An RFQ that says “three-position cam switch” and nothing else is not an order. It is the start of a week of email.
It happens every month. The buyer knows the function they want — select phase, reverse a motor, switch a heating stage — but not the vocabulary that turns it into a manufacturable part. The factory guesses, the sample comes back wrong, three weeks disappear. Almost all of that friction goes away once you can read a cam switch switching table.
A cam switch switching table is the drawing. The part number is just a name.
A rotary cam switch is a stack of rotating cams on a common shaft, and each cam pushes a contact set open or closed as the shaft turns. Change one cam profile and you have changed the device’s function without changing its appearance.
That is why a cam switch cannot be specified by appearance. Two units can look identical — same Φ22 cutout, same handle, same grey body — and behave completely differently inside. The part number is a label for one cam program. The switching table is the program itself. If you intend to reorder the part, keep the table in your own drawing system, because a part number is only as reliable as the factory’s internal records.
Five parameters that fully define the device
| Parameter | What it means | Typical values |
|---|---|---|
| Poles / terminals | How many independent circuits the switch can break | 3, 5, 6, 8, 12 terminals |
| Positions | How many discrete shaft positions exist | 2, 3, 4, or more |
| Angle per position | Shaft rotation between adjacent positions | 30°, 45°, 60°, 90° |
| Action | Whether the shaft holds or returns | Latching or spring-return |
| Contact state per position | Open or closed, per contact, per position | This is the switching table |
Get these five right and a competent factory can build anything. Get one wrong and no amount of goodwill saves the order.
How to read a cam switch switching table
A cam switch switching table is a grid: contacts down the side, positions across the top, and a mark in each cell telling you whether that contact is closed in that position.
Take a line-to-line voltmeter selector. Three incoming phases, one voltmeter, four shaft positions including OFF. Terminals 1, 2 and 3 are L1, L2 and L3; terminals 4 and 5 feed the meter.
| Contact | Terminals | Pos 0 (OFF) | Pos 1 | Pos 2 | Pos 3 |
|---|---|---|---|---|---|
| c1 | 1 → 4 | open | closed | open | open |
| c2 | 2 → 5 | open | closed | open | open |
| c3 | 2 → 4 | open | open | closed | open |
| c4 | 3 → 5 | open | open | closed | open |
| c5 | 3 → 4 | open | open | open | closed |
| c6 | 1 → 5 | open | open | open | closed |
The columns give you the function:
- Pos 1 closes c1 and c2: L1 to the meter’s U terminal, L2 to V. Reading L1–L2.
- Pos 2 closes c3 and c4: L2 to U, L3 to V. Reading L2–L3.
- Pos 3 closes c5 and c6: L3 to U, L1 to V. Reading L3–L1.
- Pos 0 closes nothing. The meter is isolated.
Six cams, four positions, one function. That grid is the cam switch contact sequence, and reading it is only half the job.
Break-before-make is not a detail. On a voltmeter switch it is the design.
During the transition from Pos 1 to Pos 2, c1 and c2 must open before c3 and c4 close. If any overlap exists — even a few milliseconds while an operator turns the handle slowly — L1 is connected to U at the same moment L2 is connected to U. That is a phase-to-phase short through your voltmeter, and the meter becomes the fuse.
A voltmeter selector must therefore be break-before-make, and the cam profiles have to guarantee a gap. That is not something you specify verbally and hope for. It belongs in the drawing.
Now flip the logic, because the same question has the opposite answer on an ammeter switch. An ammeter selector works on current transformer secondaries, and a CT secondary must never be open-circuited while primary current flows. The secondary voltage rises until something fails, and the something is usually the CT insulation or whoever is standing nearby. So an ammeter selector must be make-before-break: the incoming contact closes before the outgoing one opens, and the switch shorts the non-selected phases itself.
Same looking device. Opposite cam profile. Opposite consequence if you get it wrong.
Cam switch terminal numbering: the convention that bites on imported drawings
Terminal numbering is not standardised between manufacturers, and this is where a perfectly correct switching table still produces the wrong switch.
The most common European convention is odd/even pairing: contacts numbered 1–2, 3–4, 5–6, each pair one break, with the odd terminal usually the incoming side. A six-contact switch runs 1 through 12. The other common layout is sequential row numbering: 1, 3, 5, 7 across the top row and 2, 4, 6, 8 below, so a contact is identified by its vertical pair.
Both appear in real catalogues, and a cam switch switching table written for one reads as nonsense against the other. Two habits prevent it: name the terminal function rather than just the number, because numbers move between manufacturers and functions do not, and ask for the terminal diagram with the quotation.
