An oil safety control on a pressure-lubricated compressor does not judge oil pressure from one gauge reading alone. Copeland's FD113 instructions define effective oil pressure as the difference between oil-pump outlet pressure and crankcase pressure. That distinction helps with a common schematic and troubleshooting problem: a higher crankcase pressure can reduce the useful differential even if pump-outlet pressure looks substantial.
Which two pressures make up net oil pressure?
Use the relationship shown in Copeland's FD113 instruction sheet:
Net effective oil pressure = oil-pump outlet pressure − compressor crankcase pressure.
For a hypothetical reading of 42 psig at the pump outlet and 12 psig in the crankcase, the differential is 30 psi. If the pump outlet stays at 42 psig while crankcase pressure rises to 22 psig, the differential falls to 20 psi. The numbers illustrate subtraction only; they are not target settings or a diagnosis. Copeland specifies the connections and permissible settings for its control, and the compressor manufacturer's required minimum governs the application.
Compare the two pressure connections on the control diagram before deciding what a gauge reading means. A single oil-side pressure does not tell you the differential by itself.
Why does the control include a time delay?
Copeland describes the FD113 as starting a timer when effective oil pressure is inadequate. If pressure recovers within the delay, the compressor is not shut down by that event; if it does not recover before the model's timer expires, the control stops the compressor. The delay gives oil pressure time to establish at startup and avoids shutdown for a brief drop. The FD113-ZUK catalog gives a 20–150 second adjustment range and a factory setting for that specific model; do not treat those figures as universal settings for every compressor or oil safety.
For a study problem, trace the sequence: compare the two pressures, determine whether the differential crosses the specified threshold, then consider whether it recovers within the applicable delay. A trip says the safety detected a sustained condition under its settings; it does not identify the underlying cause by itself.
How do you use the concept without guessing at a repair?
First name the measured quantity correctly: differential oil pressure, not discharge pressure or crankcase pressure alone. Then check the equipment and control documentation for the approved settings, connections and reset procedure. Do not adjust a safety control to stop repeated trips; diagnose the cause using the manufacturer's service process.
Copeland's instructions warn that this is a control installation and service task. CCOHS says hazardous energy must be isolated, stored energy addressed and isolation verified under the applicable procedure before maintenance. This explanation is for reading and study, not a live service procedure. For another compressor schematic skill, see how a potential relay changes the start-capacitor path. The 313A guide and free sample offer further study context; the sample is original, unofficial draft material awaiting certified-mechanic review.
Sources
- Copeland: FD113 Oil Pressure Safety Control Instruction SheetNet effective oil pressure definition and control sequence; checked 2026-10-16
- Copeland: FD113-ZUK Oil Pressure Safety ControlModel-specific differential range, timer and shutdown behavior; checked 2026-10-16
- CCOHS: Lockout/Tag outHazardous-energy isolation and verification; checked 2026-10-16
- Red Seal: Refrigeration and Air Conditioning MechanicCurrent-exam standard status; checked 2026-10-16
Examen Studio is independent and not affiliated with the Red Seal Program or Skilled Trades Ontario. This article is a study aid; on the job, follow current codes, regulations and manufacturer instructions.