A safety interlock is a device that prevents a machine or process from running, or shuts it down, when a guard is opened or a safe condition is not met.
A safety interlock is a device or system that links two functions so that one cannot happen unless a safe condition is met. In machine safety, the most common example is an interlocked guard: when the guard door is opened, the interlock stops the hazardous motion, and the machine cannot start again until the guard is closed. Interlocks are also used in process safety, electrical systems and building systems, for example to stop a burner from firing until a furnace has been purged, or to prevent a high-voltage cabinet from being opened while it is energized.
Interlocks are engineering controls, which places them high on the hierarchy of controls. Unlike a warning sign or a procedure, a properly designed interlock works whether or not the operator remembers a rule. That reliability is why interlocks are used to protect against some of the most severe machine hazards, including amputation and crushing.
Interlocks are only as good as their design, installation and upkeep. Interlocks that are easy to defeat, poorly maintained or a source of daily frustration often end up bypassed, and bypassed interlocks are a recurring factor in serious machine injuries.
A shaped actuator, or tongue, mounted on the guard fits into a switch on the machine frame. Removing it when the guard opens breaks the safety circuit. These are common and inexpensive but can be defeated with a spare actuator if not properly managed.
A switch built into the hinge of a guard door detects when the door rotates open.
Magnetic or RFID-coded sensors detect the guard without physical contact. Uniquely coded versions are much harder to defeat with an ordinary magnet or a spare part, and they tolerate dirt and washdown better than mechanical switches.
Some machines keep moving after power is cut, such as a spinning saw blade or centrifuge. Guard locking interlocks hold the guard closed until the hazardous motion has stopped, using a timer, speed monitoring or a standstill signal.
A key must be released from one lock before it can be used in the next. For example, a key is released only when power is isolated, and that key is then needed to open an access door. Trapped-key systems enforce a safe sequence across several steps.
Light curtains, safety mats and laser scanners work like interlocks by stopping a machine when a person enters a danger zone, without a physical guard.
Controls that prevent unsafe combinations of conditions, such as stopping a pump from starting with a closed discharge valve or preventing a breaker from being racked in while energized.
OSHA's general machine guarding standard, 29 CFR 1910.212, requires guarding to protect operators and others from hazards such as points of operation, nip points, rotating parts and flying chips. It recognizes several methods, including guards and electronic safety devices. Interlocks are a common way to make movable guards meet this requirement, and specific standards, such as those for mechanical power presses, set additional rules.
Design standards provide detailed guidance:
An interlock is not a substitute for lockout/tagout during servicing and maintenance. Interlocks act through control circuits, which can fail or be overridden, while lockout isolates energy at the source. OSHA's lockout standard allows an exception only for minor tool changes and adjustments that are routine, repetitive and integral to production, and only when alternative measures provide effective protection.
Workers rarely bypass interlocks out of recklessness. They usually do it because the interlock gets in the way of a task, such as clearing jams, adjusting tooling or watching the process run. Preventing bypass means addressing those needs:
A packaging line had a hinged guard over a cartoner's folding section, with a simple tongue switch interlock. Jams were frequent, and operators had learned to tape a spare tongue actuator into the switch so they could open the guard and clear cartons without stopping the line. One evening an operator reached in to free a carton, and the folding arm caught his hand, causing a severe fracture.
The incident investigation found that the bypass had been in place for months and that supervisors knew about it. The plant replaced the tongue switch with a uniquely coded non-contact interlock, added a guard-locking function that held the door closed until the folding arm stopped, and installed a reduced-speed jam-clearing mode with a hold-to-run enabling device. It also worked with the carton supplier to reduce jams, added interlock checks to the start-up checklist and made it a disciplinary matter for supervisors to tolerate a known bypass.
Usually not. During servicing and maintenance, energy must normally be isolated and locked out. Interlocks protect during normal operation and some minor production tasks, but they work through control circuits that can fail or be overridden.
Most often because the interlock interferes with a frequent task, such as clearing jams, adjusting settings or observing the process. Addressing those needs with safe operating modes and better design is the most effective way to stop bypassing.
Follow the manufacturer's instructions and the risk assessment. Many employers test interlock function at the start of each shift and include them in regular inspections and preventive maintenance.
A trapped-key interlock uses a sequence of locks and keys to force steps to happen in a set order. A key can only be released from one lock when that step is safe, and it is then needed to unlock the next step.
Part of SafetyIQ's EHS glossary: plain-English definitions of workplace health and safety terms.
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