Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Notes on this paper
National Exams — December 2019 — 11-CS-2 Engineering in Society: Health and Safety. Closed book. Do any five of the seven questions (each 10 marks). Full essay-type answers are expected; sketches may support the explanations. Complete answers to all seven questions follow.
Note on the reference charts. Questions 2 and 7 depend on the two attachment pages: page 6 is the Brauer/Dalziel chart of the effects of 60 Hz AC current on a 150 lb (68 kg) man, and page 7 is the Fine risk-assessment nomograph. The nomograph prints only the qualitative descriptors (no numbers appear on its Likelihood, Exposure or Consequence axes), so the scale points used below (Likelihood 10 / 6 / 3 / 1 / 0.5 / 0.1, Exposure 10 / 6 / 3 / 2 / 1 / 0.5, Consequence 100 / 40 / 15 / 7 / 3 / 1) are the standard Fine–Kinney values that the chart's own axis spacing reproduces. Selecting a descriptor for each activity is a matter of judgement; the choices are stated explicitly below and reasonable justified alternatives are equally acceptable, as the paper's Note 8 permits.
(a) The Truck-Fire Scenario (Health 1, Flammability 3, Instability 0)
(i) Likely contents — a flammable liquid. Flammability = 3 means it ignites under almost all ambient conditions (flash point below ~38 °C); with only a slight health hazard (1), stable (0) and no special hazard, it is most likely a common flammable liquid/fuel or solvent such as gasoline, alcohol or acetone.
(ii) Firefighter action — treat as a Class B flammable-liquid fire: approach from upwind, establish an exclusion zone and evacuate; do not use a water jet (it spreads burning liquid); control/eliminate ignition sources; contain the spill and keep it from drains; cool exposed tanks/structures; identify the material from shipping papers before committing.
(iii) Protective clothing — full firefighting turnout gear with self-contained breathing apparatus (SCBA) to protect against heat and combustion products.
(iv) Extinguishing agent — foam is best (blankets the liquid surface, excluding oxygen and suppressing vapour); dry chemical or CO₂ are alternatives. Not a water jet.
(b) Wet versus Dry Sprinkler Systems
(i) Difference and mode of operation.
Wet system — the pipes are always filled with water under pressure, right up to closed sprinkler heads. Each head is held shut by a fusible link or a frangible glass bulb; when the ceiling temperature at that head reaches its rating the link melts (or the bulb bursts), the head opens and water discharges immediately over that head alone. Only the heads actually heated by the fire operate.
Dry system — the pipes downstream of the dry-pipe valve hold pressurized air or nitrogen instead of water, and that air pressure is what holds the dry-pipe valve closed against the water supply. When a head opens, the air escapes, the air pressure falls, the dry-pipe valve trips open and water then travels down the pipe to the open head — so there is a short delay (typically limited to about 60 s to the inspector's test connection) before water reaches the fire. The system is otherwise identical: closed heads, each opening only when heated.
(ii) Why one or the other is specified, and typical applications. The governing question is whether the piping can be kept above freezing.
A wet system is specified wherever the space is heated and kept above freezing. It is the simpler, cheaper and more reliable choice (fewer components, no trip valve, no air compressor) and it puts water on the fire without delay. Typical applications: offices, schools, hospitals, retail and most heated industrial occupancies.
A dry system is specified only where a wet system cannot be used, i.e. where the piping is exposed to freezing temperatures — water standing in the pipes would freeze, burst them and leave the building unprotected. Typical applications: unheated warehouses, loading docks, parking garages, attics, cold-storage rooms and exposed canopies. The penalty is the delay in water delivery, the larger design area that NFPA 13 requires to compensate for it, and the added maintenance of the air supply and trip valve.
