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BLD-HVAC07-03 · Lesson 8 · 13.5 clock hours · demo

Pressure Switches, Flow Switches, and Safety Controls

Objective. Students will identify high-pressure, low-pressure, and oil-pressure safety switches on training equipment, test switch continuity across the operating range, and explain how each protects the refrigeration system.

Why Safety Switches Exist

Refrigeration and air-conditioning systems operate under pressures and conditions that can quickly damage compressors, burst refrigerant lines, or create a safety hazard if something goes wrong. Pressure switches, flow switches, and oil-pressure safety controls are the system's first line of automated defense. Each one monitors a specific operating condition and opens or closes a circuit to shut the system down before damage occurs. Understanding how these switches work—and how to test them—is a core skill for any HVAC/R technician.

High-Pressure Switches

A high-pressure switch (HPS) monitors the discharge-side pressure of the refrigeration system. It is wired in series with the compressor control circuit. When discharge pressure rises above the switch's setpoint—caused by a blocked condenser, a failed condenser fan, or refrigerant overcharge—the switch opens the circuit and de-energizes the compressor.

High-pressure switches are normally closed (NC): under normal operating pressure the contacts are closed and current flows. When pressure reaches the trip setpoint, the contacts open. Some HPS designs are auto-reset (contacts close again once pressure drops); others are manual-reset, requiring a technician to physically reset the switch after the cause of the fault is corrected. Always find and fix the root cause before resetting a manual-reset switch.

Low-Pressure Switches

A low-pressure switch (LPS) monitors suction-side pressure. Low suction pressure can indicate a refrigerant leak, a restricted filter-drier, an evaporator iced over, or extremely low ambient temperatures. Like the HPS, the LPS is wired in series with the compressor control circuit.

Depending on application, an LPS may be wired normally closed (opens on low pressure to stop the compressor) or used as a normally open control for pump-down logic. Always check the OEM spec sheet to confirm the contact arrangement and setpoints before testing or replacing.

Oil-Pressure Differential Switch

Larger reciprocating and some scroll compressors rely on a pressurized lubrication system. The oil-pressure differential switch (OPDS) compares oil pump outlet pressure against crankcase pressure. It does not measure either pressure alone—it measures the difference (differential). If that differential falls below a safe threshold, the switch interrupts the control circuit and shuts down the compressor before bearing damage occurs.

Because it takes a few seconds for oil pressure to build on start-up, most OPDS designs include a time-delay relay: the switch is allowed a short period to prove oil pressure before the safety circuit is armed. Do not bypass or defeat this time-delay feature—it is a calibrated safety mechanism.

Normal State: NC vs. NO

Understanding the normal state of a switch is essential for diagnosis:

At atmospheric pressure on the bench, a properly functioning high-pressure switch should read closed on your multimeter (continuity present) because atmospheric pressure is far below any trip setpoint. A normally open switch at atmospheric pressure will read open (no continuity).

How to Test a Pressure Switch with a Multimeter

1. De-energize and lockout/tagout the equipment before disconnecting any wiring. Follow your facility's lockout/tagout (LOTO) procedure every time.

2. Disconnect the switch leads from the circuit.

3. Set the multimeter to the continuity or resistance (Ω) function.

4. Place probes across the switch terminals.

5. For an NC switch at rest: the meter should beep (continuity) or read near 0 Ω.

6. For an NO switch at rest: the meter should show OL (open loop / no continuity).

7. Record your reading on the service report form, note the switch's normal state from the OEM spec sheet, and compare.

Never use your meter on a live 120 V or 240 V circuit unless you are set to the correct AC voltage function and using properly rated test leads.

Diagnosing a Safety Switch Fault

When a safety switch trips, the compressor stops and often a fault code or indicator light appears on the control board. Your diagnostic steps:

1. Identify which switch tripped (check fault codes, indicator lights, or use the wiring diagram).

2. Determine the root cause—do not simply reset the switch.

3. Verify the switch is operating correctly by bench-testing continuity.

4. Repair the underlying condition (clear the blocked condenser, repair the refrigerant leak, replace the filter-drier, etc.).

