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newSchoolWorks.com › VoTechWorks › Building & Construction › HVAC/R › Commercial Refrigeration › Lesson 9
BLD-HVAC07-07 · Lesson 9 · 14 clock hours · theory

Electrical Controls and Safety Devices in Commercial Refrigeration

Objective. Students will read a commercial refrigeration wiring diagram, identify the function of high-pressure cutouts, low-pressure cutouts, oil pressure safety switches, and anti-recycle timers, and trace a control circuit from thermostat through contactor to compressor.

Electrical Controls and Safety Devices in Commercial Refrigeration

Commercial refrigeration systems protect themselves through a layered set of electrical controls. Every control either starts and stops normal operation or prevents damage and unsafe conditions. Reading a wiring diagram and understanding each device lets you diagnose faults quickly and safely.

The Wiring Diagram as a Road Map

A commercial refrigeration wiring diagram is divided into two distinct circuits that appear on the same sheet.

Line voltage circuit (also called the power circuit) carries the full supply voltage — typically 208/230 V single-phase or 208/230/460 V three-phase — from the disconnect to the compressor motor, condenser fan motor, and evaporator fan motor through a contactor or motor starter.

Control circuit operates at a lower voltage (often 24 V from a step-down transformer, or line voltage in simpler units) and energizes the coil of the contactor. The control circuit is a series loop: if any one device in that loop opens, the contactor coil de-energizes and the compressor stops.

When you read a diagram, locate the disconnect first, then follow the line voltage to the load. Then find the transformer (if present) and trace the control circuit from the 24 V hot through each safety device and the thermostat to the contactor coil and back to common. Safety devices appear in series in this loop.

The Contactor and Its Coil

The contactor is an electrically operated switch. Its heavy-duty contacts carry line voltage to the compressor motor. Its coil is part of the control circuit. When the coil is energized, the contacts close and the compressor runs. Any control device that opens the coil circuit drops the contactor and stops the compressor immediately.

High-Pressure Cutout (HPC)

The high-pressure cutout is a pressure-actuated switch wired in series in the control circuit. It opens on a rise in discharge pressure above its set point, de-energizing the contactor. Causes of high discharge pressure include a dirty or failed condenser, refrigerant overcharge, or a blocked condenser fan. Many HPCs are manual-reset: the technician must press a reset button after the pressure falls to a safe level and the root cause is corrected. Never jumper a tripped HPC without diagnosing the cause — sustained high pressure can cause a mechanical failure or rupture a weakened component.

Low-Pressure Cutout (LPC)

The low-pressure cutout opens on a drop in suction pressure below its set point. Low suction pressure can indicate a refrigerant leak, a severely restricted filter-drier, a failed expansion valve, or a loss of load (empty box). Most LPCs are automatic-reset: once suction pressure rises above the cut-in set point, the switch closes and the system can restart. Repeated LPC trips are a diagnostic flag — find the cause before returning the system to service.

Oil Pressure Safety Switch (OPSS)

Positive-displacement compressors (scroll, reciprocating) with a separate lubrication pump require a minimum net oil pressure — the difference between oil pump discharge pressure and crankcase pressure. The oil pressure safety switch monitors this differential. It includes a time-delay heater: if net oil pressure does not reach its minimum within a set time after startup (typically 90–120 seconds), the switch opens the control circuit and locks the compressor off. The OPSS is almost always manual-reset. Causes of low net oil pressure include a failing oil pump, plugged oil passages, diluted oil (refrigerant migration), or incorrect oil level.

Anti-Recycle Timer (Time-Delay Relay)

Every compressor motor requires time to decelerate before restarting, and motor windings need time to cool between starts. The anti-recycle timer (also called a time-delay-on-make relay) holds the control circuit open for a fixed period — commonly 3 to 5 minutes — after a compressor shutdown. This prevents rapid short-cycling that causes overheating, excessive current draw, and premature winding failure. The timer is wired so that after power is restored or the thermostat calls again, the contactor cannot close until the delay expires.

Tracing the Control Circuit Step by Step

From thermostat to compressor, the series path is typically:

1. 24 V hot from transformer secondary

2. Thermostat contacts (close on temperature rise in a cooler)

3. Anti-recycle timer contacts

4. High-pressure cutout contacts

5. Low-pressure cutout contacts

6. Oil pressure safety switch contacts

7. Contactor coil

8. 24 V common back to transformer

When all devices are closed and the thermostat calls, current flows through the coil, the contactor pulls in, and line voltage reaches the compressor motor.

