Danfoss Pressure Switch: 6 Questions I Ask Before I Approve a Replacement

I work in quality control at an industrial controls distributor. I review pressure switches before they ship, and I also review the units that come back. The returned units teach me more. I touch roughly 200 unique items a year—maybe 180, I would have to check the system—and most Danfoss pressure switch problems are not random breakage. They are setpoint and application mistakes.

Questions I keep answering

• What is the real difference between the Danfoss KP, RT and MBC series?
• How do I set a Danfoss pressure switch properly?
• Can I fit a cheaper third-party switch instead of a Danfoss MBC 5100?
• Why does ordering the same product code as the old switch still cause trouble?
• What is the cheapest thing to check before replacing a Danfoss high pressure switch?
• What should I verify on the data sheet before buying a Danfoss pressure switch?

1. What is the actual difference between the KP, RT and MBC series?

They are not different quality tiers. They are different design families. KP controls are compact and common in refrigeration and air-conditioning duty, particularly where panel space is tight. RT controls are a broader and generally heavier-duty industrial range, often used where you need visible field adjustment and a more substantial switch assembly. MBC controls are compact but built with an eye on marine and industrial approvals; the MBC 5100 is one of those type-approved units.

When someone says “I need a Danfoss high pressure switch,” I still ask for the product code. High pressure is not one pressure range. A KP1 and an RT116 are both specific product codes, but they are not interchangeable just because both can act as pressure controls. The pressure range, differential, reset logic, contact rating, thread and enclosure class all have to match the application. The product code on the nameplate is the fastest way to move from “this might fit” to “this is correct for the duty.”

2. How do I set a Danfoss pressure switch so I do not get a callback?

I start with a regulated pressure source and a continuity meter, not by trusting the printed scale. The scale on the housing is an indicator, not a calibration reference. What I mean is, if you move the pointer to a pressure value and never confirm when the contacts actually change state, you have not set the switch—you have only moved a number.

Check the wiring diagram for the exact product code, connect to the contacts that are used in the circuit, then raise and lower pressure slowly. Record the cut-in and cut-out pressures. Adjust the main setting and differential according to the manual, then cycle the switch at least three times and record again. The last step catches drift and wrong reset behavior. Take this with a grain of salt, but I think a third of the “failed” switches that come back to our bench were simply adjusted incorrectly or tested with the wrong contacts.

3. Can I fit a cheaper third-party switch instead of a Danfoss MBC 5100?

I have mixed feelings. On one hand, I will not say every industrial application needs an original part. On the other, I see the aftermath when someone chooses a replacement by price, thread size and a product photo. The cheaper part may look close and it may pass a slow bench test. The real question is whether it behaves the same at the operating limit: contact load, reset behavior, enclosure protection, pressure connection, temperature rating and vibration behavior.

What convinced me was a side-by-side test. We put a sample advertised as compatible with the MBC 5100 next to the Danfoss unit. On a slow pressure rise, both switched close to the setpoint. On a faster pressure rise, one switched later. It was a sample of one, so I would not make a rule from it. But it is enough to ask the supplier for the same test data before you install a whole batch. If their answer is only “it is compatible,” the risk lands on your system, not on their listing.

4. Why does ordering the same product code as the old switch still cause trouble?

Because the old switch may have been wrong from the beginning. I see this most often after nuisance trips. Someone references the existing switch, orders the same code, installs it, and still gets nuisance trips. That is not a switch problem; that is an application problem. The original range or differential was selected for conditions that do not match the actual system.

Another reason is product supersession. The code on the old nameplate may not be current. If a supplier sends a replacement or superseding variant without checking the circuit logic, a visually identical unit can have different reset behavior or electrical ratings. That is why I treat “same part number as before” as a starting point, not the end of an investigation.

5. What is the cheapest thing to check before replacing a Danfoss high pressure switch?

Whether it has actually failed. If the compressor is stopped on a high-pressure cut-out and the switch needs a manual reset, the system pressure must drop below the reset point, then someone has to reset the switch. A new switch will not fix the trip by itself.

Oh, and check the terminal numbers. Several pressure switches use one set of contacts for the control circuit and a different set for a signal circuit. If the installer used the wrong pair, the switch can look dead while it is actually operating correctly.

6. What should I verify on the data sheet before buying a Danfoss pressure switch?

If I am approving a switch for a project, I want four things: the exact product code, the current datasheet, the wiring and installation instructions, and the approval or test declaration if the application requires it. The product code tells me which variant. The datasheet gives me pressure range, differential, contact ratings, enclosure protection, ambient temperature range, connection type, reset mode and mechanical life.

If a supplier cannot provide a datasheet for the specific code, that is an immediate flag. And if I need an MBC 5100 with marine approvals, I need the MBC 5100 with the specified pressure range and approval markings, not something described as “similar” or “same fit.” Similar is not an engineering phrase. A low price does not change that.