313D exam, skill set F: Electrical Components and Controls practice questions.

Eight free practice questions on motors, capacitors, contactors, thermostats, defrost and zoning controls, for skill set F (Service or Maintain Electrical Components) of Ontario's 313D Residential Air Conditioning Systems Mechanic Certificate of Qualification exam, which Skilled Trades Ontario (STO) sets and delivers. 313D is an Ontario trade with its own exam, separate from the 313A Refrigeration and Air Conditioning Systems Mechanic. These are original questions written by TicketPrep from public sources, Ontario regulations and the trade's working practice, not questions from the exam. Each has 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. Recall · Our task F-23

   A lug in a heat pump's outdoor disconnect has worked loose, adding resistance in series with the running unit. What happens at that lug?

   1. Current falls, so the loose lug runs cooler than a tight one.
   2. Current rises with the resistance, so the breaker trips first.
   3. Current barely changes, so the lug itself heats up.
   4. Current holds, so the breaker senses the lug's heat and trips.
   Show the answer

   C. Correct. The joint's extra resistance is tiny beside the load's impedance, so the current hardly changes, but the power turned to heat at the joint is I² × R, all of it in one small spot: 20 A through 0.05 Ω gives 20 W in the lug, enough to char insulation and pit the terminal. Look for discolouration and torque lugs to the maker's value.

   Why not the others

   A. Current falls only slightly, because the joint's resistance is small beside the load's, but the joint's own heat, I² × R, goes from almost nothing in a tight lug to many watts in a loose one.

   B. Adding resistance in series can only lower the current, never raise it, and the breaker responds to current. A loose lug can cook with normal current flowing, which is why it goes unnoticed until the terminal is damaged.

   D. The breaker in the panel trips on the current through it, which stays close to normal; it cannot sense heat at a lug in a disconnect outside. The heat stays at the joint until the terminal or insulation fails.

   Source: STO 313D counselling sheet, skill set F: Service or Maintain Electrical Components (our sub-task F-23.01)

2. Critical · Our task F-24

   A compressor trips its protector a few minutes into each run. Locked out with the leads removed it reads R–C 0.5 Ω, S–C 2.9 Ω and R–S 3.4 Ω, and nothing to ground; the maker's table gives R–C 1.1 Ω and S–C 2.9 Ω at this temperature. What do the readings point to?

   1. A normal reading for a warm winding.
   2. An open circuit in the run winding.
   3. A run winding grounded to the shell.
   4. Shorted turns in the run winding.
   Show the answer

   D. Correct. The run winding reads less than half the maker's value while the start winding matches, and R–S (0.5 + 2.9 = 3.4 Ω) agrees with the two readings, so the meter is reading properly. Turns shorted together cut the winding's resistance and impedance, raise its current and heat it until the protector trips. The compressor is replaced.

   Why not the others

   A. Copper resistance rises as a winding warms, so a warm winding reads higher than the table, not lower, and the table is given for this temperature. A run winding at half its value has shorted turns.

   B. An open winding reads OL, not lower than normal. Here R–C reads 0.5 Ω against the maker's 1.1 Ω: part of the winding is being bypassed by shorted turns.

   C. A grounded winding shows as a reading from a terminal to the shell, and nothing reads to ground here. The low R–C value against the maker's figure points to shorted turns.

   Source: STO 313D counselling sheet, skill set F: Service or Maintain Electrical Components (our sub-task F-24.01)

3. Calculation · Our task F-24

   With no 45 µF run capacitor on the truck, an apprentice has wired a 25 µF and a 20 µF capacitor in series in its place. What capacitance does the compressor now see, to the nearest 0.1 µF?

   1. 5.0 µF
   2. 11.1 µF
   3. 22.5 µF
   4. 45.0 µF
   Show the answer

   B. Correct. Capacitors in series combine like resistors in parallel: 1 ÷ (1/25 + 1/20) = 1 ÷ (0.04 + 0.05) = 1 ÷ 0.09 = 11.1 µF (or 25 × 20 ÷ 45 = 500 ÷ 45 = 11.1 µF). In series the total is always below the smaller capacitor. To make 45 µF the two go in parallel, where values add: 25 + 20 = 45 µF.

   Why not the others

   A. This subtracts the smaller from the larger, 25 − 20 = 5.0 µF, which is no rule for capacitors. In series they combine as the reciprocal of the sum of reciprocals: 1 ÷ (1/25 + 1/20) = 1 ÷ 0.09 = 11.1 µF.

   C. This averages the two, (25 + 20) ÷ 2 = 22.5 µF, as if a series pair settled between its values. In series the total falls below the smaller capacitor: 1 ÷ (1/25 + 1/20) = 11.1 µF.

