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newSchoolWorks.com › VoTechWorks › Building & Construction › HVAC/R › Electrical Controls for HVAC/R Systems › Lesson 12
BLD-HVAC07-03 · Lesson 12 · 28.5 clock hours · lab

Integrated Electrical Lab: Diagnose, Repair, and Commission a Full Control Circuit

Objective. Students will independently diagnose and restore full electrical operation of a residential or light-commercial HVAC/R unit containing multiple instructor-inserted faults, completing all LOTO steps, recording all measurements, and producing a professional service report.

Integrated Electrical Lab: Diagnose, Repair, and Commission a Full Control Circuit

This lab is the capstone of the electrical controls course. You will work alone on a real unit, find every fault your instructor has hidden in the circuit, make the repairs, and prove the system runs correctly. Then you will explain every decision you made. This is the standard a field technician is held to on every service call.

Safety First: Lockout/Tagout Is Non-Negotiable

Before you touch any panel, disconnect, or wiring, you must complete a full Lockout/Tagout (LOTO) sequence. The steps are:

1. Identify every energy source feeding the unit — line-voltage supply, low-voltage transformer secondary, and any separate control-circuit breaker.

2. Notify anyone who could be affected.

3. De-energize the unit at the main disconnect.

4. Lock the disconnect in the OFF position with your personal lock; attach your name tag.

5. Release stored energy — capacitors can hold a lethal charge after power is removed. Use an insulated capacitor discharge tool or a resistor-and-probe setup to safely bleed run and start capacitors before handling them. Wait the time your instructor specifies and verify with a voltmeter that voltage is at or near zero.

6. Verify zero voltage at the load side of the disconnect with your DMM before entering the panel.

7. Get your LOTO log signed by your instructor before proceeding.

Any energized-equipment contact without a completed LOTO sequence is an automatic lab failure. Safety counseling is required before you may re-attempt. No exceptions.

Minimum PPE for this lab: safety glasses, insulated gloves rated for the voltage present, and leather-palm work gloves when handling sheet-metal panels.

Reading the Wiring Diagram Before You Touch Anything

Your instructor will hand you an OEM wiring diagram and a service order describing the customer complaint. Study both before you open the cabinet. Trace the low-voltage control circuit (typically 24 V AC from the transformer secondary) and the line-voltage load circuit separately. Note every safety switch in series with the contactor coil — high-pressure switch, low-pressure switch, thermostat contacts, fan-proving switch — because any one of them, if open, will prevent the contactor from pulling in. Circle the components you plan to test first based on the reported symptom.

Systematic Diagnosis: Work from Supply to Load

Random testing wastes time and misses faults. Use this sequence:

1. Confirm supply voltage at the disconnect load side and at the transformer primary terminals. Compare to the nameplate rating.

2. Confirm 24 V (nominal) at the transformer secondary with the thermostat calling for the mode under test.

3. Check the low-voltage fuse or fusible link if one is present. A blown fuse here kills the entire control circuit.

4. Trace the control circuit from the R terminal toward the load, measuring voltage at each safety switch terminal. A voltage reading across a switch means the switch is open (switch holds the voltage drop). A zero-volt reading across a closed switch confirms continuity.

5. Test the contactor coil — measure voltage across the coil terminals when the circuit should be energized. If voltage is present but the contactor will not pull in, the coil is faulty.

6. Test capacitors with a capacitance meter after discharge. A run capacitor that measures significantly below its rated microfarad value (typically more than 6% below) is suspect; one that reads zero or very high indicates failure.

7. Check line-voltage load side — after the contactor closes, verify voltage to the compressor and outdoor fan motor terminals.

Record every measurement in your service report as you go. Do not rely on memory.

Common Fault Types You May Encounter

Each unit in this lab has two to three inserted faults. Do not stop after finding one.

Restoring Power (LOTO Reversal)

After completing all repairs, reverse the LOTO sequence: confirm all tools and personnel are clear, remove your lock, restore power at the disconnect, and verify operation. Test every required mode — cooling, heating (if applicable), and fan-only. Record supply voltage, operating amperage (measured with a clamp ammeter), and compare amperage to nameplate RLA and LRA values. Confirm the unit runs without tripping any safety switch.

The Service Report

Your report must include: unit identification, reported complaint, LOTO log reference, each fault found with the measurement that confirmed it, the repair performed, and post-repair verification measurements. Write clearly — a technician who cannot communicate findings in writing leaves liability on the table.

Verbal Defense

You will answer five questions from your instructor covering your diagnostic logic, component function, and safety decisions. You need a score of 75% or higher to pass. Think through each question before answering; partial credit is not available.

Key terms8

Lockout/Tagout (LOTO)An OSHA-required procedure that uses physical locks and warning tags to de-energize equipment and prevent accidental re-energization while work is performed, protecting technicians from electrical, mechanical, and stored-energy hazards.
Run capacitorAn oil-filled capacitor wired permanently in the circuit to improve motor efficiency and torque by keeping the run winding current out of phase with the start winding; rated in microfarads (MFD or µF) and voltage.
ContactorA heavy-duty, magnetically operated switch used to connect and disconnect line-voltage power to compressor and fan motor loads; controlled by a low-voltage coil circuit.
Control circuitThe low-voltage (typically 24 V AC) circuit that carries thermostat signals and safety-switch logic to operate contactors, relays, and other load-switching devices in the HVAC/R system.
High-pressure switchA normally closed safety device wired in series with the contactor coil that opens and de-energizes the compressor circuit if refrigerant discharge pressure exceeds a safe threshold.
Capacitance meterA test instrument that measures capacitance in microfarads, used to verify that a run or start capacitor is within its rated value range.
RLA (Rated Load Amps)The nameplate current a compressor motor draws under normal full-load operating conditions; used as a baseline when comparing measured operating amperage.
Voltage drop across a switchA diagnostic technique: measuring voltage across a series switch with the circuit energized; a reading equal to supply voltage indicates the switch is open (breaking the circuit).