5. Reset the switch if it is manual-reset, or confirm auto-reset has occurred.

6. Document all findings and actions on the service report form.

If the switch itself has failed (will not reset, contacts welded shut, or opens at the wrong pressure), replace it with an OEM-equivalent rated switch.

Safety Reminders

Key terms9

High-Pressure Switch (HPS)A normally closed safety switch that monitors discharge-side refrigerant pressure and opens the compressor control circuit when pressure exceeds the trip setpoint.
Low-Pressure Switch (LPS)A safety switch that monitors suction-side refrigerant pressure and shuts down the compressor when pressure drops below a safe threshold, protecting against conditions such as refrigerant loss or evaporator icing.
Oil-Pressure Differential Switch (OPDS)A safety device that measures the difference between oil pump outlet pressure and crankcase pressure; shuts the compressor down if the differential falls below the minimum needed for safe lubrication.
Normally Closed (NC)A switch contact arrangement in which the circuit is complete (contacts touching) under normal, un-tripped conditions; the circuit opens when the trip condition is reached.
Normally Open (NO)A switch contact arrangement in which the circuit is open (contacts separated) under normal conditions; the circuit closes when the actuating condition is reached.
SetpointThe pressure (or other measured value) at which a safety switch is designed to change state, as specified by the original equipment manufacturer.
Manual ResetA switch design that requires a technician to physically reset the device after it trips; it will not automatically return to the closed position when the condition clears.
Lockout/Tagout (LOTO)An OSHA-required safety procedure that physically isolates and de-energizes equipment and applies a lock and warning tag to prevent accidental re-energization while work is performed.
Continuity TestA multimeter function that checks whether a complete electrical path exists between two points; used to verify the normal state of switch contacts.

