TSSA G3 basic electricity practice questions (Module 5).

Five free practice questions on basic electricity: module 5 of the nine modules of Ontario's Gas Technician 3 (G3) certificate exam, which TSSA runs. The exam is all multiple choice; these have four options and one correct answer, as every question in our bank does. Answer each one before opening the explanation. No account, no email.

01The questions

1. Procedural · Safe working techniques: de-energize, isolate, prove dead, lock and tag

   In a small apartment building's boiler room, the supervising G.2 has locked and tagged the breaker feeding a boiler's 120 V circuit. The G.3 on the job will also reach into the boiler's control cabinet. What lockout practice applies to the G.3?

   1. The G.3 relies on the G.2's lock, since the G.2 supervises the job
   2. The G.3 holds the key to the G.2's lock while working in the cabinet
   3. The G.3 tapes the breaker handle off and writes a name on the tape
   4. The G.3 applies a personal lock and tag to the same breaker
   Show the answer

   D. Correct. Each worker exposed to the hazard puts a personal lock and tag on the isolating device (with a multi-lock hasp when needed), so the breaker cannot be switched on until every worker has removed their own lock.

   Why not the others

   A. A supervisor's lock protects the supervisor. If the G.2 removes it, for example to test, the G.3 is unprotected; each worker exposed to the hazard applies a personal lock.

   B. Holding someone else's key does not stop that person, or anyone with a spare key, from re-energizing the circuit. Each worker places a personal lock and tag on the isolating device.

   C. Tape and a written name are a warning only: anyone can peel them off. A lock and tag on the isolating device are what keep the circuit off while the G.3 works.

2. Procedural · Ohm's law

   Once hot, a furnace's 120 V hot-surface igniter draws a steady 1.25 A. What is its hot resistance?

   1. 0.010 Ω
   2. 19.2 Ω
   3. 96 Ω
   4. 150 Ω
   Show the answer

   C. Correct. R = V ÷ I = 120 V ÷ 1.25 A = 96 Ω. An igniter's resistance changes as it heats, so a hot value found this way will not match a cold ohmmeter reading; use the manufacturer's stated range for the cold check.

   Why not the others

   A. This divides current by voltage: 1.25 ÷ 120 = 0.010. Resistance is voltage divided by current: 120 ÷ 1.25 = 96 Ω.

   B. This uses the 24 V control voltage: 24 ÷ 1.25 = 19.2 Ω. The igniter is powered at 120 V, so R = 120 ÷ 1.25 = 96 Ω.

   D. This multiplies: 120 × 1.25 = 150, which is the igniter's power in watts, not its resistance. R = V ÷ I = 120 ÷ 1.25 = 96 Ω.

3. Procedural · Parallel circuits

   During ignition, a furnace's 120 V inducer motor draws 0.9 A and its hot-surface igniter draws 3.1 A, both connected in parallel across the supply. Treating both loads as resistive, what current flows in the furnace's supply conductor?

   1. 0.70 A
   2. 2.0 A
   3. 3.1 A
   4. 4.0 A
   Show the answer

   D. Correct. In a parallel circuit the total current is the sum of the branch currents: 0.9 + 3.1 = 4.0 A, while each branch gets the full 120 V. Each added load in parallel increases the supply current.

   Why not the others

   A. This applies the parallel-resistance formula to the currents: 1 ÷ (1/0.9 + 1/3.1) = 0.70. Branch currents in parallel simply add: 0.9 + 3.1 = 4.0 A.

   B. This averages the two branch currents. The supply conductor carries the current of every branch at once: 0.9 + 3.1 = 4.0 A.

   C. This is the igniter's branch alone. The inducer's 0.9 A also returns through the same supply conductor, so the total is 4.0 A.

4. Recall · Induction: a changing magnetic field induces a voltage

   A furnace's control transformer has no electrical connection between its 120 V primary winding and its 24 V secondary winding. How does energy reach the secondary?

   1. The primary's changing field in the core induces a secondary voltage
   2. Electrons pass from the primary to the secondary through the iron core
   3. The core stores charge from the primary and releases it to the secondary
   4. The windings share the 120 V neutral, which carries the current across
   Show the answer

   A. Correct. Alternating current in the primary sets up a magnetic field in the iron core that keeps changing. A changing field through the secondary turns induces a voltage in them, and the ratio of the turns sets how large that voltage is.

