General Plumbing Knowledge and Safety
Water in, waste out, air behind it
Every plumbing question on this exam traces back to three ideas: potable supply must never be contaminated, drain lines must move solids by gravity, and a trap only holds its seal if the vent lets air in behind the flow.
Drainage slope is a favorite trap. Horizontal drainage pipe 2 1/2 in. and smaller falls at 1/4 in. per foot, while 3 in. and larger may run at 1/8 in. per foot. Know which direction the rule runs - an answer offering 1/8 in./ft for a 1 1/2 in. line is wrong. Trap seals hold between 2 and 4 in. of depth; too shallow and evaporation or pressure breaks the seal, too deep and it is not a permitted trap.
Backflow protection is a safety concept, not an accessory. An air gap is the only truly positive protection; an RPZ, double check, or vacuum breaker is chosen by hazard level and must be tested by a certified backflow assembly tester.
On site, safety questions are blunt. Trenches 5 ft deep or more need protective systems, a ladder within 25 ft of lateral travel, and spoil piles kept back. Confined spaces require atmospheric testing and a rescue plan before entry. Soldering requires a fire watch and cleared combustibles; gas piping requires a leak test with air or inert gas, never a flame, and purging before light-off. Lock out the circuit before working on a disposer or water heater.
Worked example: 32 ft of 3 in. building drain at 1/8 in. per foot needs 32 × 0.125 = 4 in. of fall end to end.