   D. This adds them as if they were resistors in series: 25 + 20 = 45 µF. Capacitors add only in parallel; in series they combine as the reciprocal of the sum of reciprocals, 11.1 µF, and the start winding is starved of current.

   Source: STO 313D counselling sheet, skill set F: Service or Maintain Electrical Components (our sub-task F-24.02)

4. Procedural · Our task F-25

   A condensing unit on a 240 V single-phase supply uses a single-pole contactor; its other line passes straight through to the compressor and fan on a shunt bar. With the thermostat satisfied and the contactor open, what does a meter read from any compressor terminal to ground?

   1. 0 V
   2. 24 V
   3. 120 V
   4. 240 V
   Show the answer

   C. Correct. The shunt bar keeps one line connected to the compressor and fan at all times. With the contactor open no current flows, so every point on the windings sits at that line's potential, 120 V to ground on a 120/240 V supply. An open single-pole contactor is never isolation: open and lock the disconnect, then test.

   Why not the others

   A. Opening one pole stops the current, but the shunted line is still connected to the compressor and fan, and with no current flowing every point on those windings sits at that line's voltage. Each terminal reads 120 V to ground: the unit is not dead until the disconnect is open and locked.

   B. 24 V is the control circuit's voltage, which drives the contactor coil; it does not appear on the compressor terminals. With the shunted line still connected, every compressor terminal reads 120 V to ground.

   D. 240 V appears between the two lines. One terminal to ground can see only one line's voltage, 120 V on a 120/240 V supply, and here every compressor terminal sits at the shunted line's 120 V.

   Source: STO 313D counselling sheet, skill set F: Service or Maintain Electrical Components (our sub-task F-25.01)

5. Recall · Our task F-26

   A single-stage air-to-air heat pump with heat strips is wired to a conventional heat-pump thermostat. On a first-stage heating call, which thermostat output starts the compressor?

   1. W1, the output for the first stage of heat.
   2. Y, the output for heating and for cooling.
   3. W2, the output for the second stage of heat.
   4. E, the output for the emergency heat mode.
   Show the answer

   B. Correct. On a heat pump the compressor is the first stage of heat as well as of cooling, so Y starts it in both modes; the reversing-valve output (O or B, as the maker requires) decides which way the refrigerant flows. The strips come on from the auxiliary output as a later stage.

   Why not the others

   A. On a heat-pump thermostat the first stage of heat is the compressor, called by Y. A W output there calls the backup heat; wiring the compressor to it would leave the heat pump off in cooling.

   C. W2 (AUX) is the backup-heat stage: it brings the strips on with the heat pump when the room keeps falling behind. It does not start the compressor.

   D. E brings on the backup heat when the user selects emergency heat, with the compressor held off. It is not used on a normal first-stage call.

   Source: STO 313D counselling sheet, skill set F: Service or Maintain Electrical Components (our sub-task F-26.01)

6. Procedural · Our task F-26

   A house has two condensing units, one for each floor. After every power outage both compressors start at the same moment and the lights dim hard. Each unit's board has a field-set delay that holds its compressor off for a chosen time after power returns. What is done?

   1. Lengthen both units' minimum off time by the same amount.
   2. Set the two units' power-up delays to different lengths.
   3. Set both units' power-up delays to their shortest setting.
   4. Fit a hard-start kit to each compressor to shorten its start.
   Show the answer

   B. Correct. With different delays the compressors start one after the other, so their starting currents never add on the house supply. Record the settings on each unit for the next service call.

   Why not the others

   A. After an outage both boards start counting at the same moment, so equal off-time settings still release both compressors together. Only different delays separate the starts.

   C. Equal delays, short or long, still start both compressors together. The fix is to make the delays differ.

   D. A start kit helps a compressor that struggles to start; both compressors would still draw their starting current at the same instant. Staggering the power-up delays separates them.

   Source: STO 313D counselling sheet, skill set F: Service or Maintain Electrical Components (our sub-task F-26.02)

7. Recall · Our task F-26

   A heat pump's time-temperature defrost board has started a defrost. What ends it?

   1. The run timer reaching the next interval it was set to.
   2. The suction pressure falling to the low-pressure switch cut-out.
   3. The outdoor air sensor rising above freezing, or the time limit.
   4. The coil sensor reaching its terminate point, or the time limit.
   Show the answer

   D. Correct. Once in defrost the board watches the outdoor coil sensor and returns to heating when the coil reaches its terminate temperature, meaning the frost is gone; a maximum defrost time ends it if the sensor never gets there. Both settings are the maker's.

   Why not the others

   A. The run timer counts heating run time to decide when a defrost may start. It does not end one.

   B. A low-pressure trip stops the compressor as a fault, not as the normal end of a defrost. Termination comes from coil temperature or the time limit.

