How to Check a Micro Switch with a Multimeter
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How to check a micro switch with a multimeter takes three readings per plunger state: common to normally closed, common to normally open, and normally closed to normally open. Take them with the power off and the switch out of its circuit, once at rest and once with the plunger or lever pressed past the point where it changes over. This guide covers loose micro switches and ones in low-voltage circuits. It also shows how plunger travel changes what the meter reads and which datasheet figures a multimeter cannot confirm.
Quick answer
- Safety first: mains work can kill, the CAT rating of your meter must suit the circuit, and panel or service work belongs to a licensed electrician.
- Turn the power off, disconnect the switch, and use continuity mode or a low resistance range.
- At rest, a typical three-terminal switch should connect common to normally closed and leave common to normally open open. Pressing the plunger past its operating point should swap those two results.
- The normally closed and normally open terminals should never connect to each other through the switch.
- A multimeter confirms that the contacts change over. It cannot confirm the datasheet’s contact resistance, insulation resistance, force or life figures.
Electrical safety before you touch a micro switch
Work on mains circuits can kill, so this guide covers only switches on the bench or in low-voltage DC circuits that are turned off. Fluke’s manual for its 15B and 17B meters says to use caution above 30 V ac rms, 42 V ac peak or 60 V dc, because these voltages pose a shock hazard. The manufacturer Crouzet notes that its micro switches are, unless stated otherwise, intended for Class I devices and “their envelopes provide basic insulation”, and it leaves protection against shock to whoever installs the switch. In our reading, that is a reason not to probe a switch that is still installed in powered equipment. Panel, breaker and service work belongs to a licensed electrician.
The meter’s CAT rating must suit the circuit it touches, and the rating comes from your meter’s manual or listing. Fluke’s manual describes CAT II as equipment supplied from the fixed installation, such as TVs, PCs and portable tools, and CAT III as distribution panels, feeders and short branch circuits. If the listing does not state a CAT rating, do not treat the meter as safe for household mains. None of the steps here need a live circuit.
What a micro switch is and how its terminals are labeled
Crouzet calls its micro switches “high-precision, snap-action switches” and lists fast switching “largely independent of actuating speed”, low operating force and short pre-travel with large overtravel. The Rockwell limit switch guide explains the mechanism: movement of the actuator loads an overcenter mechanism, which creates a fast change in contact state once the overcenter position is passed. So the contacts do not drift from one state to the other. They flip.
The common three-terminal version is a single-pole, double-throw changeover. Crouzet numbers its single-break example 1 (C), 2 (NC) and 4 (NO). The Multicomp Pro datasheets list a COM, an NO and an NC terminal on their parts. Numbering differs between makers, so identify the common terminal from the markings or the datasheet before you probe. Crouzet also shows normally-open-only and normally-closed-only versions, which have two terminals.
| Construction (Crouzet) | Terminals | Note |
|---|---|---|
| Single-break changeover | 1 (C), 2 (NC), 4 (NO) | One moving contact between two fixed contacts |
| Double-break changeover | 1 and 2 (NC), 3 and 4 (NO) | “NO and NC circuits must be used at same polarity” |
| Double-break, separated circuits | 13 and 14 (NO), 21 and 22 (NC) | NO and NC circuits are electrically separated |
| Normally open or normally closed only | Two | Form A is normally open, Form B is normally closed |
Expected readings at rest and pressed
The Iowa State industrial-controls textbook defines the normal position as where a spring returns the switch with no applied force. Contacts that are open there are normally open, and contacts that are closed there are normally closed. For a three-terminal switch that gives the pattern below. It is our deduction from those definitions and not a figure from a datasheet.
| Terminal pair | Plunger free | Plunger pressed past the operating point |
|---|---|---|
| Common to NC | Closed (beep, low reading) | Open |
| Common to NO | Open | Closed (beep, low reading) |
| NC to NO | Open | Open |
On a four-terminal double-break switch, read the two terminals of each pair as one pair. At rest the NC pair is closed and the NO pair is open, and pressing swaps them. Crouzet gives the changeover time as under 20 ms and the contact bounce as under 5 ms. This check looks at the state before and after the changeover and does not try to capture the bounce.
Set up the meter
Fluke’s continuity guide gives the order. Turn the dial to continuity, which is usually combined with resistance, and press the continuity button if your model has one. Put the black lead in COM and the red lead in the VΩ jack. The position of the test leads is arbitrary. Connect the leads “With the circuit de-energized”, and make sure the component is isolated from other components in the circuit.
A beep is not a measurement. Fluke says that on a 400.0 Ω range a meter typically beeps at 40 Ω or less, and the 15B and 17B manual puts its threshold at under 50 Ω, so the two sources disagree. For a micro switch, the resistance range is more useful, because you can watch the number change as you press. Fluke recommends the lowest range for switch contacts. Touch the probe tips together first so you know what the leads alone read.
