313A safety, tools and trade skills practice questions.

Five free practice questions on safety, tools and trade skills: block A of the Red Seal 313A Refrigeration & AC Systems Mechanic exam, about 13 of its 125 questions. They are in the exam's format: four options, one correct answer. Answer each one before opening the explanation. No account, no email.

01The questions

1. Procedural · Uses charging tools and equipment

   With both hoses open to atmosphere, the low-side compound gauge on your manifold reads 20 kPa (3 psig). What should you do before using it to check superheat on an R-410A system?

   1. Leave it; the offset reflects local atmospheric pressure above sea level.
   2. Subtract 3 psig mentally from every reading taken with it.
   3. Reset the pointer to 0 psig with the gauge's calibration screw and verify it against a P/T reference.
   4. Use the high-side gauge for suction pressure instead; only one gauge needs to be accurate.
   Show the answer

   C. Correct. Gauges are zeroed open to atmosphere using the adjusting screw (the RSOS calls this calibrating by adjusting the zero point), then checked against a known pressure: a cylinder of pure refrigerant at rest at a known temperature should read the saturation pressure from the P/T chart. On R-410A near 40 °F saturated suction, a 3 psig error shifts the saturated suction temperature by about 1.3 °F (0.7 K) and throws the superheat reading off by the same amount. Confirm the manifold and hoses are rated for R-410A pressures as well.

   Why not the others

   A. A gauge measures pressure relative to the local atmosphere, so open to the air it reads 0 psig at any elevation. The 14.7 on the formula sheet converts psig to psia; it never appears as a reading on an open gauge. A 3 psig reading is an error to correct.

   B. A zero offset is not necessarily constant across the scale, and mental corrections get forgotten under a hot rooftop. The gauge has an adjusting screw so it can be set right; use it, then confirm against a reference.

   D. A high-side gauge's scale is too coarse for suction pressures and it usually has no vacuum range. Both gauges are zeroed and checked; superheat needs an accurate low-side reading.

2. Procedural · Performs lock-out, tag-out and isolation procedures

   You have opened the disconnect on a rooftop unit, applied your lock and tag, and are about to open the control panel to change a contactor. What is the required next step?

   1. Test for zero voltage with a meter proven on a live source, then try a start.
   2. Check that the unit's status LED on the control board has gone out.
   3. Have the building operator confirm from the BAS that the unit reads 'off'.
   4. Confirm the disconnect handle is in the OFF position and the padlock is snug.
   Show the answer

   A. Correct. Isolation is not complete until it is verified: prove the meter on a known live source, test every load-side conductor to ground and to each other for zero voltage, re-prove the meter, then attempt a start from the normal controls to confirm nothing can energize. Only then is the zero-energy state established that the RSOS requires.

   Why not the others

   B. A dark LED shows the board lost its low-voltage supply, not that the line-voltage terminals you are about to work near are dead. Verification is done with a proven meter at the point of work.

   C. A BAS point shows the control state, not the electrical state at the panel, and it can be stale or mis-mapped. It is useful for coordinating with the operator, but it never replaces a meter test by the person doing the work.

   D. A handle in the OFF position proves only that the handle moved. Blades can weld or stick, the wrong disconnect can be locked, or a second feed (control power, a heater circuit) can exist. Verification means measuring for absence of voltage, not looking at the handle.

3. Recall · Uses documentation and reference material

   On a pressure-enthalpy (P/E) diagram, what does the net refrigeration effect (NRE) represent?

   1. The enthalpy rejected in the condenser, in BTU/lb.
   2. The enthalpy added by the compressor, in BTU/lb.
   3. The pressure difference between the high and low sides, in psia.
   4. The enthalpy gained by the refrigerant across the evaporator, in BTU/lb.
   Show the answer

   D. Correct. NRE is the enthalpy change from the metering-device outlet to the evaporator outlet, the heat each pound of refrigerant absorbs from the load. On the P/E diagram it is the horizontal length of the evaporation line. Dividing the load in Btuh by the NRE gives the mass flow in lb/h, and the formula sheet's COP entry is written in terms of THR and NRE.

   Why not the others

   A. That is the total heat of rejection (THR): desuperheating, condensing and subcooling combined, which equals NRE plus the heat of compression. THR is larger than NRE; the difference is the compressor's contribution.

   B. That is the heat of compression, the rise in enthalpy from suction to discharge along the compression line. It is work put in, not refrigeration delivered.

   C. The vertical distance on a P/E diagram is pressure and gives the compression ratio, not the refrigeration effect. NRE is read along the horizontal enthalpy axis.