   C. Outdoor air does not warm during a defrost; the coil does. Termination is read from the coil sensor.

   Source: STO 313D counselling sheet, skill set F: Service or Maintain Electrical Components (our sub-task F-26.03)

8. Recall · Our task F-26

   A zone damper's actuator is the three-wire, power-open and power-close type, with no return spring. If the zone panel loses power while that damper is halfway through its stroke, where does the damper end up?

   1. Fully open, so that no zone is starved of air.
   2. Fully closed, so that the coil is protected from it.
   3. At the end stop its last command was driving it to.
   4. Where it was at the moment the power was lost.
   Show the answer

   D. Correct. A power-open, power-close actuator is driven both ways and has no spring, so it holds its last position when power is removed. A spring-return (two-wire) actuator is the type that goes to a set position; check which type a zone uses before judging a damper found part-open.

   Why not the others

   A. Going fully open on power loss is what a spring-open actuator does. With no spring, nothing drives this damper once its power is gone.

   B. Spring-closed actuators fail shut; this one has no spring. Without power it simply stops.

   C. The motor moves only while one of its drive wires is powered. When the panel loses power the motor stops at once and does not finish the stroke.

   Source: STO 313D counselling sheet, skill set F: Service or Maintain Electrical Components (our sub-task F-26.04)

02Answer key

03What skill set F covers

Skill set F is the electrical side of residential cooling and heat-pump systems. Our six tasks for it are applying electrical fundamentals and testing safely; servicing motors and their components; servicing contactors, relays, transformers and protective devices; servicing controls and control circuits; servicing electronic devices and controls; and repairing wiring and servicing electrical accessories. Expect Ohm's law at a loose terminal, compressor winding resistance, capacitor arithmetic, the voltage left on a unit by a single-pole contactor, heat-pump thermostat terminals, defrost boards, staggered start delays and zone damper actuators. Where a question turns on the Ontario Electrical Safety Code, the code is cited by name and the requirement explained in our own words; no code text is reproduced.

STO's 313D exam counselling sheet lists six weighted skill sets and gives Service or Maintain Electrical Components 20 per cent of the exam, with a pass mark of 70 per cent. The sheet's body says “Updated as of December 5, 2016” and follows the older training standard, not the one in force since December 2023, so its weights may not match today's exam. STO publishes no question count for the 313D exam, only that its exams have between 90 and 150 questions. Our mock is 130 questions in 4 hours, and its draw of 26 questions from skill set F is ours, worked out from the sheet's percentages. The six tasks below and the right-hand column are TicketPrep's own grouping: the column is how many questions our bank holds on each task, Standard and Advanced together, not a count from the exam.

04What the questions turn on

Resistance where it does not belong, and where it is missing

A loose lug adds a resistance so small beside the load that the current barely changes, but the power turned to heat at that one spot is I² × R, enough to char insulation. On a compressor, compare each winding's resistance with the maker's table and check that R–S equals R–C plus S–C: a run winding reading well under its rated value while the start winding matches points to shorted turns.

Capacitors and contactors

Capacitors in series combine like resistors in parallel, so the total is always below the smaller one; to make a larger value from two, they go in parallel, where values add. A single-pole contactor opens only one line, so with the thermostat satisfied the compressor terminals still sit at 120 V to ground on a 120/240 V supply. An open contactor is never isolation: open and lock the disconnect, then test.

Heat-pump control logic

On a heat pump the compressor is the first stage of heat as well as of cooling, so the thermostat's Y output starts it in both modes, the O or B output sets the reversing valve, and the strips come on from the auxiliary output as a later stage. A time-temperature defrost board ends defrost when the outdoor coil sensor reaches its terminate temperature, or when the maximum defrost time runs out.

Start delays and damper actuators

Two condensing units on one house supply that restart together after every outage are separated by setting their power-up delays to different lengths, so their starting currents never add. On zoned systems, a three-wire power-open, power-close damper actuator has no spring and holds wherever it was when power was lost; a spring-return actuator is the type that goes to a set position.

05The other skill sets

STO publishes no minimum score for any one skill set, only the 70 per cent pass mark on the whole exam, so every skill set's marks count toward it. Each of the six skill sets has its own page of free questions:

The 313D practice exam page describes the full question bank. Working toward the full-scope trade as well? See the 313A practice exam; the two trades share their first-level training. More free questions for every exam we cover: free practice questions. How these questions are written and checked: how our questions are made. Found a mistake? Tell us and we will fix it. TicketPrep is not affiliated with Skilled Trades Ontario or CSA Group. CSA B52, CSA F280 and the Ontario Electrical Safety Code are cited by name only and never reproduced; Ontario regulations such as O. Reg. 875/21 and O. Reg. 463/10 are quoted only briefly.