The same applies to key-operated types. A 3 terminal key type cam switch and a 5 terminal key type cam switch may share a housing and a key profile while breaking completely different circuits, and the switching table is the only reliable way to tell them apart.
Worked example: a three-lamp selector with two supplies
The most frequent custom request we receive is a selector that also indicates what is selected. This is where the three-lamp cam switch and dual-indicator cam switch families live.
Say you need three positions for a dual-supply changeover: mains, generator, OFF. Each live position must light a lamp, and the two supplies must never connect together, not even momentarily. That produces two requirements pulling in opposite directions. The main contacts must be break-before-make, or you bridge two independent supplies. The lamp contacts should be make-before-break or at least simultaneous, because a visible gap between selecting a supply and lighting its lamp reads to an operator as a fault.
| Function | Terminals | Pos 1 | Pos 2 | Pos 3 (OFF) |
|---|---|---|---|---|
| Mains to load | 1 → 2 | closed | open | open |
| Generator to load | 3 → 4 | open | closed | open |
| Mains lamp | 5 → 6 | closed | open | open |
| Generator lamp | 7 → 8 | open | closed | open |
| Neutral reference | 9 → 10 | closed | closed | open |
Two sentences must accompany that table: “break-before-make on 1–2 and 3–4, no overlap permitted”, and “lamp contacts close at the same shaft angle as the corresponding main contact, ±3°”. An OFF position closes nothing at all, and it is the parameter customers most often forget to specify.
Electrical data you must supply, or the factory will guess
The switching table defines the logic. It says nothing about whether the switch survives the load.
Rated thermal current (Ith) is the current the contacts carry continuously without exceeding temperature rise limits — trivial for instrumentation switching, decisive for motor switching.
Rated operational current (Ie) at your voltage. Always state the voltage, because a contact rated 20 A at 230 V AC is not a 20 A contact at 690 V.
Utilisation category is the one buyers skip and the one that causes field failures. AC-21A is resistive, easy duty. AC-22A adds inductive load. AC-23A means motor starting and plugging, typically rated at half the AC-21A current or less for the same contact. If the switch starts a motor and the drawing says AC-21A, the contacts will weld within months.
DC specifics. Interrupting DC is far harder because there is no natural current zero, so a cam switch rated 10 A AC may be good for 1–2 A DC at the same voltage. For 110 V DC control circuits, ask for a DC rating, not an AC rating with an asterisk.
What to send the factory for a custom cam program
- Positions, including whether OFF counts as one and which position is the resting position.
- Angle per position, and whether the handle is latching or spring-return.
- The switching table — contacts as rows, positions as columns, open or closed in every cell.
- Break-before-make or make-before-break, stated per contact rather than once for the whole switch.
- Terminal numbering you require, with functions named.
- Electrical data — voltage, current, AC or DC, utilisation category, and whether a motor is involved.
- Mechanical and marking data — cutout diameter, handle or key type, shaft length, legend plate text and language.
For key-operated versions, add the key profile and whether keys must be removable in specific positions. A panel key cam switch where the key can be withdrawn in a live position is a very different safety story from one where it can only be withdrawn at OFF, and buyers forget to mention it surprisingly often.
Where a switch simply bolts into an existing cutout with no custom cam work, the standard families are faster and cheaper. A general type cam switch covers most on/off and changeover duties; the 3 terminal general type and 5 terminal general type versions handle single-phase and three-phase circuits. The central mounting cam switch suits mechanisms behind the panel centre. For motor duty above a few kilowatts, see our guide to selecting cam switches for motor control; for high pole counts, the 6-pole rotary cam switch notes.
Three errors that show up in most cam switch RFQs
A part number with no table. The number means nothing outside the factory that issued it. Position count without angle is equally thin — half a specification, and it hides the OFF question.
An AC rating on a DC circuit. Common on 110 V DC panels and battery circuits, and the failure is contact welding or an arc that will not extinguish.
Break-before-make assumed but not written down. Every engineer knows it is needed; almost nobody states it. The one order in fifty built the other way takes out a transformer.
Frequently asked questions
Can a cam switch be modified in the field to change its function?
Sometimes, if the frame is a modular stack with replaceable cam elements. Many compact Φ22 units are not, so treat anything safety-related as a sealed assembly.
How many positions can one cam switch have?
Most panel-mount cam switches run two to four positions; larger frames go further. Past four, the handle throw gets long and the operating force rises, so two switches are often the better ergonomic answer.