CSLB C-20 HVAC ContractorCooling and RefrigerationMedium
A rooftop package unit's compressor control circuit includes a high-pressure switch. What is the primary function of this safety device?
- ARegulates head pressure by cycling the condenser fan motor
- BDelays compressor startup for a set time after a power interruption
- CPrevents the compressor from short cycling during defrost
- DOpens the compressor circuit when discharge pressure exceeds a preset limit, protecting the compressor from damage
Show answer & explanationAnswer & explanation
Correct answer: D. Opens the compressor circuit when discharge pressure exceeds a preset limit, protecting the compressor from damage
A high-pressure switch is a safety control wired in series with the compressor contactor coil. It opens the circuit and shuts down the compressor if discharge (head) pressure rises above a safe factory-set limit, protecting components from overpressure damage such as a ruptured discharge line or blown gasket.
Why the other options are wrong
- A. Fan cycling for head pressure control is typically managed by a fan cycling pressure switch or head pressure control valve, a separate control.
- B. Startup delay after power loss is the function of a time-delay relay, not the high-pressure switch.
- C. That function describes an anti-short-cycle timer, not a pressure switch.
High-Pressure Switch
A safety control that opens the compressor circuit when discharge pressure exceeds a factory-set limit, protecting the compressor and system from overpressure damage.
- Wired in series with the compressor contactor coil
- Trips on high discharge pressure (e.g., dirty condenser, overcharge, fan failure)
- Manual-reset types require investigation before resetting
Memory trick: High pressure hits the brakes before things blow.