An interlock stops a machine or process from running until required conditions are met, such as a guard being in place. They can be mechanical, electrical or software based. They fail most often when people bypass them, which is why design, maintenance and procedure matter as much as the device.
Three kinds
Mechanical. Physical parts that block movement or access until the right tool or condition is present. A trapped key system on switchgear doors is a classic example.
Electrical. Sensors, relays and switches that interrupt operation when a guard, gate or access panel is open.
Software based. Control logic, for example in a PLC, that refuses to start or stops when an input says a guard is open.
A machine that cannot start with its guard out of place, or an interlocked gate around a robot cell, are everyday examples.
What OSHA says about guarding
OSHA states that any machine part, function or process that may injure someone must be safeguarded. Injuries named on its machine guarding page include crushed fingers or hands, amputations, burns and blindness. OSHA highlights a 2025 instruction that continues a National Emphasis Program on amputation hazards in manufacturing.
Why bypassing happens
The interlocks reference notes that interlocks are not fully effective if damaged, bypassed, poorly maintained or left out of a wider safeguarding plan, and that many documented accidents involve operators deliberately bypassing one while working on machinery.
People bypass for understandable reasons: a nuisance trip, a slow clear jam routine, or pressure to keep output up. Treat each bypass as feedback about a design problem.
Designing against defeat
ISO 14119:2024 addresses designing and selecting interlocking devices, including how to prevent defeat or bypass. Practical ideas:
- Make the correct way the easy way, for example a fast, safe jam clearing procedure.
- Choose devices that are hard to fool with a spare piece of metal or a taped switch.
- Log and alert when a safety input is forced or overridden in the control software.
- Review nuisance trips and fix their cause.
- Check interlocks in the periodic inspection, not only at commissioning.
Software changes the risk
When an interlock is a line of logic, anyone who can edit the program can change it, including through a remote connection. Control the right to change safety logic, track every change, and compare the running program with an approved copy. The Therac-25 case shows what happens when software alone carries the protection (details here).
Interlocks and lockout are different
Interlocks reduce unexpected startups, hazardous motion and release of stored energy. They do not replace procedures such as lockout tagout. For service work, isolate energy under the procedure described in our LOTO guide.
Quick check
If a machine has an interlock, do we still need lockout tagout?
Yes, where the standard applies. Interlocks help but do not replace energy control procedures.
Is a software interlock acceptable?
It can be, depending on the risk level and the applicable standard, but it needs the lifecycle rigour such functions require. Ask a qualified safety engineer.
Sources
Last reviewed: 19 October 2026.
