Angle Grinder, Petrol Spill and a Ten-Minute Gap in Supervision
During a first-year practical class, 18 students are working in a bay that is also being used to drain a moped fuel tank. A student is cleaning a rusty bracket with a 100 mm angle grinder. The grinder has a cracked disc and the guard has been removed because it 'gets in the way'; the student is wearing cotton gloves, safety glasses and a loose overall. Three metres away, another student is siphoning petrol from the tank into an open plastic can. The bay has one dry-powder extinguisher near the door, a compressed-air line hissing at 7 bar, and no local exhaust ventilation. The instructor has been called to the office for ten minutes and has told the class to 'carry on'. A small pool of petrol is visible under the moped, and the grinder is throwing sparks towards it.
1. Rank the hazards you would deal with in the first 30 seconds and state exactly what you would stop or isolate first. Justify the ranking.
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The ranking is (1) petrol vapour plus grinding sparks, (2) cracked and unguarded disc, (3) loose cotton gloves and overall, (4) compressed-air line. Petrol is first because the pool and open can are producing vapour, and the grinder is an ignition source; a flash fire can involve the whole bay. My first action would be to stop the grinder at its switch and unplug it, then stop the fuel transfer and move the open can away from the pool without creating sparks. I would open the bay doors, evacuate students, and use the dry-powder extinguisher only if a fire has started. The cracked disc is second because a bonded disc can burst at high speed and throw fragments; the missing guard removes containment. Gloves and overall are third because loose cloth can be pulled into the spindle. The air line is last but should be isolated to prevent whipping and vapour movement.
2. Explain why the cracked disc and the cotton gloves are each unacceptable, and describe the checks and PPE required before grinding resumes.
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A bonded abrasive disc is under tensile stress from centrifugal force. A crack concentrates stress; at high speed the disc can separate and throw pieces with enough energy to injure the operator or bystanders. The guard is not optional: it contains fragments and deflects sparks. Cotton gloves are unacceptable at a rotating tool because the fabric can catch on the spindle or a burr, pulling the hand in; loose overall sleeves are similar. Before restarting: discard the cracked disc, fit a disc rated for the grinder's maximum rpm, check for cracks, chips and expiry, fit the guard with the correct opening, check flanges and spindle thread, run the tool in a safe direction for 30 seconds, and use safety glasses plus a face shield, close-fitting overalls and hearing protection. If it is a vitrified bonded wheel, perform the ring test; do not rely on a visual check alone.
3. The instructor wants to restart work in the same bay within 15 minutes. State whether this is acceptable and list the controls you would require.
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Fifteen minutes is not acceptable unless the petrol has been removed and the bay has been made safe. Controls: stop all ignition sources, including the grinder, compressor and any switches; remove the moped and fuel can to a designated outdoor fuel area; absorb the spill with inert absorbent and dispose of it as hazardous waste; ventilate the bay with doors open and local exhaust; test for vapour if a meter is available; refit the guard and a sound disc; change to close-fitting PPE; place a dry-powder or foam extinguisher within reach; assign one person to supervise fuel transfer and one to hot work. The class should be briefed before restart. If any petrol smell remains, work should move to another bay.