Choosing a push button involves more than matching a colour or actuator diameter: contact behaviour, electrical duty, safety function, panel fit and environmental exposure all affect whether the control works reliably. By the end, you will be able to distinguish operating types, read contact and rating information, separate ordinary controls from emergency stops, and specify a button that fits both the circuit and the panel.
Key takeaways
- Match momentary or maintained action to the machine state you need.
- Use NO, NC or changeover contacts according to the control-circuit logic.
- Treat emergency stops as safety devices, not ordinary machine-control buttons.
- Verify contact ratings, panel cutout size, IP rating and operator access before buying.
Match the Button’s Action to the Machine’s Required State
Choose the operator action from the machine’s required state, not from the button’s appearance. A momentary push button changes its contacts only while you press it, then spring-returns when released.
Use it for a command that must disappear when the operator lets go; replacing it with a maintained type can change restart behaviour despite identical contact ratings.
Treat operator action and contact arrangement as separate decisions.
| Design | Behaviour after release | Suitable use |
|---|---|---|
| Momentary push button | Returns to its normal position immediately | Start, jog or reset commands that must not remain active |
| Maintained push button | Stays in the changed position until pressed or turned back | A state that must remain selected |
| Latching push button | Remains mechanically latched after actuation and releases through a second action or mechanism | A persistent command where a toggle-like action is acceptable |
| Changeover push button | Transfers a common contact between its NC and NO paths; the mechanism can be momentary or maintained | Switching between two control paths |
A fast PLC input can see several transitions from one press because contact bounce is normal. Add debounce in the PLC program, input filter, timer or suitable hardware when the logic is sensitive; do not hide the problem by arbitrarily specifying a slower button.
Choose a maintained selector switch when the operator must see and deliberately choose a mode such as Hand or Auto, or when accidental changes require more resistance. A key-operated selector adds control over who can change that state, while a guarded or flush operator reduces unintended actuation in a crowded or vibrating panel.
Read NO, NC and Changeover Contacts in the Control Circuit
A typical start-stop circuit sends control voltage through the stop device, overload contact and start button to the contactor coil. Pressing the momentary start button energises the coil; its auxiliary normally open contact seals the circuit around the start button. Releasing the button leaves the coil energised until the stop button or overload contact opens.
| Arrangement | Released state | Pressed state | If damaged or failed |
|---|---|---|---|
| Normally open contact | Circuit open; no coil current | Circuit closes and permits operation | An open failure prevents starting; welded contacts can cause an unexpected start |
| Normally closed contact | Circuit closed; control circuit can operate | Circuit opens and stops the coil | An open failure prevents operation; welded contacts can defeat the stop function |
| Changeover contact | Common connects to NC terminal | Common transfers to NO terminal | The machine may lose the intended interlock or receive the wrong signal |
Specify the operator action separately from the contact arrangement. A maintained button stays changed after release, while a momentary button returns; use a maintained selector switch when mode or enable status must remain visible and deliberate.
A push button normally switches the contactor coil, not the motor. Check the contact block’s rated operational current Ie and voltage Ue for the actual duty, such as IEC 60947-5-1 AC-15 for an AC coil or DC-13 for a DC coil. Do not substitute thermal current Ith for that rating.
Compare electrical life as well as mechanical life, because coil inrush, inductance and arcing reduce switching endurance.
An NC contact is not automatically safety-rated: verify positive-opening marking and certification on the contact block.
Separate Emergency-Stop Functions from Ordinary Machine Controls
A standard red push button commands a process; an emergency-stop device initiates a safety function. The emergency version uses a red mushroom actuator on a yellow background, latches when pressed, and requires twist-release or pull-release. Its contact element must provide positive opening: the mechanism forcibly opens NC contacts instead of relying only on spring return.
ISO 13850 sets the design principles; red colour alone proves nothing.
- Define the stop category: Category 0 removes power immediately, while Category 1 performs a controlled stop before power removal.
- Use a contact block marked for positive opening and the required safety approval; NC alone is not a safety rating.
- Route the circuit through a suitable safety relay or safety controller, with wiring that detects broken conductors and short circuits.
- Provide diagnostics for contact disagreement, welded contacts and loss of channel continuity.
- Place the reset outside the hazard zone, make it deliberate, and require a separate start command.
