MP 54 and MP 55 oil differential pressure switches are used as safety switches to protect refrigeration compressors against low lubricating oil pressure. If the oil pressure fails, the oil differential pressure switch stops the compressor after a certain time period. MP 54 and MP 55 are used in refrigerating systems using HCFC and non-flammable HFC refrigerants. MP 55A is used in refrigerating systems with R717. MP 55A can also be used in systems with HCFC and non-flammable HFC refrigerants. MP 54 has a fixed differential pressure setting. It also incorporates a thermal time relay with a fixed release time setting. MP 55 and MP 55A have adjustable differential pressure and are available both with and without thermal time relay.