4. Procedural · Uses mentoring techniques

   You are teaching a second-year apprentice to braze a 22 mm (7/8 in) suction line with a nitrogen purge. Which order of steps follows good on-the-job teaching practice?

   1. Demonstrate three joints in silence, then have them copy you on the live line.
   2. Hand them the torch and let them try, then explain what went wrong afterwards.
   3. Explain the theory of capillary flow in detail and let them braze unsupervised the next day.
   4. State the lesson's point, demonstrate while explaining, guide their practice, then give feedback and follow up.
   Show the answer

   D. Correct. The RSOS teaching sequence is: identify the point of the lesson (why the purge and the alloy matter), link it to what they already know, demonstrate the skill while explaining the who-what-where-when-why-how, set up safe guided practice on scrap before the live line, give supportive and corrective feedback, and follow up to confirm the skill sticks and to move them toward independence.

   Why not the others

   A. Showing without explaining leaves out the reasoning (why the purge flow is low, why the heat goes on the fitting) that lets the apprentice adapt to the next joint. Practice should also start on scrap under guidance, not on the system.

   B. Trial and error on a live suction line risks a scaled, leaking joint and a burn. Practice belongs after a demonstration and on scrap first; feedback after the fact without a model to compare to teaches little.

   C. Theory alone skips the demonstration and the guided practice, and unsupervised practice on a system is unsafe for a first attempt. The apprentice needs to see it, try it with you watching, and get feedback.

5. Recall · Uses evacuation tools and equipment

   What is the purpose of the gas ballast valve on a two-stage vacuum pump?

   1. Cool the pump by drawing air across the motor windings during long evacuations.
   2. Vent the oil reservoir so the oil can be drained and replaced.
   3. Admit a little outside air so water vapour is discharged instead of condensing in the oil.
   4. Let the pump reach a deeper ultimate vacuum by bypassing the first stage.
   Show the answer

   C. Correct. On the compression stroke, water vapour pulled from a wet system would condense in the pump and emulsify the oil. A partly open gas ballast bleeds a small amount of atmospheric air into the final stage so the vapour is swept out the exhaust before it reaches its dew point. It is opened during the early, wet stage of an evacuation and closed for the final pull to the target micron level, because the bleed air limits the ultimate vacuum.

   Why not the others

   A. Motor cooling is done by the motor fan. The ballast admits air into the pumping chamber, not over the windings, and it is there to keep water vapour from condensing in the oil.

   B. Oil is drained through the drain plug with the pump warm and stopped; the ballast valve is not part of the oil change. Its job is moisture control during operation.

   D. The ballast does the opposite: while open it raises the pressure the pump can reach. It is closed for the deep, final part of the evacuation. Nothing on a two-stage pump bypasses a stage.

02Answer key

03What block A covers

This block covers the skills the rest of the trade relies on. It includes refrigerant hazards in machinery rooms and pits, lock-out and capacitor discharge, WHMIS labels, fall protection, oxy-acetylene brazing outfits, recovery cylinders and vacuum pumps, gauge and thermometer checks, ladders and rigging, drawings and equipment schedules, P/T charts and pressure-enthalpy diagrams, halocarbon records, and mentoring apprentices.

The Red Seal Occupational Standard names this block “Performs common occupational skills”. It carries 10.4% of the exam, about 13 of the 125 questions, split across four tasks. The share column is each task's weight within the block, as the standard publishes it.

04What the questions turn on

Refrigerant leaks in low spaces

An A1 refrigerant is non-flammable and low in toxicity, but its vapour is three to four times denser than air and pools in pits and below-grade rooms. An oxygen reading under 19.5 % means no entry: ventilate and re-test from outside.

Proving zero energy

Locking out is not proof. Prove the meter on a live source, test for zero, prove it again, then try a start. Bleed a run capacitor through a resistor of about 20 kilohms, then measure. VFD bus capacitors need their stated discharge time.

Recovery cylinder fill limit

Recovery cylinders are filled to no more than 80 % of water capacity, by weight. A 22.7 kg WC cylinder with an 11.3 kg tare holds 18.2 kg of refrigerant, a 29.5 kg maximum on the scale. Keep one refrigerant per cylinder.

Superheat from a P/T chart

Superheat is the suction-line temperature minus the saturation temperature the P/T chart gives at suction pressure. On a zeotropic blend, read the dew-point column for superheat and the bubble-point column for subcooling. Zero the gauges and check probes in ice water first.

05The other blocks

The 313A exam has six blocks. Each has its own page of free questions:

The 313A Refrigeration & AC Systems Mechanic practice exam 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.