Industrial safety and accident prevention

Why it matters

Every serious accident costs a life or a livelihood, stops production, brings legal action under the Factories Act and damages a firm's reputation. Most accidents are preventable, and the engineer who designs the layout, machines and methods is in the best position to design the hazards out. Plants also report safety performance with standard rates, so you must be able to calculate and interpret them.

Key ideas

Definitions. A hazard is a source of potential harm (a rotating shaft, a solvent, a height). Risk combines the likelihood of harm and its severity. An accident is an unplanned event that interrupts work and may cause injury or damage; a near miss is one that could have but did not. A lost-time injury keeps the worker off work beyond the day or shift of the accident.

Causes of accidents

• Unsafe acts: removing guards, operating without authority, not using PPE, horseplay, unsafe lifting, working on live equipment.
• Unsafe conditions: unguarded machinery, poor housekeeping, bad lighting, slippery floors, defective tools, poor ventilation.
• Heinrich's studies attributed most accidents to unsafe acts (often quoted as 88 % unsafe acts, 10 % unsafe conditions, 2 % unavoidable); modern thinking stresses that unsafe acts usually have management and design causes behind them.
• Domino theory (Heinrich): ancestry and social environment, fault of the person, unsafe act or condition, accident, injury. Remove the central domino (the unsafe act or condition) and the chain is broken.
• Accident pyramid: for each major injury there are many minor injuries and far more no-injury incidents (Heinrich's ratio 1 : 29 : 300). Reporting and acting on near misses prevents the serious accidents.

Hierarchy of controls (most to least effective)

1. Elimination: remove the hazard (redesign so the task is not needed).
2. Substitution: use something less hazardous (water-based instead of solvent paint).
3. Engineering controls: guards, interlocks, light curtains, two-hand controls, local exhaust ventilation, enclosures.
4. Administrative controls: procedures, training, permits to work, lockout–tagout, job rotation, signage, housekeeping (5S).
5. Personal protective equipment: helmets, goggles, gloves, ear protection, safety shoes, harnesses; the last line of defence.

Hazard analysis tools: job safety analysis (JSA), checklists and inspections, HAZOP for process plants, failure mode and effects analysis (FMEA), fault tree analysis, and risk matrices that rank risk = likelihood × severity.

Machine guarding. Fixed guards, interlocked guards (machine cannot run while open), adjustable and self-adjusting guards, and devices such as presence-sensing light curtains and two-hand controls. Guards must not create new hazards and must allow lubrication and maintenance safely.

Legal framework in India. The Factories Act, 1948 covers fencing of machinery, work on or near moving machinery, hoists and lifts, pressure plant, fire precautions, safety officers (for factories employing 1,000 or more workers, or as notified) and reporting of accidents. The Occupational Safety, Health and Working Conditions Code, 2020 consolidates these laws. IS 3786 gives the method of computing and recording injury rates, including standard time charges for deaths and disabilities; take those values from the standard.

Fire classes (Indian practice): A ordinary combustibles (wood, paper); B flammable liquids; C flammable gases; D combustible metals; electrical fires need non-conducting agents (CO₂, dry powder). Never use water on electrical or class B fires.

Formulas

Frequency rate FR = (Number of lost-time injuries × 10⁶) / Man-hours worked

Severity rate SR = (Man-days lost × 10⁶) / Man-hours worked

• Man-days lost include standard charges for deaths and permanent disabilities from IS 3786 (given in questions).

Frequency–severity index FSI = √(FR × SR / 1000)

Incidence rate IR = (Number of injuries × 1000) / Average number of workers

Man-hours worked = Workers × weeks × hours per week (or from attendance records)

Risk score R = Likelihood × Severity (on the scales of the chosen risk matrix)

Worked examples

Example 1 (standard). A factory with 1,000 workers works 48 weeks of 48 h in a year. There were 12 lost-time injuries with 300 man-days lost. Find FR, SR and FSI.

1. Man-hours = 1,000 × 48 × 48 = 23,04,000 h.
2. FR = 12 × 10⁶ / 23,04,000 = 5.21.
3. SR = 300 × 10⁶ / 23,04,000 = 130.2.
4. FSI = √(5.21 × 130.2 / 1000) = √0.678 = 0.82.

Example 2 (GATE level). Year 1: 18 lakh man-hours, 9 lost-time injuries, 270 man-days lost. In Year 2, after machine guarding was improved: 20 lakh man-hours, 5 lost-time injuries with 120 man-days lost, plus one of them caused a permanent partial disability charged at 1,200 man-days (given from the standard). Compare FR and SR.

1. Year 1: FR = 9 × 10⁶ / 18,00,000 = 5.0; SR = 270 × 10⁶ / 18,00,000 = 150.
2. Year 2: FR = 5 × 10⁶ / 20,00,000 = 2.5; SR = (120 + 1,200) × 10⁶ / 20,00,000 = 660.
3. FR fell by 50 % (5.0 → 2.5), but SR rose 340 % (150 → 660).
4. Interpretation: guarding cut the number of injuries, but one serious injury dominated the severity. Investigate it as a priority; both rates must be watched, never one alone.

Common mistakes

• Using 10⁵ or 10³ instead of 10⁶ man-hours in FR and SR (check the convention the question uses).
• Counting all injuries in FR when the definition asks for lost-time injuries only.
• Forgetting the standard time charges for deaths and permanent disabilities in SR.
• Treating PPE as the first control; it is the last.
• Judging safety by FR alone; a low FR can hide a high SR.

For GATE PI

Expect NAT questions on frequency rate, severity rate and incidence rate, and one-mark questions on the hierarchy of controls, causes of accidents, the domino theory and accident pyramid, and fire classes. Practise the man-hour calculation and the 10⁶ factor until they are automatic.

Quick check

1. 4 lost-time injuries in 4,00,000 man-hours. FR?
2. 6 man-days lost in 2,50,000 man-hours. SR?
3. Which control comes immediately after substitution in the hierarchy?
4. What does Heinrich's 1 : 29 : 300 ratio describe?

Answers: 1. 10; 2. 24; 3. Engineering controls; 4. For every major injury, about 29 minor injuries and 300 no-injury incidents.