ITI Plumber Interview Questions and Answers

Which pipe materials have you worked with, and where is each used?

CPVC for hot and cold water supply inside buildings, UPVC for cold water and for drainage (soil, waste and rain-water pipes), GI pipes in older water lines, HDPE for underground and long supply lines, and PPR in some hot-water systems. Name only the ones you have joined yourself.

How to say it: Pair each material with where it is used. That is the real question.

How do you join CPVC and UPVC pipes?

With solvent cement: cut the pipe square, deburr and chamfer, clean, apply the correct solvent cement to pipe and fitting, push in fully with a slight turn and hold. Let it cure for the time on the tin before pressure testing. CPVC and UPVC use different cements.

How do you make a threaded GI joint?

Cut the pipe square, thread it with a die set, clean the threads, wrap thread-seal (PTFE) tape in the direction of the thread, and tighten the fitting with pipe wrenches without over-tightening.

Name common pipe fittings.

Elbow (90° and 45°), tee, reducer, coupling or socket, union, nipple, end cap or plug, and valves such as gate, ball and non-return valves. A union lets you remove a section without cutting the pipe.

What is the difference between a gate valve and a ball valve?

A ball valve opens or closes fully with a quarter turn and is quick to operate. A gate valve opens with several turns and is used where the line is fully open or fully shut most of the time.

What is a trap and why is it needed?

A trap (P-trap, S-trap, bottle trap or floor trap) holds a water seal that stops sewer gases and smells coming back into the room. If the seal dries out or is siphoned away, the smell returns.

Why is a vent pipe provided in a drainage system?

To let air into the system so flowing water does not siphon the water out of the traps, and to let sewer gases escape above the roof.

Why must a drain pipe have a slope?

So waste flows by gravity and solids do not settle and block the pipe. Too little slope blocks; too much can leave solids behind as the water runs ahead. Follow the slope on the drawing.

What is the difference between a soil pipe and a waste pipe?

A soil pipe carries discharge from WCs and urinals. A waste pipe carries water from basins, sinks, baths and floor drains.

How do you test a water supply line before closing the wall?

Cap the open ends, fill the line and pressurise it with a hydraulic test pump to the test pressure in the specification, then hold it for the specified time and check the gauge does not drop and no joint weeps.

How to say it: "Test before you close the wall" is the line supervisors want to hear.

Water pressure is low at a tap. What will you check?

The tap aerator or filter for dirt, a partly closed valve, the tank level and float valve, a leak in the line, an undersized pipe, or air lock in the line.

How do you clear a blocked drain?

Start with a plunger, then a drain auger or spring rod from the nearest cleaning eye, and check the trap. Find out why it blocked (slope, grease, or something flushed), so it does not happen again.

Which tools are in your kit?

Pipe wrenches, adjustable spanner, pipe cutter and hacksaw, threading die set, pipe vice, measuring tape, spirit level, PTFE tape, plunger, drain auger and a hydraulic test pump. Name each with one use.

What safety do you follow?

Shut and drain the line before cutting, wear gloves and goggles when using solvent cement and cutting, ventilate when using solvents, take care with ladders and work at height, and never enter a manhole or septic tank without permit, gas testing and a standby person.

What is the usual practical test?

Commonly: cutting and joining CPVC or GI pipe with fittings to a sketch, fixing a tap or a wash-basin connection, or assembling a small line and pressure-testing it for leaks.

How to say it: Measure twice, keep joints square, and test before you say you are done.