Step-by-step check
- Read the rest state. With the plunger or lever free, read common to NC, common to NO, and NC to NO. Compare with the table.
- Press slowly with the meter on common to NO. Watch the display while you press. The reading should jump from open to a low value at one point, not drift. A flicker right at the changeover can be normal, as explained below.
- Hold the plunger at full travel. Read common to NO and common to NC again. Crouzet’s low contact resistance figure applies at rest or at total travel, so the reading should be at its lowest and steadiest here.
- Release slowly. The contacts should return to the rest state. Note that they return at a different plunger position than where they changed over.
- Repeat several times, including with a different actuator speed. The Multicomp Pro datasheets state an operating speed of 1 to 500 mm/sec, and Crouzet describes the switching as independent of actuating speed, so the changeover point should not depend on how fast you press.
Why the changeover can flicker
Crouzet describes the contact resistance as generally less than 20 mΩ when the plunger is at rest or at total travel. It adds that near the operating and release positions the contact force decreases and “the resistance may increase substantially”. So a brief wobble right at the changeover is normal for a micro switch. A reading that stays high or noisy once the plunger is fully pressed or fully free is not.
Why it releases at a different point
ABB’s limit switch terminology page says snap action contacts have a release position that is distinct from the operating position, a gap it calls differential travel. Slow action contacts release at the same position. E-Switch describes differential travel as the distance from the make point to the release point. So when you ease the plunger back, the contacts reset a little farther out than where they changed over, and that is by design.
A travel map from one datasheet
The Multicomp Pro DM3-1501EZ hook-lever switch lists a free position of 12.5 mm maximum, an operating position of 10.5 ±1.2 mm, a movement differential of 1.3 mm maximum, over travel of 0.6 mm minimum and pre travel of 3.4 mm maximum. The datasheet does not say where the measurements start. For this example we assume they are plunger heights, so a smaller number means more pressed. The sums are ours. Rockwell defines total travel as the sum of pretravel and overtravel.
| Quantity | Calculation | Result |
|---|---|---|
| Pretravel at the nominal operating position | 12.5 − 10.5 | 2.0 mm (datasheet maximum: 3.4 mm) |
| Release position at the maximum differential | 10.5 + 1.3 | 11.8 mm |
| End of over travel at the minimum over travel | 10.5 − 0.6 | 9.9 mm |
| Total travel | 2.0 + 0.6 | 2.6 mm |
The table says that you must press roughly 2 mm before the contacts change over, and let the plunger return to about 11.8 mm before they reset. If you press only 1 mm, the contacts may not move, and a tester might blame the switch when the lever was not pressed far enough. Different actuators change the numbers a great deal. The same family’s other datasheets show the spread.
| Part (Multicomp Pro) | Actuator | Operating force | Release force | Free position | Operating position |
|---|---|---|---|---|---|
| DM3-1501AZ | Pin plunger | 250 gf max | 50 gf min | 7.8 mm max | 7 ±0.3 mm |
| DM3-1501EZ | Hook lever | 45 ±30 gf | 5 gf min | 12.5 mm max | 10.5 ±1.2 mm |
| DM3-1501MZ | Long lever | 45 ±30 gf | 5 gf min | 12.4 mm max | 9.2 ±0.8 mm |
In ounces, using 28.35 g per ounce, 250 gf is about 8.8 oz and 45 gf is about 1.6 oz. That conversion is ours. The point is that a light lever may need very little force, while a pin plunger can need much more, so the same finger press may work on one and not the other.
What a multimeter cannot confirm
A multimeter proves function: contacts that open, close and change over. It does not verify the other datasheet figures, and the comparison below uses the Fluke 15B and 17B specifications as an example of a typical hand meter.
| Datasheet item | Value | Why a multimeter falls short |
|---|---|---|
| Contact resistance | Crouzet: generally under 20 mΩ. Multicomp Pro: 100 mΩ initial | The lowest range resolves 0.1 Ω with an accuracy of 0.5 % + 3 digits, so 100 mΩ is one count and 20 mΩ is below it |
| Insulation resistance | Multicomp Pro: 100 MΩ min at 500 V DC | The highest range tops out at 40.00 MΩ |
| Dielectric strength | Multicomp Pro: 600 V AC between non-continuous terminals | This is a high-voltage test and not a multimeter job |
| Force and travel | See the travel table | Needs a force gauge and a ruler |
| Service life | Multicomp Pro: 200,000 mechanical and 30,000 electrical operations | Cannot be checked with one session of presses |
The arithmetic on the first row is ours: 100 mΩ is 0.1 Ω, which is the meter’s smallest step on that range, and the three-digit term alone is 0.3 Ω. Fluke’s resistance guide puts typical test lead resistance at 0.2 Ω to 0.5 Ω, which is more than either figure. So a reading of 0.0 or 0.1 Ω says the switch is not badly worn, and it cannot say the switch meets 20 mΩ or 100 mΩ. If you need a figure, use a milliohm meter or compare the reading against a known-good identical switch.