Worked examples4

1. A run capacitor is rated 45 MFD ±6%. Your capacitance meter reads 40.2 MFD after the capacitor has been safely discharged. Is the capacitor within tolerance?
  1. Step 1 – Find the lower tolerance limit: 6% of 45 MFD = 0.06 × 45 = 2.7 MFD.
  2. Step 2 – Subtract from rated value: 45 − 2.7 = 42.3 MFD is the minimum acceptable reading.
  3. Step 3 – Compare: 40.2 MFD is below 42.3 MFD.
Answer: The capacitor is out of tolerance and must be replaced. It reads 4.8 MFD below the rated value, exceeding the 2.7 MFD allowable deviation.
2. With the thermostat calling for cooling, you place your DMM leads across the high-pressure switch terminals and read 24 V AC. What does this tell you, and what is your next action?
  1. Step 1 – Interpret the reading: voltage measured across a series switch in an energized circuit means the switch is holding that voltage drop, so current is not passing through it — the switch is open.
  2. Step 2 – Determine cause: the high-pressure switch has tripped or failed open, breaking the control circuit before it reaches the contactor coil.
  3. Step 3 – Next action: before replacing the switch, determine why it tripped (high head pressure due to dirty condenser coil, refrigerant overcharge, blocked airflow, etc.). If it is a manual-reset switch, identify and correct the root cause, then reset it and retest.
Answer: A 24 V reading across the switch confirms it is open. The high-pressure switch has interrupted the control circuit. Identify and correct the underlying cause before resetting or replacing the switch.
3. After repairs, you clamp your ammeter around one leg of the compressor supply wires and read 18.5 A. The nameplate shows RLA = 15.2 A and LRA = 88 A. Is the operating current acceptable?
  1. Step 1 – Compare measured current to RLA: 18.5 A vs. 15.2 A RLA.
  2. Step 2 – Calculate the percentage above RLA: (18.5 − 15.2) / 15.2 × 100 = 3.3 / 15.2 × 100 ≈ 21.7% over RLA.
  3. Step 3 – Evaluate: steady-state current running more than roughly 10% above RLA warrants investigation. At 21.7% over, the condition is abnormal — possible low refrigerant charge, high suction pressure, weak capacitor (if a start capacitor is involved), or mechanical issue.
Answer: 18.5 A is approximately 22% above nameplate RLA and is not acceptable for steady-state operation. Further diagnosis is required; do not return the unit to the customer until the cause is identified and corrected.
4. You need to confirm that the 24 V control transformer secondary is functioning. With the thermostat off, you measure across the transformer secondary terminals and read 25.8 V AC. Is this a concern?
  1. Step 1 – Recognize that 24 V AC control transformers are nominal ratings; actual no-load secondary voltage is typically somewhat higher than 24 V.
  2. Step 2 – A reading of 25.8 V at no load is within the normal range for a 24 V nominal transformer and indicates the transformer secondary is functioning.
  3. Step 3 – To fully verify transformer health, also check that voltage does not collapse significantly when the contactor coil load is applied (a severely loaded or failing transformer will show a large voltage drop under load).
Answer: 25.8 V AC at no load is normal for a 24 V nominal transformer. The transformer secondary is functioning. Monitor voltage under load to confirm the transformer can sustain the circuit.

The math in this lesson1

Learners check capacitor MFD against ±6% and compare running amps to nameplate RLA.
7.RP.3
Brush up in the math halls at newSchoolWorks.com:
In class: Individual pre-task briefing: each student receives a unit assignment, an OEM wiring diagram, and a service order describing reported symptoms; student performs a complete LOTO sequence before opening any panel, verified by instructor signature on the LOTO log. · Diagnosis and repair phase (unassisted): student systematically tests the control circuit, measures capacitors, checks contactors and safety switches, identifies all faults (two to three per unit), replaces or repairs each, then re-energizes following LOTO reversal protocol and verifies normal system operation in all required modes. · Service report and verbal defense: student submits a complete service report and answers five instructor questions about the diagnostic process, component function, and safety decisions made during the lab; answers are scored against a key.
Materials: Residential split-system and/or light-commercial packaged units (operational, fault-capable), digital multimeters, clamp ammeters, capacitance meters, replacement capacitors, contactors, and relays, LOTO hardware sets, OEM service manuals and wiring diagrams, service report forms, verbal-defense scoring rubric
Check: All faults correctly identified and repaired, unit operating normally in all tested modes, LOTO log signed at start and reversal, service report complete, and verbal-defense score ≥ 75% = pass. Any energized-equipment contact without completed LOTO = automatic fail with safety counseling required before re-attempt.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

Safety and OSHA compliance
1. Before opening any electrical panel on a unit, the FIRST action a technician must take is:
Electrical diagnosis
2. You measure 24 V AC across a normally closed safety switch while the control circuit is energized. This reading means the switch is:
Electrical components and measurement
3. A run capacitor is rated 35 MFD ±6%. What is the minimum acceptable capacitance reading?
Safety and PPE
4. Which tool is used to safely discharge a capacitor before handling it?
Electrical diagnosis
5. The contactor coil measures 24 V AC across its terminals, but the contactor will not pull in. The most likely cause is:
System commissioning and verification
6. During post-repair verification, you should compare the measured operating amperage to the nameplate value called:
Diagnostic strategy
7. A technician finds one fault (an open control fuse) and replaces it, then re-energizes the unit. The unit still will not cool. What is the most likely explanation?
Professional practices and documentation
8. Which of the following is a required element of a complete service report for this lab?
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