Electrical Safety on the Job

Before working inside any panel or on any wiring: de-energize at the disconnect and apply lockout/tagout (LOTO) per OSHA requirements. Attach your personal lock; verify zero energy with a tested meter before touching conductors. On three-phase equipment, verify all three phases are dead. Wear safety glasses and arc-rated PPE appropriate for the panel's incident-energy level when working near energized parts. Use a CAT III or CAT IV rated multimeter for measurements on commercial equipment. Never assume a circuit is de-energized because a disconnect looks open — verify with your meter.

Voltage-Drop Testing to Find an Open

With the thermostat calling and the contactor failing to pull in, measure voltage across each series device in the control circuit. Place one meter lead on the 24 V hot side of a device and the other on its load side. A device that is open will show the full supply voltage across it (all voltage drops across the open). A closed device shows near zero volts across it. Work systematically from one end of the loop to the other; the open is at the point where voltage appears across the device.

Key terms8

ContactorAn electrically operated switching device whose heavy contacts carry line voltage to the compressor motor; controlled by a low-current coil in the control circuit.
High-Pressure Cutout (HPC)A pressure-actuated switch that opens the control circuit when discharge pressure exceeds its set point, stopping the compressor; often requires manual reset.
Low-Pressure Cutout (LPC)A pressure-actuated switch that opens the control circuit when suction pressure falls below its set point; typically automatic-reset when pressure recovers.
Oil Pressure Safety Switch (OPSS)A differential pressure switch with a built-in time-delay heater that locks out the compressor if net oil pressure (oil pump discharge minus crankcase pressure) does not reach minimum within a set time after startup.
Anti-Recycle TimerA time-delay relay that prevents the compressor from restarting immediately after shutdown, protecting the motor from short-cycling and thermal damage.
Control CircuitThe low-current series loop — often at 24 V — that energizes the contactor coil; all safety devices and the thermostat are wired in series within this loop.
Line Voltage CircuitThe high-current path that carries full supply voltage from the disconnect through the contactor contacts to the compressor and fan motors.
Voltage-Drop TestA diagnostic method in which a meter is placed across each device in a series circuit; the device with full supply voltage across it is the open (faulty) component.

Worked examples4

1. A technician measures voltages across each device in a 24 V control circuit while the thermostat is calling and the contactor will not close. Readings are: across thermostat contacts = 0 V; across anti-recycle timer contacts = 0 V; across HPC contacts = 24 V; across LPC contacts = 0 V; across OPSS contacts = 0 V. Which device is open and why?
  1. Step 1 — Recall the rule: in a series circuit with a voltage source, the full source voltage appears across the open device because no current flows and there is no drop elsewhere.
  2. Step 2 — Review the readings: thermostat = 0 V (closed), anti-recycle timer = 0 V (closed), HPC = 24 V, LPC = 0 V, OPSS = 0 V.
  3. Step 3 — The HPC contacts show the full 24 V across them while all other devices show 0 V. This confirms the HPC contacts are open.
  4. Step 4 — Conclude: the high-pressure cutout has tripped. Before resetting, identify and correct the root cause (e.g., dirty condenser coil, failed condenser fan) and confirm discharge pressure has returned to a safe level.
Answer: The high-pressure cutout (HPC) is open. It shows 24 V across its contacts while all other devices in the series loop show 0 V, confirming it is the only break in the circuit.
2. A net oil pressure calculation: The oil pump discharge pressure gauge reads 55 psig. The crankcase (suction side) pressure gauge reads 20 psig. The OPSS is set to open if net oil pressure drops below 25 psid. Is the compressor protected or at risk of lockout?
  1. Step 1 — Calculate net oil pressure: Net oil pressure = Oil pump discharge pressure − Crankcase pressure.
  2. Step 2 — Substitute values: Net oil pressure = 55 psig − 20 psig = 35 psid.
  3. Step 3 — Compare to the OPSS set point: 35 psid is greater than the 25 psid minimum.
  4. Step 4 — Conclude: the net oil pressure is adequate; the OPSS will remain closed and the compressor will continue to run.
Answer: Net oil pressure is 35 psid, which exceeds the 25 psid minimum set point. The OPSS remains closed and the compressor is protected.
3. A technician uses a clamp ammeter on one leg of a three-phase compressor and reads 18 A. The nameplate full-load amperage (FLA) is 20 A. The nameplate also lists a service factor of 1.10. Calculate the maximum allowable continuous current and determine if the compressor is operating within limits.
  1. Step 1 — Recall that maximum allowable continuous current for a motor with a service factor ≥ 1.15 is typically 125% of FLA per NEC, but for motors with a lower service factor or for comparison to nameplate, use FLA × service factor as the upper operating limit the manufacturer allows.
  2. Step 2 — Calculate the service-factor current: 20 A × 1.10 = 22 A.
  3. Step 3 — Compare measured current to FLA and service-factor current: 18 A < 20 A FLA < 22 A service-factor current.
  4. Step 4 — Conclude: at 18 A the compressor is drawing less than its FLA, well within normal operating limits. No overcurrent concern exists at this reading.
Answer: Maximum allowable current at service factor is 22 A. The measured 18 A is below FLA (20 A), so the compressor is operating well within its rated limits.
4. A technician is troubleshooting a compressor that short-cycles on and off roughly every 90 seconds. The anti-recycle timer is set for 4 minutes. A voltage check shows 24 V present at the contactor coil terminal each time the compressor tries to start. What does this indicate about the anti-recycle timer, and what is the likely fault?
  1. Step 1 — Understand the timer's purpose: the anti-recycle timer should hold the control circuit open for 4 minutes after each shutdown before allowing the contactor coil to be energized again.
  2. Step 2 — Observe that voltage is reaching the coil within 90 seconds — far less than the 4-minute delay — which means the timer contacts are closing prematurely.
  3. Step 3 — A timer that closes before its set delay expires indicates a failed or mis-set timer relay.
  4. Step 4 — Confirm by timing the interval from shutdown to coil energization with a stopwatch. If it is consistently less than 4 minutes, the timer is defective or set incorrectly. Replace or correctly adjust the timer, then recheck cycle intervals.
Answer: The anti-recycle timer is defective or improperly set; it is allowing the contactor to close far sooner than the 4-minute delay, causing short-cycling. The timer should be replaced or adjusted and retested.