   Why not the others

   B. The windings are insulated from the core and from each other; no current crosses. The core only carries magnetic flux from one winding to the other.

   C. Storing charge is what a capacitor does. A transformer core carries a changing magnetic field, and it is the change that induces the secondary voltage.

   D. The primary and secondary are separate windings with no conductor shared between them; even where one side of the secondary is bonded to the cabinet, no current passes from the primary to the secondary through a wire. Energy crosses only by the changing magnetic field in the core.

5. Procedural · Transformers: step-down to 24 V control circuits, turns ratio, VA rating

   A replacement 120 V to 24 V control transformer has 700 turns on its primary winding. How many turns does its secondary winding have?

   1. 29 turns
   2. 140 turns
   3. 3 500 turns
   4. 16 800 turns
   Show the answer

   B. Correct. The turns ratio equals the voltage ratio: 120 ÷ 24 = 5, so the secondary has 700 ÷ 5 = 140 turns. A step-down transformer has fewer turns on the secondary.

   Why not the others

   A. This divides the primary turns by the secondary voltage: 700 ÷ 24 = 29. Turns follow the voltage ratio: 700 × 24 ÷ 120 = 140 turns.

   C. This applies the 5:1 ratio the wrong way, as for a step-up: 700 × 5 = 3 500. Stepping 120 V down to 24 V needs fewer secondary turns: 700 ÷ 5 = 140.

   D. This multiplies the primary turns by the secondary voltage: 700 × 24 = 16 800. The secondary turns are 700 × (24 ÷ 120) = 140.

02Answer key

03What module 5 covers

The electricity a gas technician meets on appliances: shock and arc hazards and safe isolation, what a gas technician may wire, grounding and polarity, voltage, current, resistance and power and Ohm's law, series and parallel circuits and their faults, magnetism, solenoids and relays, alternating and direct current, control transformers, thermocouples and thermopiles, and taking meter readings and reading them to find a fault.

TSSA names module 5 “Introduction to Electricity”. It gives its G3 curriculum only to accredited training providers, so the six objectives below are TicketPrep's own, built from public sources such as college course outlines and Ontario's regulations, not TSSA's. TSSA does not publish the G3 question count or how many exam questions come from each module: its prep guide gives no G3 count and no per-module count, and candidates who fail get a mastery report broken down by module. The right-hand column is how many questions our bank holds on each objective, Standard and Advanced together, not a count from the exam.

04What the questions turn on

Prove it dead at the appliance

Locking off a breaker is not proof. If a tester checked on a live source still reads 120 V at the furnace, another breaker feeds it and must be found. Reversed polarity is a trap of its own: a switch in the grounded conductor leaves the motor and board live with the switch off. A glowing status light means the board has power, whatever the thermostat is set to.

Ohm's law at 24 volts and below

Current is voltage divided by resistance, so a 60 Ω valve coil on 24 V draws 0.40 A. Thermostat wiring has resistance too: 0.50 A through 3.0 Ω of wire drops 1.5 V, leaving 22.5 V at the valve. Milliamperes are turned into amperes before they go into the formula.

Safeties in series, loads in parallel

Safety switches are wired in series with the gas valve, so any one of them opening shuts it. Loads go in parallel: wired in series by mistake, a 40 Ω valve coil and a 20 Ω solenoid share the 24 V and the valve gets only 16 V. In parallel the branch currents add, so a total that is short by one branch's current points to that branch being open.

Volts across, amps in series

Voltage is read across two points with the circuit energized, such as across a gas valve's coil terminals during a call for heat. An ammeter goes in series, with the circuit opened at one terminal. Holding one lead on C and stepping along a string of safety switches, every point before an open reads 24 V and every point after it reads 0 V.

05The other modules

The G3 exam covers nine modules, 1 to 9. Each has its own page of free questions:

The TSSA G3 practice test page describes the full question bank. How these questions are written and checked: how our questions are made. Found a mistake? Tell us and we will fix it.