The push button switch is one of the most common human-machine interface devices on any control panel, yet its specification involves more decisions than its simple appearance suggests. Choosing the wrong action, contact arrangement, or colour can compromise both usability and compliance with machinery safety standards. This guide walks through the anatomy, action types, contact options, illumination, sealing, and colour conventions that control and design engineers should weigh before committing to a part.
Anatomy of a Push Button
A modern industrial push button is a modular assembly rather than a single component. Understanding its parts makes selection and ordering far easier, because the actuator and the contact block are frequently purchased separately and combined to suit the application.
- Actuator (operator): The part the operator presses — a button, mushroom head, key, or selector knob. It sits on the front face of the panel.
- Bezel / collar: The mounting ring and front trim that secures the actuator to the panel and provides the front seal. Bezels may be plastic or chrome-plated metal.
- Contact block: The switching element mounted behind the panel. Blocks carry the electrical contacts and snap or screw onto the rear of the actuator. They are typically stackable.
- Mounting: Most industrial push buttons are built around the 22 mm panel cut-out standard (a 22.3 mm hole), which is near-universal across European and Asian product ranges. A 30 mm cut-out standard is also common in North American (NEMA) equipment, while compact 16 mm and 19 mm formats serve space-constrained applications.
Because the rear assembly is modular, a single 22 mm actuator can often host one, two, or more contact blocks depending on the holder, allowing the switching capacity to be tailored on site.
Momentary vs Maintained Action
The single most important functional choice is the actuation behaviour.
- Momentary action returns to its rest position the instant the operator releases pressure. It produces a switching pulse for as long as the button is held. Momentary buttons are used for start commands, jog functions, lamp test, acknowledge/reset, and any situation where a control system latches the command in software.
- Maintained (latching) action stays in the operated position after release and must be pressed again — or twisted, pulled, or turned with a key — to return. Maintained operators suit on/off control, mode selection, and emergency stop devices, where the physical state must persist without continuous holding.
A common design pattern is a momentary button feeding a relay or PLC seal-in circuit, giving the tactile feel of a momentary press with a maintained logical state.
NO vs NC Contacts and Stackable Blocks
Each contact block provides one or more contact elements in one of two rest states:
- Normally Open (NO): The circuit is open at rest and closes when actuated. Used for start, run, and energising commands.
- Normally Closed (NC): The circuit is closed at rest and opens when actuated. Used for stop commands and safety functions, where loss of continuity (including a broken wire) should de-energise the load — a fail-safe behaviour.
Blocks are commonly stackable, so a single actuator can drive combinations such as 1 NO + 1 NC, 2 NO, or 2 NC. This lets one button simultaneously signal a PLC input and break a hardwired interlock. For safety-related stop functions, NC contacts must use positive (direct) opening operation per IEC 60947-5-1 — a mechanism that forces the contacts apart rather than relying on a spring, indicated by the ⊖ symbol.
Actuator Styles
The actuator shape determines how the button is operated, how easily it is pressed accidentally, and what safety function it can serve. The table below summarises the main styles and their typical uses.
| Actuator style | Description | Typical use |
|---|---|---|
| Flush | Button sits level with or just below the bezel | General commands where accidental operation must be avoided |
| Extended (raised/projecting) | Button protrudes above the bezel | Frequent operation; easy to find by feel |
| Mushroom head | Large domed cap, easy to strike | Stop functions and palm operation; high-visibility actions |
| Emergency stop (twist / pull release) | Red mushroom, latching, twist or pull to reset | Machinery emergency stop circuits |
| Illuminated | Integral LED behind a translucent cap | Status indication combined with command (e.g. lit “run”) |
| Key-operated | Keyswitch actuator | Authorised access, mode selection, lockout of functions |
| Selector | Rotary knob with two or three positions | Mode/source selection (e.g. Auto–Off–Manual) |
Illuminated Push Buttons
Illuminated operators combine a command function with built-in status indication, reducing panel real estate by merging a button and a pilot lamp. Modern units use LED elements for low current draw and long life, available in common supply voltages such as 24 V DC, 24 V AC/DC, 110 V, and 230 V AC — confirm the LED block voltage matches your control supply, as a mismatch will fail to illuminate or burn out the element. Colours typically match the button cap (green, red, white, blue, yellow). Some designs split the cap into illuminated and non-illuminated zones, or pair illumination with the contact state for at-a-glance feedback.
