control high-power circuits based on low-power signals; and control circuits that integrate these elements to enforce sequential or conditional operations. These components work together to create fail-safe systems where, for instance, opening a guard door triggers a switch to de-energize a motor circuit. The operation of electrical interlocks relies on contact configurations and logic principles to ensure reliable control. Normally closed (NC) contacts are closed in their default state, allowing current to flow until an interlock condition opens them, providing a interruption if power is lost; conversely, normally open (NO) contacts are open by default and close only when energized, suitable for permissive circuits that enable operations under specific conditions. In , these contacts form AND or OR conditions—for example, an AND interlock requires multiple NC contacts in series to remain closed for the circuit to energize, ensuring all safeguards are in place, while OR logic uses parallel NO contacts to allow activation from any one of several inputs. This setup prevents simultaneous activation of conflicting devices, such as forward and reverse motor directions, by using auxiliary NC contacts from one to block the other. Electrical interlocks offer advantages such as faster response times compared to purely mechanical systems, often in milliseconds via electronic relays, and the ability to support remote monitoring through integrated sensors and control units. However, they are vulnerable to electrical faults like wiring failures or , which can lead to unintended operation or failure to interlock. To mitigate these limitations, fault-tolerant designs incorporate dual-channel redundancy, where two independent circuits monitor the same conditions and cross-check each other; if one channel detects a discrepancy, the system defaults to a safe state, achieving higher safety integrity levels as defined in standards like ISO 13849-1. Common applications of electrical interlocks include integration with programmable logic controllers (PLCs) in manufacturing environments, where they sequence robotic arms or conveyor systems to avoid collisions by verifying positions via proximity sensors before advancing operations. In emergency stop (e-stop) circuits, NC contacts from e-stop buttons are wired in series with safety relays to immediately de-energize equipment across an entire , ensuring rapid shutdown in hazardous situations. These systems are prevalent in industries like automotive assembly, where dual-channel e-stop interlocks provide to maintain operation even if a single fault occurs.

Specialized Interlocking Mechanisms

Trapped-Key Interlocking

Trapped-key interlocking systems employ a series of mechanical locks and keys to enforce a predetermined sequence of operations in industrial machinery and processes, ensuring that hazardous equipment cannot be accessed or restarted until prerequisites are met. These systems typically involve multiple interlocking devices, such as control locks, transfer blocks, and access locks, where keys are physically trapped within one lock until specific conditions—like shutdown or guard closure—are satisfied, thereby preventing unauthorized or unsafe actions. In system design, trapped-key mechanisms often utilize a primary control key (sometimes called a master key) that interacts with an isolation device to de-energize equipment, releasing the key only after power is cut off via a safety relay or switch. This key then transfers to a secondary lock or exchange unit, which captures it and liberates one or more access keys (slave keys) to unlock guards or panels, creating a chained dependency that mandates procedural compliance. Complex setups may incorporate key exchange blocks for multi-step processes, aligning with standards like ISO 14119, which classifies these as Type 5 interlocking devices for their reliance on key trapping without electrical integration. Historical implementations, such as the Kirk-key systems developed in the early 20th century by R.L. Kirk—who filed the first patent in 1932—pioneered this approach for electrical and mechanical safety in industrial settings. The operation follows a strict step-by-step sequence to maintain : first, an operator inserts the into the isolation lock and turns it to the "off" position, which stops the and releases the key while it in place until shutdown is confirmed; second, the released key is inserted into a transfer interlock, which captures it and frees an ; third, the unlocks the relevant guard or valve, allowing while the remains isolated; finally, after work completion, the is reinserted and turned to relock the guard, releasing it back to the transfer unit, which then frees the to restart the equipment only when all elements are secured. This sequential ensures no shortcuts, as each key remains captive until the prior step is reversed. Advantages of trapped-key interlocking include high reliability in harsh environments due to their purely mechanical nature, requiring no or , which makes them ideal for dusty, explosive, or remote applications and reduces failure points from electrical faults. They promote strict adherence to lockout-tagout (LOTO) procedures beyond basic protocols, enhancing personnel protection and preventing equipment damage from improper sequencing, while being cost-effective for low-frequency access scenarios. However, limitations arise from their complexity in design and installation, necessitating meticulous to avoid loss or duplication, along with higher upfront costs for multiple components and the absence of diagnostic feedback compared to electronic systems. Specific examples illustrate their utility: in elevator controls, trapped-key systems sequence the isolation of power before allowing access to the shaft, ensuring technicians cannot enter while the car is operational and preventing accidental restarts during maintenance. In chemical plants, they manage valve sequencing to avoid forming explosive mixtures, such as requiring a shutdown key to be trapped before unlocking valves for nitrogen purging in hydrogen-cooled generators, thereby controlling hazardous energy flows.

