The 5×5 Matrix That Scored a Jam Clearance as Tolerable
At a flour mill in Kano, a 12 m inclined screw conveyor at the base of a silo jams two or three times per shift. Operators lift a bolted guard and clear the jam with a steel rod while the conveyor is still running, because stopping the line costs about 40 minutes of production. In the last eight months two operators have needed treatment for hand lacerations. A newly appointed HSE officer scores the task as likelihood 3 × severity 3 = 9 on the site's 5×5 matrix; the site's own action threshold is 15. The production manager therefore argues that no further controls are justified and that the existing cut-resistant gloves are sufficient. The HSE officer is unsure whether to accept the score or challenge it, and has been given two weeks to close the item on the action tracker.
1. Should the HSE officer accept the score of 9 and close the action? Justify your answer using the evidence in the setup.
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No. The score is indefensible on its own facts, and the argument that 'below 15 means no action' misuses the matrix.
Challenge the likelihood. The task is performed two or three times per shift, every shift. A likelihood rating of 3 implies an occasional event; the realistic rating is 4–5.
Challenge the severity. The credible worst outcome of reaching into a running screw conveyor is entanglement, amputation or a fatality, not a laceration. Severity must be rated on the worst credible consequence, not the outcome that happens to have occurred. A realistic rating is 4–5.
The re-scored risk is roughly 16–25, above the site's own threshold of 15.
Challenge the method. A 5×5 index is a ranking aid, not a decision rule. It is uncalibrated, highly assessor-dependent, and gives false precision. Two lacerations in eight months is a lagging indicator that the real risk is higher than the paper score. The assessment also fails to consider that jam clearing is an unplanned, rushed task with no written procedure and no isolation - the conditions that produce fatalities in silos.
Conclusion. The item stays open. The correct action is to refuse sign-off, re-score with severity against the worst credible outcome, and escalate to the site manager in writing with the injury history attached.
2. Recommend controls in order of the hierarchy, and explain what to do about the 40-minute downtime argument.
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Eliminate / design out: fit a self-clearing or reversing screw, a rotary valve, or a hydraulically operated de-blocking door that clears jams without anyone reaching into the trough. Address the root cause of the jam (material bridging, moisture, over-filling) rather than the symptom.
Engineering: interlocked guarding so the conveyor cannot run with the guard open; a lockable local isolator at the jam point; a reversing function operated from a safe position; a fixed clearing tool so the operator cannot get a body part near the auger.
Administrative: a written lockout/tagout (LOTO) procedure naming who isolates, who verifies, and how the guard is refitted; authorisation and training; a review of the task as a management of change item.
PPE: last line only. Note the trap: heavy cut-resistant gloves near rotating equipment can increase entanglement risk, so gloves are not a substitute for isolation and may themselves be a hazard here.
On downtime: the 40 minutes is a production problem, not a safety justification. Solve it with a surge bin or buffer stock sized to absorb the stoppage, or by scheduling planned clearing, so that the safe method no longer carries a production penalty. If safe clearing really costs 40 minutes each time, that cost belongs in the business case - not in the risk register as a reason to keep the unsafe method.
3. How should the outcome be recorded and communicated, and who should sign it off?
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Recording: update the risk assessment with the original score, the re-scored value, the controls selected, the residual risk and the reason each control was chosen. Name an owner and a review date. Attach the injury history and the LOTO procedure reference. The record must be retrievable and shown to an inspector.
Sign-off: by the site manager or the competent person with authority over production, countersigned by the HSE officer. A control that changes a production task needs production's accountable signature, otherwise it is quietly abandoned at the first busy week.
Communication: toolbox talk at the conveyor in the language the operators actually use, plus a practical demonstration of the isolation and guard refit; briefing to shift supervisors; signage at the jam point; and a check of understanding - ask operators to walk through the new sequence rather than asking 'any questions?'.
Assurance: add a monthly audit of guard interlock function and LOTO compliance, and track leading indicators (isolation permits issued, interlock defects) alongside the lagging injury record. Review the assessment on any further jam, injury or equipment change.