Hazard Note Before the Next Practical Class
A government higher secondary school workshop has 30 students in a 12 m × 8 m room. It contains a 250 mm tilting-arbour circular saw (1.5 kW), a 300 mm thickness planer (2.2 kW) and two 150 mm random orbital sanders. The saw has lost its riving knife and its guard is tied back with wire; the planer's dust hood is disconnected and the chip outlet blows into the room. Both machines are fed from one 15 A socket through two extension boards joined together, and the earth wire at the socket is loose. Noise at the operator's ear measures 95 dB(A).
Students wear open sandals and loose kurtas; one student is being treated for asthma. Oily rags used for French polish are kept in a cardboard box in the corner, and the CO₂ extinguisher by the door carries a test tag dated March 2019. There is no eyewash and the only water tap is outside the room. The instructor asks you to write a hazard note before the next practical class.
1. Rank the hazards in this workshop by risk and match each to the level of the hierarchy of controls you would apply. State which items must stop work immediately and why.
Read the worked answer
Risk is likelihood × severity, so each item has to be judged on both.
Stop work immediately.
- Guard tied back, no riving knife. Contact with a 250 mm blade at several thousand rpm is fatal or amputating, and without a riving knife a short offcut will be thrown back in kickback. Engineering controls: refit the crown guard and a riving knife set 2–3 mm below the tooth tips and within 8 mm of the blade, add anti-kickback fingers and a short fence. No sawing until then.
- The wiring. Two machines totalling 3.7 kW on one 15 A socket through two extension boards, with a loose earth, is a fire and electrocution risk. See the separate calculation.
- Oily rags in a cardboard box. Linseed and French-polish rags self-heat and can ignite spontaneously, and the cardboard is fuel sitting underneath them.
High priority. 4. Wood dust. A Group 1 carcinogen, plus a dust-explosion risk in a confined room. The planer blowing chips into the room is an engineering failure, not a housekeeping problem. 5. Noise at 95 dB(A). Hearing damage is cumulative and irreversible; at 95 dB the permitted unprotected exposure is roughly 4 hours on a 3 dB exchange rate, and a double practical period with two machines running exceeds that.
Medium priority. Open sandals and loose kurtas (closed leather shoes, aprons, no loose sleeves near rotating parts), the extinguisher with a 2019 tag, and no eyewash.
Hierarchy applied. Eliminate - do not run the planer until the hood is reconnected. Substitute - water-based finishes instead of solvent polish in the teaching room. Engineering - guards, extraction, separate circuits, sound enclosure. Administrative - supervised machine use, rotation, training records, a written rule that no student operates a machine without the instructor present. PPE last: dust masks, goggles, ear muffs and shoes are the final layer, never the answer on their own.
2. Work out the electrical demand of the saw and planer and comment on the 15 A socket, the daisy-chained extension boards and the loose earth. State what must be installed before the machines are used again.
Read the worked answer
Current demand. Saw 1.5 kW + planer 2.2 kW = 3.7 kW. At 230 V the current is 3,700 ÷ 230 ≈ 16 A at unity power factor; with a realistic motor power factor of about 0.8 it is nearer 20 A, and an induction motor draws 3–6 times its running current for a second or two at start-up. A 15 A socket is therefore at or beyond its rating before the sanders are added, which is why the extension boards warm up.
Extension boards. Two boards joined in series add plug-and-socket resistance and voltage drop, lengthen the cable run and, if a drum is left coiled, trap heat. This is a fire waiting for a reason and must not be used as permanent workshop wiring.
Loose earth. This is the most dangerous single fault. If a live conductor touches the metal frame while the earth is open, the frame sits at mains potential and the operator becomes the path to earth. An RCCB will not protect a person if the earth path is not sound.
Required before further use. A dedicated circuit for each machine, or one circuit sized for the combined load with a 20–25 A MCB; a 30 mA RCCB on the workshop circuits; fixed wiring in conduit with correctly sized copper conductors and a verified earth electrode or earth bus, with a recorded earth continuity test; no daisy-chained extension boards; a lockable isolator beside each machine; and a written test certificate from a qualified electrician before the class uses the room.
3. Set out the dust controls - health effects, extraction requirements, housekeeping and PPE - and address the asthmatic student and the box of oily rags.
Read the worked answer
Health effects. Wood dust is classified as a Group 1 carcinogen, causing nasal and sinus cancer, and it also causes asthma, dermatitis and eye irritation; hardwood dust such as shisham is the greater risk.
Explosion risk. A settled layer of fine dust disturbed into a cloud at roughly 40–50 g/m³ in a confined room can ignite from a spark or a hot bearing, and a first small explosion can loft the settled dust for a much larger second one.
Extraction. Reconnect the planer hood and connect the saw's extraction port to a dust collector sited outside the room. Design for a duct velocity of 15–20 m/s in horizontal runs so dust does not settle, and a capture velocity of about 0.5–1 m/s at the hood. Keep the dust layer below about 1 mm.
Housekeeping. No dry sweeping and no compressed air, which simply puts the dust back into the air - use a brush and a vacuum with a fine filter, and clean at the end of every session rather than at the end of the week. No smoking or grinding near the collector.
Asthmatic student. With 95 dB(A) and airborne dust, this student should not be in the machine room while the saw and planer run. A dust mask is not adequate protection for someone with existing respiratory disease. Put a written individual plan in place: hand-tool and finishing work in a ventilated area, or drawing and estimation, and record the decision so it is not left to whoever is on duty.
Oily rags. Spread them flat to dry outdoors, then keep them in a metal bin with a self-closing lid, or switch to water-based finishes. The CO₂ extinguisher is unsuitable for a person on fire and its 2019 tag means it must be re-tested or replaced; fit an ABC dry-powder extinguisher, provide an eyewash or at least clean water at the tap inside the room, and stock a first-aid box.