Materials and IP Sealing
The construction material affects durability and the environment the button can survive.
- Plastic (engineered thermoplastic): Cost-effective, corrosion-free, and electrically insulating. Suitable for the majority of indoor control panels.
- Metal / anti-vandal: Stainless steel or zinc-alloy bezels and caps resist impact, tampering, and outdoor exposure. Preferred for public-access, transport, and heavy-duty applications.
Ingress protection is rated to IEC 60529. A front-of-panel rating of IP65 (dust-tight, protected against water jets) or IP67 is common for wash-down, dusty, or outdoor locations; lighter-duty indoor units may be IP40 or IP54. Note that the IP rating usually applies to the front face when correctly mounted with its seal — the rear contact block is protected by the enclosure. For oily or chemically aggressive atmospheres, verify cap and seal material compatibility as well.
Colour and Marking Conventions
Colour coding is not cosmetic — it is governed by IEC 60073 (coding of indicators and actuators) and IEC 60204-1 (electrical equipment of machines). Consistent colour use prevents operator error and is expected by safety auditors. Key conventions include:
- Green — start / on / energise (a permitted, safe condition).
- Red — stop / off / emergency.
- Yellow — abnormal condition or intervention to suppress an abnormal state.
- Blue — mandatory action requiring operator response.
- White, grey, black — no specific assigned meaning; used for general functions where green and red are reserved (black is often preferred for start).
For emergency stop, IEC 60204-1 requires a red mushroom-head actuator on a yellow background, with maintained (latching) action so the machine stays stopped until deliberately reset, and positive (direct) opening of the NC contacts. The reset must require a separate, intentional action (twist, pull, or key). These requirements are mandatory, not stylistic — an E-stop that springs back or relies on spring-only contacts does not comply.
Contact Ratings and Utilisation Categories
Beyond the simple “10 A” headline figure, switching capability is defined by utilisation categories in IEC 60947-5-1, which describe the type of load:
- AC-15 — switching of electromagnetic AC loads such as contactor and relay coils, where inrush current is high.
- DC-13 — switching of DC electromagnetic loads, which are demanding because of the inductive energy stored at break.
A button feeding a PLC digital input is a light, near-resistive load; one directly switching a contactor coil should be specified to the appropriate AC-15 or DC-13 rating at the working voltage. Where the device will be used on UL-listed equipment, check the corresponding UL ratings (e.g. utilisation category and pilot-duty rating) in addition to the IEC figures.
Selection Checklist
Before finalising a part number, confirm:
- Panel cut-out — 22 mm (most common), 30 mm, or compact 16/19 mm.
- Action — momentary or maintained; for E-stop, maintained with twist/pull/key reset.
- Contact arrangement — required combination of NO and NC blocks; positive opening for safety NC.
- Actuator style — flush, extended, mushroom, E-stop, illuminated, key, or selector.
- Illumination — needed or not; if yes, match LED voltage to the control supply.
- Material — plastic for general use, metal/anti-vandal for harsh or public environments.
- IP rating — match to the operating environment (e.g. IP65/IP67 for wash-down or outdoor).
- Colour — per IEC 60073 / 60204-1 (green start, red stop, red-on-yellow E-stop).
- Ratings — utilisation category (AC-15 / DC-13) and voltage for the actual load; UL where applicable.
- Legend/marking — engraved or insert legends for function clarity.
Choosing the Right Push Button
A well-chosen push button balances ergonomics, electrical capability, environmental protection, and standards compliance. Start from the function and its safety implications, settle the action and contact arrangement, then confirm the actuator style, sealing, and colour against IEC 60073 and IEC 60204-1. Getting the contact rating and positive-opening requirements right is what separates a compliant panel from one that fails inspection. Unison Connectors manufactures push button switches for industrial control across these common formats and configurations, supporting OEMs and panel builders who need parts matched to the application from the outset.