Trade safety, hand tools, measuring instruments and workshop practice
What this domain tests
AITT questions here are practical: which extinguisher for a live electrical fire, how to store an R290 cylinder, which tool deburrs a copper tube, and what a vernier actually reads. These marks are easy to secure and easy to lose through careless wording.
Safety rules
- Live electrical fire: isolate, then use CO2 or dry powder. Water and foam conduct.
- Oxy-acetylene brazing: flashback arrestors on both regulators, red hose for acetylene, no oil or grease on oxygen fittings, acetylene gauge never above 1 bar, cylinders upright and secured.
- Refrigerant cylinders: upright, valve cap on, below 50 °C, never flame-heated, never filled above 80 % capacity, never vented.
- Flammable refrigerants R290, R600a and A2L R32: no spark, ventilate the area, use an intrinsically safe leak detector.
Tools and instruments
Tube cutter then reamer to deburr, flaring block with a 45° SAE cone, swaging punch, spring bender for soft copper, torque wrench for flare nuts, tube brush, feeler gauge, dial gauge, steel rule, vernier caliper, outside micrometer. Least count: vernier = 1 MSD minus 1 VSD; micrometer = pitch divided by thimble divisions.
Worked example
A vernier has 0.5 mm main-scale divisions and 50 vernier divisions, so LC = 0.5 / 50 = 0.02 mm. The main scale shows 12 mm and vernier line 24 coincides: reading = 12 + 24 x 0.02 = 12.48 mm. With the jaws closed the instrument reads 0.04 mm, so the true size is 12.48 minus 0.04 = 12.44 mm.
Traps
Hacksawing copper tube and leaving swarf inside the circuit; over-tightening flares until they split; bending hard-drawn tube without annealing; forgetting zero error; trusting cylinder colour instead of the label.