Defeatable Interlocks

Defeatable interlocks, also known as manually suspendable safeguards, are engineered mechanisms that incorporate deliberate provisions for temporary bypass during , , or situations, while incorporating measures to mitigate misuse. These systems typically feature override switches, keyed access mechanisms, or software-enabled temporary defeats that allow authorized personnel to suspend the interlock function without permanent alterations. In modern implementations, such designs often include electronic logging capabilities, such as defeat counters or audit trails in safety programmable logic controllers (PLCs), to record bypass events for compliance and review purposes. Protocols for employing defeatable interlocks emphasize strict procedural controls to ensure safety, integrating with lockout-tagout (LOTO) processes under OSHA 29 CFR 1910.147 for minor servicing where full energy isolation is impractical. Before activation, a risk assessment must identify hazards, implement compensating measures like reduced machine speeds or additional supervision, and obtain multi-level approvals via a formal permit system detailing the bypass duration, purpose, and restoration steps. Regulatory standards, such as ANSI B11.19-2019, mandate that manual suspension (bypassing) of interlocks be limited to short durations, with requirements for design features that discourage unauthorized or easy defeat, including redundancy in control circuits and post-bypass verification testing. Weekly audits of bypass logs and annual program reviews are recommended to prevent habitual misuse. The primary advantage of defeatable interlocks lies in their provision of operational flexibility, enabling efficient diagnostics and repairs on complex machinery without necessitating complete shutdowns, thereby minimizing production downtime. However, this capability introduces elevated risks if protocols are not rigorously followed, as suspended safeguards can expose workers to hazards like or energy release, potentially leading to incidents if bypasses extend beyond approved limits. Since the early , these systems have evolved from basic mechanical override switches to sophisticated electronic variants with integrated auditing, driven by advancements in safety-rated PLCs and standards updates like ANSI B11.19-2010, which first formalized manual suspension requirements to balance accessibility with risk control. Representative examples include maintenance modes on computer (CNC) machines, where keyed switches permit guarded door access for tool changes while limiting spindle speeds to safe levels, and defeatable guards on automated assembly lines that allow temporary suspension for jam clearance using hold-to-run devices. Unlike non-defeatable alternatives such as trapped-key systems, which enforce sequential operations without bypass options, defeatable interlocks prioritize controlled flexibility in dynamic industrial environments.

Applications and Implementations

In Security Systems

In security systems, interlocks serve as critical components for physical access control, integrating with alarms, closed-circuit television (CCTV), and badge-based authentication to prevent unauthorized entry into protected areas. Door interlocks, often electrical in nature, link multiple entry points such that only one door can open at a time, triggering alarms if a breach attempt occurs and coordinating with CCTV for real-time monitoring of access events. For high-security environments like data centers and financial institutions, man-trap systems employ pairs of interlocked doors forming a vestibule, where the inner door remains secured until the outer door closes and credentials—such as access badges or key fobs—are verified, effectively isolating and verifying entrants. These systems operate in fail-secure or modes to balance security and emergency response. Fail-secure interlocks maintain a locked state during power loss, prioritizing perimeter defense by preventing forced entry, while configurations default to unlocked for rapid egress in fires or evacuations. In response to breaches, interlocks—such as door contact sensors—detect forced openings by monitoring the gap between door and frame, immediately activating alarms or locking adjacent doors to contain intruders. Interlocks enhance overall perimeter defense by enforcing and reducing risks, but they present limitations, such as potentially trapping occupants during emergencies if fail-secure modes are over-applied without adequate overrides. Compliance with standards like UL 294 ensures reliability, evaluating units for endurance, electrical integrity, and resistance to tampering in interlocked setups.

In Microprocessor and Control Systems

In and control systems, hardware interlocks serve as essential safeguards to maintain integrity by preventing invalid operational states caused by faults, hangs, or transient errors. Watchdog timers, a prominent example, are dedicated hardware circuits integrated into that initiate a reset if not periodically "kicked" or serviced by software within a predefined timeout period, thereby detecting and recovering from malfunctions such as infinite loops or hardware failures. This mechanism operates through a counter that decrements from an initial value unless refreshed, ensuring the processor remains responsive; for instance, in the MC6809E , watchdog timers have been evaluated to cover 62% of transient faults in benchmark programs by triggering resets on timeouts. These hardware interlocks provide low-latency , often implemented at the circuit level to interface with electrical controls, but require careful tuning to balance sensitivity against false positives. Software interlocks extend these protections into programmable logic, particularly in embedded systems where resource constraints demand efficient concurrency management. Mutexes (mutual exclusion locks) ensure only one task accesses a at a time, preventing in shared resources, while semaphores—either binary for exclusion or counting for resource pooling—facilitate signaling between tasks or , enabling coordinated execution without busy-waiting. In real-time embedded applications, handlers often incorporate these primitives to prioritize safety-critical operations, such as suspending lower-priority tasks during fault recovery. further bolster reliability by enforcing resource allocation policies, like adaptations of the that pre-validate requests to avoid circular waits, tailored for distributed control systems with limited . For example, in RTOS-based environments, these mechanisms detect potential deadlocks through resource graphs and resolve them via preemption or , ensuring deterministic behavior in time-sensitive . The advantages of interlocks in and control systems lie in their for increasingly complex architectures, allowing modular integration in multicore processors and networked controllers, though limitations include vulnerability to software bugs that bypass checks or introduce new overheads. Since the , the evolution of real-time operating systems (RTOS) has significantly advanced these interlocks; early systems like the TRON RTOS family, introduced in 1984, pioneered standardized primitives for embedded applications, influencing modern RTOS such as and with built-in mutexes and semaphores certified for safety standards like IEC 61508. This progression has enabled robust handling of concurrency in resource-constrained devices, reducing mean time to failure by orders of magnitude in fault-prone environments. Representative examples illustrate their practical impact. In CPU pipelines, interlocks address data hazards by stalling instruction fetch until dependencies resolve, as seen in load-use interlock (LUI) designs where hardware detects operand mismatches and inserts no-op cycles, minimizing performance penalties in architectures like early MIPS processors ( without Interlocked Pipeline Stages, which relied on compiler avoidance but inspired hardware solutions). In programmable logic controllers (PLCs) for Industry 4.0, safety interlocks integrate software mutexes with IoT sensors to enforce sequential operations in cyber-physical systems, such as halting robotic arms on anomaly detection via , enhancing reliability in smart factories while supporting exchange over protocols like OPC UA. These implementations underscore interlocks' role in bridging hardware reliability with software flexibility, though ongoing challenges include verifying bug-free code in evolving IoT ecosystems.

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