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Machine Safety: Risk Assessment, Guarding and Safety PLCs
Machine safety starts with a documented risk assessment that identifies each hazard, estimates its risk and selects measures in order: eliminate the hazard by design, then guard it, then warn and train. Where safety depends on the control system, ISO 13849-1 sets the required performance level, and the safety circuit or safety PLC is designed and verified to meet it.
Published · Last reviewed by Geek Engineering senior engineering review.
The risk assessment comes first
Every safeguarding decision should trace back to a risk assessment. ANSI B11.0 and ISO 12100 describe the method: define the machine's limits and tasks, including maintenance and cleaning, identify hazards, estimate severity and likelihood, then reduce risk and re-assess. OSHA 29 CFR 1910.212 requires machine guarding in general terms; the standards describe how to meet that requirement.
The assessment must include tasks beyond normal production, such as clearing jams, changeovers and maintenance, because that is where many injuries happen.
The hierarchy of risk reduction
- Eliminate or reduce the hazard by design: lower force, lower speed, remove pinch points.
- Guard it: fixed guards, interlocked doors, light curtains, area scanners, two-hand controls.
- Use complementary measures: emergency stops, safe energy isolation (lockout/tagout under OSHA 1910.147).
- Warn and train: labels, procedures and training, which are the least reliable measures and never the only one for a serious hazard.
Performance levels and safety-rated control
The required level comes from the risk assessment, not from preference. The designed circuit is then verified, for example with a calculation tool such as SISTEMA, and validated by testing each safety function.
| Required level | Typical meaning | Typical implementation |
|---|---|---|
| PL a – PL b | Low risk: minor, reversible injury | Single-channel circuit with well-tried components |
| PL c | Moderate risk | Monitored single or dual channel, depending on design |
| PL d | Serious injury possible, common for guard interlocks and light curtains | Dual-channel, monitored safety relay or safety PLC |
| PL e | Serious, irreversible injury with frequent exposure | Dual-channel with high diagnostic coverage, usually a safety PLC |
Safety relay or safety PLC
Safety relays suit machines with a few simple safety functions. A safety PLC suits machines with many zones, muting, safe speed monitoring or functions that change by mode, because the logic is programmable, diagnosable and documented in one place. Robots add their own standards: ANSI/RIA R15.06 and ISO 10218 for industrial robot systems, with additional guidance for collaborative applications.
Key takeaways
- Every safeguard should trace back to a documented risk assessment.
- Design out hazards first; warnings and training come last.
- The risk assessment sets the required performance level.
- Verify the safety circuit by calculation and validate it by test.
Related questions
Is a machine risk assessment required by law?
OSHA requires hazards to be guarded, and the consensus standards OSHA and courts refer to treat a documented risk assessment as the way to show that was done. In practice, a written assessment is expected for new and modified machines.
What is a safety PLC?
A safety PLC is a controller certified for safety functions, with redundant processing and self-diagnostics. It runs safety logic separately from standard logic and is used when a machine has many safety functions or complex modes.
Does modifying a machine require a new risk assessment?
Yes, for any change that affects hazards, tasks or safety functions. A controls upgrade or added automation can change how people interact with the machine, so the assessment and safety validation should be updated.
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