Clearing a Bird's Nest on Second Shift
During second shift an operator finds a bird's nest at the torch of a robotic GMAW cell running 350 A at 28 V. To clear it, he turns the mode selector to maintenance, which mutes the 2 m light curtain and caps robot speed at 250 mm/s, then walks into the cell while the robot is still in automatic. The fume extraction is switched off because the hood was in the way, and he is wearing safety glasses but no arc-rated clothing. The cell has an interlocked gate that was bypassed last week because the switch was misaligned. The plant's written policy requires lockout/tagout for entry beyond the curtain, and the robot controller has a teach-pendant enabling switch that is not being used.
1. Identify the safety violations in this scenario and rank them by severity, explaining the reasoning for the ranking.
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Highest: the bypassed interlocked gate. A defeated interlock removes the last engineering barrier that stops the robot regardless of human behaviour. It is a latent fault that turns every other error into a potential fatality, so it ranks first.
Second: entering the muted curtain zone with the robot in automatic. Maintenance mode reduced speed to 250 mm/s but did not disable the drives, and the enabling switch was not held. At 250 mm/s the robot can still reach the torch path before the operator reacts. Reduced speed is not a substitute for a hold-to-run device in the operator's hand.
Third: no lockout/tagout, which is a direct violation of the written policy and is the control that would have made the entry safe.
Fourth: fume extraction off and no arc-rated clothing. If an arc is struck, 350 A at 28 V is a serious arc-flash and UV hazard, and welding fume in an enclosed cell accumulates quickly. These are serious but secondary to the motion hazards.
2. State what must happen before the operator enters the cell, and whether maintenance mode alone is ever acceptable.
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The preferred sequence is: stop the program at a safe point, remove the robot from automatic, isolate and lock out the drives at the disconnect, verify zero energy, open the gate, and then enter. If the task genuinely requires the robot powered, the robot must be in teach mode with the operator holding the enabling switch, one person only in the cell, the light curtain muted only by that person, and a risk assessment that specifically permits the reduced-speed entry.
Maintenance mode alone is not enough because it changes speed but not the source of motion. It is acceptable only where the risk assessment says the mode gives reduced speed and restricted space, the entry is by a single trained person holding the enabling device, and the person's own key controls the mute. Here none of those conditions were met.
Before entry, also restore fume extraction, put on arc-rated clothing and a welding helmet if any arc is possible, and correct and test the misaligned gate switch.
3. Recommend engineering and administrative changes that would prevent the bypass from recurring.
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Engineering controls
- Replace the misaligned switch with a non-contact, coded interlock that cannot be defeated by simple alignment and that the controller monitors for faults.
- Add a physical service position outside the curtain, for example a torch park station, so the gun can be cleaned without entry.
- Add a bird's-nest detection feature to the wire-feed drive and a wire break sensor, so the fault stops the cell and alarms before the operator discovers it.
- Provide a slide or lift gate with its own interlock rather than relying only on the light curtain for entry.
Administrative controls
- Rewrite the entry procedure with a permit and clear LOTO steps, and train both shifts on it.
- Require a functional test of interlocks at each shift start, recorded on a checklist.
- Review maintenance-mode use with a documented risk assessment and restrict the mute key to one named person per shift.
- Audit bypasses weekly; any interlock defeat should be a stop-work condition, not a workaround.