The Warning Label That Said Nothing
A robot cell had a warning sign on the fence. It read: “Warning: Robot. Authorized Personnel Only.” The sign was black and yellow, faded, mounted 2 meters up on the fence. A new operator, who did not speak fluent English, walked past it. He did not know what “authorized” meant. He reached over the fence to clear a jam. The robot was in auto mode. The cell had a warning sign — but the sign did not tell him what to do, what the hazard was, or what would happen. It was decoration. A proper safety label would have shown a pictogram (a robot arm crushing a figure), the word “DANGER” in red, and the instruction “Do not enter — press e-stop first.” Signs that nobody reads are not safety controls.
Safety signage is a weak control (it depends on people reading and understanding). But when it is needed, it must be clear, standardized, and unmissable. Here is how to do it right.
The Three Signal Words: Danger, Warning, Caution
ANSI Z535 (the US signage standard) defines three signal words, and they are not interchangeable:
- DANGER (red): Imminent hazard that will cause death or serious injury. Use for the worst hazards — “Do not enter the robot cell while auto.”
- WARNING (orange): Potential hazard that could cause death or serious injury. Use for “watch your fingers at the nip point.”
- CAUTION (yellow): Potential hazard that may cause minor injury. Use for “hot surface” or “wear eye protection.”
The faded “Warning: Authorized Personnel Only” was the wrong word (it should have been DANGER), the wrong color (it was black/yellow, not red/orange), and the wrong message (it said nothing actionable). A sign that does not tell you the hazard and the consequence is not a sign — it is wallpaper.
The signage rule: Every sign must have: (1) the signal word (DANGER/WARNING/CAUTION) in the correct color, (2) the hazard in plain language, (3) the consequence (“crush injury”), and (4) the action to take (“press e-stop before entering”). If any of the four is missing, the sign is decoration.
Pictograms: Universal Language
Not every worker reads English fluently. The standard uses pictograms (graphic symbols) that convey the hazard without words: a hand being crushed, an electric arc, a falling object. The pictogram is the most important element — a Chinese-speaking operator, a Spanish-speaking operator, or a child intern can all read a picture. The “Authorized Personnel Only” sign had no pictogram. A robot-arm-crushing-figure would have told the story.
Where to Place the Sign
A sign that is not seen is not a sign. Place it:
- At the point of hazard (on the guard, at the gate, near the nip point).
- At eye level (1.4–1.7 m), not 2 meters up.
- On a contrasting background (the standard says a specific layout: white panel with a color header).
- Where the operator looks before acting (not on the back of the fence).
- Large enough to read from 1 meter away.
The Label That Speaks the Operator’s Language
In a multi-language plant, use bilingual signs (English + the local language). But pictograms are the primary language — they transcend translation. If the sign must be read in words, write them in the language the operators actually speak. The “Authorized Personnel Only” sign, in English only, was useless to a Cantonese-speaking worker who had been on the line for a month.
A Signage Audit Checklist
- Walk the plant; list every hazard point.
- For each, is there a sign at the point of hazard?
- Does it use the correct signal word (DANGER/WARNING/CAUTION)?
- Is the color correct (red/orange/yellow header)?
- Does it have a pictogram (graphic)?
- Does it state the hazard and consequence?
- Does it state the action to take?
- Is it at eye level, readable from 1 m?
- Is it in the language operators actually speak?
- Is it clean, not faded, not covered by grime?
The Bottom Line
The faded “Authorized Personnel Only” sign was not a safety control. It was a sign that said nothing to the person who needed it. Every sign must have the right signal word, a pictogram, the hazard and consequence, and the action to take — at eye level, in a language the worker reads. The sign that prevents injury is the one that tells the operator what will hurt them and what to do about it, not the one that says “authorized personnel only.”
