High pressure and oil pressure switch troubleshooting 🔴 RED CIRCLE Danfoss High-Pressure (HP) Safety Control (Common models: Danfoss KP7 / KP15 / KP17 depending on range) What it is • Mechanical high-pressure cutout • Senses discharge pressure only • Wired in series with the compressor safety circuit What it does • Protects the compressor from excessive discharge pressure • Opens the control circuit when pressure exceeds its setpoint How it operates • Internally uses a diaphragm & spring • Discharge pressure pushes against the diaphragm • When pressure > setpoint → contacts open → compressor stops • Some models are: • Manual reset • Others auto-reset (not common on racks) Typical trip range • Medium temp: ~350–400 psi • Low temp: ~300–350 psi (exact value depends on refrigerant) What causes HP to trip (REAL rack causes) 1. Condenser airflow problems • Dirty condenser coils • Failed condenser fans • VFD fan stuck at low speed • Winter fan control stuck closed 2. Over-condensing / split rack issues • Too many compressors running on one side • Other side unloaded • Poor heat rejection balance 3. Non-condensables • Air in system • Bad evacuation after repairs • Nitrogen contamination 4. Liquid line restrictions • Plugged filter drier • Closed or partially closed king valve • Collapsed line How to troubleshoot HP trips 1. Check actual discharge pressure with gauges 2. Compare to HP cutout rating 3. Verify: • Condenser fans running • VFD speed command 4. Check coil condition 5. If pressure is normal but HP trips: • Bad HP control • Misadjusted setpoint • Kinked sensing line Key field rule: HP trips are almost always a heat rejection problem, not a compressor problem. 🔵 BLUE CIRCLE Danfoss Oil Differential Pressure Safety Control (Typically Danfoss MP54 / MP55) What it is • Net oil pressure safety • Compares oil pump pressure vs suction (crankcase) pressure What it does • Protects compressor from oil starvation • Shuts compressor down if oil pressure differential is too low