Machine safety

Why it matters

Automated machines move fast and do not notice people. Amputations and crush injuries from unguarded or bypassed machinery are among the most common serious injuries in manufacturing. Controls technicians design, install, test and maintain the safety systems that prevent them, and they are often asked to bypass them. Knowing how they work, and why they must not be defeated, is part of the job.

Risk assessment first

Machine safety starts with a risk assessment (ISO 12100 describes the method): identify the hazards in every task, including setup, cleaning and maintenance, estimate the risk, and reduce it using the same hierarchy as in workplace safety:

1. Inherently safe design: remove the hazard (lower force, rounded edges, no pinch point).
2. Safeguarding: guards and protective devices.
3. Information and training: warnings, procedures, PPE.

Standards such as ISO 13849-1 then set how reliable each safety function must be, expressed as a performance level (PLa to PLe) or, under IEC 62061, a SIL. In the US, OSHA requires machine guarding (29 CFR 1910.212) and NFPA 79 covers electrical equipment of industrial machinery (https://www.nfpa.org/codes-and-standards/nfpa-79-standard-development/79). Checked on 2026-10-23.

Guards

• Fixed guards: bolted or welded, need a tool to remove. Best where access is rarely needed.
• Interlocked guards: doors or covers with a safety switch; opening the guard stops the hazardous motion, and the machine cannot start while it is open. Guard locking keeps the door locked until motion has stopped, for machines that coast.
• Adjustable and self-adjusting guards: common on saws and similar tools.
• Guards must prevent reaching around, under, over or through to the hazard. Openings and distances are set by standards tables.

Protective devices

• Light curtains: rows of infrared beams; breaking any beam stops the machine. They must be mounted at a safety distance that allows the machine to stop before a hand reaches the hazard:

safety distance = hand speed × total stopping time + intrusion allowance

With a typical hand speed of 63 in/s (1.6 m/s) and a stopping time of 0.25 s, the distance is 15.75 in plus the allowance for the curtain's resolution. A machine whose brakes wear and stop more slowly makes that distance unsafe, which is why stopping time is tested.

• Safety mats and laser scanners detect presence in an area.
• Two-hand controls keep both hands on buttons during the hazardous motion.
• Enabling switches (three-position) for teaching robots.

Emergency stops

An E-stop is a red mushroom-head button on a yellow background that stops hazardous motion. Key points:

• It is a complementary measure, not a substitute for guarding.
• It must latch in until deliberately reset, and resetting it must not restart the machine by itself.
• Stop categories (NFPA 79 / IEC 60204-1): Category 0 removes power immediately (uncontrolled stop); Category 1 brakes under power, then removes it (controlled stop); Category 2 is a controlled stop with power kept on, not allowed for E-stops.
• Every E-stop must be hard-wired through safety-rated devices, never only through an HMI button or ordinary PLC logic.

Safety relays and safety PLCs

An ordinary relay can weld its contacts closed and nobody would know. A safety relay (safety module) is designed so a single fault does not cause loss of the safety function:

• Dual-channel inputs: the E-stop or guard switch has two contacts wired separately; the relay checks they agree.
• Redundant, monitored outputs: two force-guided contactors in series with the motor, and their NC auxiliary contacts fed back to the relay (the feedback loop or EDM).
• Cross-fault detection: a short between the channels is detected.
• A monitored manual reset: the machine only resets when the button is pressed and released and everything checks out.

Larger machines use safety PLCs (safety controllers), which run certified safety logic separately from the standard program.

Safety functions vs lockout

Guards, light curtains and E-stops protect people during normal production. They do not isolate energy. For maintenance, repair, clearing jams inside the machine or servicing, workers use lockout/tagout (OSHA 29 CFR 1910.147): isolate every energy source, lock it, release stored energy and verify zero energy. An E-stop is not a lockout device. See https://www.osha.gov/control-hazardous-energy and https://www.osha.gov/machine-guarding. Checked on 2026-10-23.

Worked example: a guard door circuit

A packaging machine has an interlocked door with a coded safety switch (two channels) wired to a safety relay. The relay drives two contactors in series feeding the motor, with their NC auxiliary contacts in the relay's feedback loop. One contactor welds shut. When the door opens, the second contactor still stops the motor (redundancy). At the next reset, the relay sees the welded contactor's feedback contact open and refuses to reset (monitoring). The fault is found before the machine can run again with only one working channel.

Never defeat a safety device

Taped-down interlock switches, light curtains muted with jumpers and guards left off are common causes of serious injuries. If a safety device stops production too often, the fix is a design change through the proper process, not a bypass.

Safety

Control panels contain voltages that can kill, stored energy in drives and capacitors, and machines that can start unexpectedly. Everything in this course is practised on paper or in simulators. Working in real panels requires employer training, supervision by a qualified person, lockout/tagout and the arc-flash practices in NFPA 70E (https://www.nfpa.org/codes-and-standards/nfpa-70e-standard-development/70e), and for electricians a registered apprenticeship and state or local licence. Checked on 2026-10-23.

Recap

• Risk assessment, then design out hazards, then guard, then inform.
• Guards: fixed, interlocked (with locking if motion coasts), adjustable.
• Light curtain safety distance depends on stopping time.
• E-stops: latching, hard-wired, Category 0 or 1, complementary to guarding.
• Safety relays: dual channel, redundant monitored outputs, feedback loop.
• Lockout, not E-stop, for maintenance.

Try it

Open the Do tab and take the quiz on guarding and safety relays.