Worked examples4

1. You remove a high-pressure switch from a training unit and place it on the bench. You set your multimeter to continuity mode and touch the probes to the two switch terminals. The meter reads OL (no continuity). The OEM spec sheet states this switch is normally closed with a trip pressure of 400 psig. Is the switch good or faulty?
  1. Step 1 – Identify the normal state from the spec sheet: the switch is normally closed (NC), meaning contacts should be closed (continuity present) at atmospheric pressure, which is far below the 400 psig trip setpoint.
  2. Step 2 – Compare the meter reading to the expected state: the meter reads OL, indicating open contacts—no continuity.
  3. Step 3 – Draw a conclusion: at atmospheric pressure an NC switch must show continuity. Reading OL means the contacts are open when they should be closed, which is not the normal state.
  4. Step 4 – Determine action: the switch contacts have failed open. The switch is faulty and must be replaced with an OEM-equivalent unit.
Answer: The switch is faulty. An NC switch at atmospheric pressure must show continuity; OL indicates the contacts are stuck open and the switch must be replaced.
2. A compressor wiring diagram shows a low-pressure switch wired NC in series with the compressor contactor coil. The suction pressure is currently 68 psig (normal range). A technician measures 0 Ω across the switch terminals. Later, a refrigerant leak causes suction pressure to fall to 10 psig, tripping the switch. What will the multimeter read across the switch terminals after the trip, and why?
  1. Step 1 – Recall the NC behavior: normally closed contacts are closed (0 Ω / continuity) during normal operation.
  2. Step 2 – Understand the trip action: when suction pressure falls to the trip setpoint, the NC switch opens its contacts to break the control circuit and stop the compressor.
  3. Step 3 – Predict the post-trip reading: after the switch trips, its contacts are open—the multimeter will read OL (no continuity).
  4. Step 4 – Relate to system effect: with contacts open, no current flows through the contactor coil, so the compressor is de-energized and protected from operating with insufficient refrigerant.
Answer: After the trip the multimeter reads OL (open/no continuity) because the NC contacts opened when suction pressure fell to the trip setpoint, breaking the control circuit.
3. An oil-pressure differential switch has a minimum required differential of 9 psig. The oil pump outlet pressure is 42 psig and crankcase pressure is 35 psig. Is the compressor protected or at risk?
  1. Step 1 – Calculate the actual oil pressure differential: Differential = Oil pump outlet pressure − Crankcase pressure = 42 psig − 35 psig = 7 psig.
  2. Step 2 – Compare to the minimum required differential: 7 psig actual vs. 9 psig required minimum.
  3. Step 3 – Determine status: 7 psig is less than the 9 psig minimum. The differential is below the safe threshold.
  4. Step 4 – Draw a conclusion: the oil-pressure differential switch should trip, shutting down the compressor to prevent bearing damage from inadequate lubrication.
Answer: The actual differential is 7 psig, which is below the 9 psig minimum. The OPDS should trip and stop the compressor to prevent lubrication failure.
4. During a bench test you need to verify a pressure switch sample with two terminals. You set the multimeter to resistance (Ω) mode. With probes on the terminals you read 0.3 Ω. The spec sheet lists the switch as normally closed. Is this reading acceptable, and what does it indicate?
  1. Step 1 – Understand what a low resistance reading means: 0.3 Ω is essentially zero resistance, indicating the circuit between the terminals is complete (closed contacts with normal contact resistance).
  2. Step 2 – Compare to the expected normal state: the spec sheet says NC, so contacts should be closed—low resistance is the expected result.
  3. Step 3 – Assess acceptability: a reading very close to 0 Ω (the slight resistance is from contact material and lead resistance) confirms the switch contacts are properly closed.
  4. Step 4 – Conclude: the reading is acceptable and confirms the switch is functioning correctly in its normal state at atmospheric pressure.
Answer: A reading of 0.3 Ω is acceptable for a normally closed switch; it confirms the contacts are properly closed. Near-zero resistance is expected—the tiny value comes from contact and lead resistance, not a fault.
In class: Instructor-led identification walk-around on a trainer refrigeration unit: students locate and label each safety switch on a diagram, recording setpoint data from the nameplate or OEM spec sheet. · Bench continuity test: students use a multimeter to verify the normally-closed or normally-open status of a high-pressure switch and a low-pressure switch at atmospheric pressure, then predict the state change that would occur at trip pressure. · Simulated fault scenario: instructor disables the low-pressure switch signal on the training unit (using a switch bypass tool, unit off); students diagnose the resulting control circuit fault, determine whether to replace or reset the switch, and document the service call on a report form.
Materials: Training refrigeration unit with accessible safety switches, high-pressure and low-pressure switch samples, oil-pressure differential switch sample, digital multimeters, OEM spec sheets, switch bypass tool, service report forms, student workbook
Check: Students correctly identify each switch by function, record proper normal state (NO or NC), and complete the simulated-fault diagnosis with correct root cause and corrective action documented = pass at ≥ 80% accuracy on the report form.

Practice check8

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

Electrical controls and components
1. A high-pressure switch is wired in series with a compressor contactor coil. During normal operation, which statement correctly describes the switch contacts?
Electrical diagnosis and testing
2. A technician performs a continuity test on a low-pressure switch sample at atmospheric pressure. The multimeter reads OL (open loop). The OEM spec sheet states the switch is normally closed. What should the technician conclude?
Refrigeration system components and operation
3. What condition does an oil-pressure differential switch monitor?
System troubleshooting and service
4. A compressor trips on its manual-reset high-pressure switch. After resetting the switch, the compressor trips again within five minutes. What is the MOST appropriate next step?
Safety and work practices
5. Before disconnecting the wiring to a pressure switch for bench testing, a technician MUST first:
Electrical controls and components
6. An oil-pressure differential switch has a minimum required differential of 8 psig. Oil pump outlet pressure reads 40 psig and crankcase pressure reads 34 psig. What is the actual differential, and should the switch trip?
Safety and work practices
7. During a simulated fault exercise, a student finds the low-pressure switch signal has been disabled. The student bypasses the switch temporarily with a bypass tool for diagnosis. Which action is REQUIRED once the diagnosis is complete?
Electrical fundamentals
8. Which of the following BEST describes a normally open (NO) pressure switch contact at rest (no actuating pressure applied)?
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