Up front, so there's no confusion: this isn't a how-to for designing safety circuits. Safety is the one area where a "close enough" answer can hurt somebody, and the right answer depends on your actual machine, your actual hazards and a proper risk assessment. What I can do is lay out how it works and clear up a few mix-ups I hear a lot, so you know what to ask for.
Everything starts with a risk assessment. You look at the machine, figure out what could hurt someone and how (pinch points, moving parts, stored energy, heat, flying debris), estimate how bad and how likely each one is, and then reduce the risk. The order matters. First, design the hazard out if you can. Then guard it with fixed guards, interlocked doors, light curtains or scanners. Then warnings, training, procedures and PPE for whatever's left. Warnings are the last resort, not the plan.
There's a whole family of standards behind this, and it helps to know the names when you're talking to an engineer or a vendor. ISO 12100 covers risk assessment and risk reduction in general, and ANSI B11.0 is the US counterpart. ISO 13849-1 (current edition 2023) and IEC 62061 cover how reliable the safety-related parts of the control system have to be. NFPA 79 covers the electrical side of industrial machines in the US. For robots, ISO 10218 was revised in 2025, and the US version is ANSI/A3 R15.06-2025. And OSHA has the rules that are actually law in the US, including 1910.212 for machine guarding and 1910.147 for lockout/tagout.
Now the mix-ups.
"We have an E-stop, so it's safe." An emergency stop is a backup, not a safeguard. It only helps if somebody sees trouble coming and can reach the button in time. Guards and safety devices are what keep people out of the hazard in the first place.
"It came with guards from the builder, so we're covered." Maybe when it shipped. But if the machine has been modified, moved into a different layout, sped up, or had guards taken off "just for a minute" years ago, the original assessment may not match what's on your floor today. Changes to a machine usually mean the risk assessment needs another look.
"I'll just handle the safety in the PLC." A standard PLC isn't designed to be a safety device. Safety functions belong in safety-rated hardware, like safety relays, safety controllers or safety PLCs, designed and validated to the level the risk assessment calls for.
"Hit the E-stop and it's safe to reach in." Not for maintenance. Stopping a machine isn't the same as isolating its energy. Electrical, pneumatic, hydraulic, gravity and stored energy can all still hurt someone. Servicing almost always calls for lockout/tagout.
"We've never had an accident on that machine." That's luck, not a safety system.
So when should you call an engineer? Pretty much any time you're adding or changing a safeguard, modifying a machine, building a new cell, integrating a robot, or you're just not sure whether what you have is enough. Bring whatever you've got: the machine, the manuals, any existing risk assessment and the changes you're planning. Someone qualified, looking at the real machine, is the only way to get this right.
If that's where you are, that's exactly what the "Talk to Arpan" button is for.