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  • Time delay to avoid
  • nuisance shutdown
  • Choice of safety timings

The P30 oil differential pressure Switch is for use with all refrigeration with compressors using external oil pumps with refrigerants.
It protects the compressor against loss of lubricating oil pressure which could otherwise cause severe bearing damage resulting in a compressor breakdown.
Connected to the crankcase and oil pump by capillaries, the P30 senses the difference between the pressure in the crankcase and that of the outlet side of the oil pump.


  • two opposed pressure bellows (one sensing crankcase pressure, one sensing oil pump pressure);
  • a bimetal manual reset "delay-relay".


The pressure bellows measure the effective oil pressure available (for lubricating the bearings within the compressor). The opposed bellows operate a switch which breaks on rise of effective oil pressure.
According to model, the time delay of the delay-relay can be (factory set) between 60 and 120 seconds. This value will be selected by the compressor manufacturer and should not be substituted by a control of different time delay.
Two important terms need defining:
C.O.P.D. (cut-out pressure differential) - this is the pressure difference (between crankcase and oil pump pressures) at which the time delay-relay is energised to cut-out (i.e. shut down the system);
C.I.P.D. (cut-in pressure differential) - this is the pressure difference required to de-energise the time delay-relay.
The explanations of operation should be read in conjunction with the wiring diagram and illustrations overleaf.