The math in this lesson2

Learners compute net oil pressure as pump discharge minus crankcase pressure.
7.NS.17.RP.3
Brush up in the math halls at newSchoolWorks.com:
In class: Wiring diagram annotation exercise: students receive a printed three-phase condensing unit wiring diagram and use colored pencils to trace the line voltage circuit, control circuit, and safety circuit, labeling each device and its function. · Instructor demonstration on a live training panel: each safety device is manually tripped (simulated) in turn; students observe the compressor response, record which control opened, and write the reset procedure for each device. · Control circuit fault simulation: instructor inserts a wiring fault (open wire in control circuit) in the training panel; students use a multimeter to perform a systematic voltage-drop test, locate the open, and document the diagnosis on a wiring diagram mark-up.
Materials: Commercial refrigeration wiring diagrams (three-phase), colored pencils, training electrical control panel, digital multimeters, clamp ammeter, high-pressure/low-pressure cutout training components, NCCER HVAC Trainee Guide Level 3
Check: Fault diagnosis documentation reviewed by instructor; student must correctly identify the open circuit location, show the voltage readings that led to the diagnosis, and write the correct corrective action. Pass requires correct fault location with supporting measurements.

Practice check8

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

Electrical Theory and Controls
1. In a commercial refrigeration control circuit, all safety devices are wired in which configuration with the contactor coil?
Troubleshooting and Diagnostics
2. A high-pressure cutout trips on a condensing unit. Before resetting the device, what is the technician's most important first action?
Electrical Troubleshooting
3. A technician performs a voltage-drop test on a 24 V control circuit. The meter reads 24 V across the oil pressure safety switch contacts and 0 V across all other series devices. What does this indicate?
Electrical Controls
4. What is the primary purpose of the anti-recycle timer in a commercial refrigeration system?
Refrigeration System Components
5. An oil pressure safety switch monitors which pressure value to protect the compressor?
Troubleshooting and Diagnostics
6. Which condition would most likely cause a low-pressure cutout to trip repeatedly on a walk-in cooler?
Safety and OSHA Regulations
7. Before opening an energized commercial refrigeration control panel to measure voltages, a technician must:
Electrical Troubleshooting
8. On a wiring diagram, a technician traces the control circuit and finds the following series path: thermostat → anti-recycle timer contacts → HPC contacts → LPC contacts → OPSS contacts → contactor coil. The thermostat is calling, the timer has timed out, the HPC and LPC are closed, and the OPSS has been closed for over two minutes. The contactor will not pull in. Using a meter, the technician measures 0 V across every device in the loop. What is the most likely cause?
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