Low-voltage and low-current circuits
If the switch sits in a sensor or logic line, the meter check says little about how it behaves at a few milliamps. Crouzet recommends gold-coated contacts for applications below 20 V or 100 mA, especially under 0.3 VA, and says proper functioning can usually be assumed down to 4 V and 1 mA. Below that, the circuit should pass at least a few mA through the contacts, at least while they close. The Iowa State text gives the same reason: a contact that runs on too little current accumulates resistance. So a silver-contact switch that passes your bench check may still be a poor choice for a very low-energy line.
The contacts in a relay obey the same rules, so the ideas carry over to our relay test guide. For general continuity practice, see how to use a continuity tester and the continuity checker guide. When you are choosing a meter for this kind of bench work, the multimeter comparison lists the options.
How to decide: keep, replace or look further
This table is our own reading of the results above.
| What you see | What it suggests | Next step |
|---|---|---|
| States match the table, jump cleanly, steady at full travel | The contacts change over as designed | Look at the wiring, actuator and load |
| No change when pressed | Not enough travel, or a stuck mechanism | Press farther, then replace if it still does not move |
| Common to NC stays closed when pressed | Welded contact or a stuck blade | Replace the switch |
| Reading stays noisy at full travel or at rest | Worn or contaminated contact | Replace the switch |
| NC to NO conducts | Internal short, or a double-break type read wrongly | Recheck the terminal layout against the datasheet |
How we chose
We kept a step only when a page opened on 2026-10-21 supports it. The pages are a Crouzet technical guide, three Multicomp Pro datasheets, an E-Switch article, an ABB terminology page, a Rockwell Automation limit switch guide, a Fluke continuity guide, resistance guide and meter manual, and an Iowa State textbook chapter. The expected-reading table, the travel arithmetic and the unit conversion are our own deductions from those pages, and the plunger-height convention in the travel example is an assumption.
What we could not verify (what the sources do not say)
The pages we opened do not say how micro switches usually fail, what test current a meter sends in resistance mode, or how makers outside Crouzet and Multicomp Pro number their terminals. The datasheets do not state where their travel measurements start. The Crouzet guide describes its own switch range, so its 20 mΩ figure is not a limit for every micro switch. Nothing here covers switches inside mains-powered appliances.
Frequently asked questions
Which terminal is common on a micro switch?
Read the markings or the datasheet. Crouzet numbers its common terminal 1 on the single-break example, and Multicomp Pro labels its terminals COM, NO and NC.
Should the reading change when I press the plunger?
Yes, on a changeover switch. Common to NC should open and common to NO should close once the plunger passes the operating point. If nothing changes, make sure you are pressing far enough.
Why does the switch reset at a different point than it operated?
Snap action contacts have a release position that differs from the operating position. ABB calls this differential travel.
Can a multimeter measure a micro switch’s contact resistance?
Only roughly. The datasheet figures are in milliohms, and a typical hand meter’s smallest step on its lowest range is 0.1 Ω.
Bottom line
Power off, switch isolated: a good changeover micro switch swaps common-to-NC for common-to-NO at a clean point as you press, and a multimeter shows that it works but not that it meets its datasheet.
Sources
- https://zoro.com/static/cms/enhanced_pdf/Waldom%20Electronics_SWxxbasicxxtechnicalxxconceptsxxENxx679897.pdf (opened 2026-10-21)
- https://www.farnell.com/datasheets/4788651.pdf (opened 2026-10-21)
- https://www.farnell.com/datasheets/4788649.pdf (opened 2026-10-21)
- https://www.farnell.com/datasheets/4788652.pdf (opened 2026-10-21)
- https://e-switch.com/blog/switch-pretravel-overtravel-differential-travel-meaning-explained (opened 2026-10-21)
- https://library.e.abb.com/public/3bc07b949ed045eeb142e05426863e59/1SXU000023C0202_09_General_tech_data.pdf (opened 2026-10-21)
- https://documents.mrosupply.com/product_documents/57/16/5716418/limit-td001_-en-p.pdf (opened 2026-10-21)
- https://www.fluke.com/en-us/learn/blog/digital-multimeters/how-to-test-for-continuity (opened 2026-10-21)
- https://assets.fluke.com/manuals/15b17b__umeng0400.pdf (opened 2026-10-21)
- https://iastate.pressbooks.pub/electriccircuits/?p=810 (opened 2026-10-21)
- https://www.fluke.com/en-us/learn/blog/digital-multimeters/how-to-measure-resistance (opened 2026-10-21)