- Prevent automatic restart when the actuator is reset, while retaining guards, interlocks and other protective measures.
Two ordinary NC contacts do not establish a Performance Level or SIL. The machinery risk assessment determines the required architecture, monitoring and stop category, and you must verify those details on the contact block and controller documentation.
If a fast PLC input reads the device directly, add an appropriate input filter or debounce logic so contact bounce cannot create repeated commands.
Check Panel Fit, Environmental Protection and Operator Access
Before ordering, compare the operator’s actual cut-out, panel thickness and installation conditions with the series datasheet—not its nominal size. A 22 mm mounting hole is not always correct for a 22.5 mm push button, and a 30 mm mounting system may require a hole near 30.5 mm.
1. Confirm the drilling diameter, anti-rotation notch or pin, maximum panel thickness, and clearance behind the panel. Check that the contact block, lamp module and wiring will not strike rails, ducts or the enclosure door.
2. Verify the complete installed assembly’s ingress protection. Under IEC 60529, IP65 is dust-tight and protected against water jets; IP66 also withstands powerful water jets. The gasket, cut-out, tightening torque, cable entry and unused-hole plugs determine whether the installed panel achieves that rating.
3. Check the rear terminals separately. A front-face IP rating does not protect exposed contact blocks, so confirm finger protection, creepage, clearance, conductor size, terminal torque and separation from power wiring.
4. Match the actuator to access conditions. A projecting or mushroom head helps a gloved operator find it, while a flush or guarded button reduces accidental operation. Check actuation force, glove compatibility, vibration, oil or coolant exposure, and nearby moving parts before choosing a raised actuator.
A button that fits the hole can still fail in service if water enters around a poorly tightened gasket or vibration loosens its terminals. Treat the enclosure, operator, seal and rear wiring as one installation—not as separate catalogue features.
Compare Designs and Confirm the Datasheet Before Purchase
Compare the complete assembly, not the actuator material or headline amperage. PVC suits general indoor panels; metal tolerates harsher handling. An enclosed unit adds protection around the operator but can restrict access and increase panel depth.
For options from Dollex Control Panel Accessories, request the individual contact-block and illumination datasheets rather than relying on the product title.
| Design | Choose it when | Check before ordering |
|---|---|---|
| PVC | Cost, insulation and ordinary indoor use matter | UV, impact and temperature limits |
| Metal | Repeated impact or a robust operator face is needed | Corrosion resistance, earthing and panel thickness |
| Illuminated push button | The operator needs visual status or location feedback | LED AC/DC voltage, polarity, resistor or module, and lamp life |
| Key-operated push button | Accidental or unauthorised operation must be limited | Key removal position, key retention and electrical life |
| Twin push button | Two related commands must share one station | Independent contacts, actuator spacing and simultaneous operation |
| Enclosed | Dust, washdown or accidental contact threatens the rear assembly | Complete IP rating, cable entries, terminal access and depth |
Verify these figures before purchase:
- Mechanical life counts unloaded actuator cycles; electrical life reflects arcing, inrush, inductance and duty cycle. A high mechanical figure does not prove suitability for frequent contactor or solenoid switching.
- Match the contact-block rating to Ie and Ue under IEC 60947-5-1: AC-15 for AC coils and DC-13 for DC coils. Do not substitute Ith for either rating.
- For a PLC input, check minimum switching voltage and current against the input’s ON-current and leakage limits. A contact that switches a 230 V relay can give intermittent low-current signals.
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Frequently asked questions
How do you match a push button’s action to a machine state?
Use a momentary button when the machine should respond only while the operator presses it. Use a maintained action when the selected state must remain after release.
What do NO, NC and changeover contacts mean?
Normally open contacts close when the button is pressed. Normally closed contacts open when pressed. Changeover contacts transfer a common connection between normally closed and normally open terminals.
How is an emergency-stop button different from an ordinary push button?
An emergency stop is designed to stop hazardous motion quickly and remain actuated until deliberate reset. Select it as part of the machine’s safety circuit rather than substituting an ordinary control button.
What should you check before installing a push button in a panel?
Confirm the panel cutout diameter, mounting depth, contact ratings, terminal type, IP rating, temperature range and access for the operator.
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