Both controls sense high-side pressure, but the words open-high and open-low describe opposite switch actions. A 313A study problem becomes easier when you name the controlled load first: is the switch stopping the compressor on excessive pressure, or cycling a condenser fan as pressure changes? This is an original explanation, not a recalled exam item or a setting instruction.
Which control protects the compressor, and which cycles the fan?
Johnson Controls' P70/P72/P170 guide, revision G dated 2026-04-28 and checked 2026-09-29, describes P70C/P70D and P170C/P170D types as open-high controls typically used for high-pressure cutout. It describes P70A/P170A types as open-low controls typically used for condenser-fan cycling. These are examples from one product family; identify the actual model and wiring before interpreting equipment.
An open-high cutout opens its relevant circuit on a high-pressure event. An open-low fan control is open below its low event, then closes as pressure rises to its cut-in. The action tells you what the switch does; the wiring tells you what load that action affects. A fan control is not automatically a pressure safety device.
What happens in a rising-pressure study example?
Imagine two separate, correctly specified controls sensing the same high side. At a low pressure, the open-low fan contact is open. As pressure rises through its fan cut-in, that contact closes and the fan can run if the rest of its circuit permits. If pressure continues rising to the separate open-high cutout's cut-out, that contact opens and can stop the compressor circuit. The points are deliberately unnamed: they are not universal settings.
On the way down, an automatic-reset high-pressure cutout may close again at its lower cut-in; a manual-reset model instead needs its specified recovery condition and a deliberate reset after the cause is found. Johnson Controls lists automatic and manual-reset cutout models separately. Do not assume that every open-high switch resets by itself.
How do cut-in, cut-out and differential differ here?
For an automatic high-pressure cutout, Johnson Controls' P77 revision D guide states cut-in = cut-out − differential. That is a model-specific way to see why the reset point is below the high trip point. Its separate fan-cycling controls use different switch action and scale labels, so copying a cutout setting rule onto a fan control would be a mistake.
Ask three questions when reading a schematic or a written scenario: which pressure side is sensed, does the contact open on a rise or on a fall, and what does that contact control? The low-pressure cut-in/cut-out guide applies the same reasoning on the suction side, where the typical sequence is different. The contactor glossary entry helps separate a control contact from the motor circuit it commands.
Why should this remain a paper exercise?
A pressure-trip symptom alone does not prove a failed switch. Equipment design, refrigerant, load, actual pressures and the manufacturer's instructions matter. The Johnson Controls guide warns that most models in this family are operating controls unless specifically listed for pressure cutout. It also warns against using a control without further precautions when its failure could cause injury or damage. This article does not specify setpoints, bypasses or a field test.
CCOHS lockout guidance, checked 2026-09-29, covers hazardous-energy control and authorized work. For exam preparation, sketch the two contact states on paper and explain the sequence aloud. Check the Ontario exam outline and current-standard note; then use the free sample as unofficial draft practice, not a pass prediction.
Sources
- Johnson Controls: P70/P72/P170 high-pressure applicationsOpen-high cutout and open-low fan-cycling model roles; revision G 2026-04-28, checked 2026-09-29
- Johnson Controls: P77 high-pressure cutout adjustmentsAutomatic-reset cut-in, cut-out and differential relationship; revision D 2024-10-09, checked 2026-09-29
- CCOHS: Lockout/Tag outHazardous-energy controls and authorized work; checked 2026-09-29
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.