How to Choose a Signal Tower: Tiers, Colours, Sound and Mounting
A signal tower (also called a stack light or andon light) tells operators, maintenance staff and supervisors what a machine is doing without anyone walking up to the HMI. Choosing one is mostly about matching the number of tiers, the colours, the sound and the mounting to how the machine is used and where people stand. This guide walks through those decisions using the Werma range as examples.
1. Signal tower, beacon or integrated light?
- Signal towers stack two to five light elements, often with a buzzer or siren on top, and show several machine states at once. They suit CNC machines, assembly stations, packaging lines and anywhere status must be seen across the shop floor.
- Beacons are single-colour or multicolour units used as warning lights and alarms on cranes, process plant and hazardous areas. The Werma D-Series combines optical and audible signals in one beacon.
- Recessed and integrated lights sit flush in a panel or machine frame. Compact 35 mm units such as the Werma C MC35 beacon combine red, yellow and green with an 80 dB buzzer and an M12 connector, and Werma LineLIGHT Fusion integrates status lighting into the machine guarding itself.
2. Colours and what they mean
Standard colour meanings make a machine readable to anyone in the plant. IEC 60204-1, the standard for electrical equipment of machines, gives a widely used convention for indicator lights:
| Colour | Meaning | Typical use on a tower |
|---|---|---|
| Red | Emergency or hazardous condition | Machine fault, stopped on alarm, immediate action needed |
| Yellow | Abnormal condition, critical condition approaching | Warning, low material, tool life ending, door open |
| Green | Normal condition | Running in automatic |
| Blue | Mandatory action by the operator | Operator call, material or quality check request |
| White | Neutral, other information | Machine powered, manual mode, cycle complete |
Keep red at the top, as on traffic signals, so the most urgent state is the most visible. Write the colour assignments into the machine documentation and use the same scheme on every machine in the plant.
3. How many tiers?
Three tiers (red, yellow, green) cover most machines: fault, warning and running. Add blue for an operator-call or material-request signal on assembly lines, and white for a neutral state such as manual mode. More than four or five tiers tends to confuse rather than inform; if you need more states, use flashing and steady light on the same colour instead of adding tiers.
A ready-configured tower saves selection and assembly time. The Werma KombiSIGN 72 comes as a complete red, yellow and green tower under one order number, and its TwinLIGHT elements combine continuous and flashing light in one element. Modular towers such as the KombiSIGN 40 can be retrofitted with further elements, for example an acoustic element, later. The Werma RST 56 is available with 2 to 5 tiers in a slim 57 mm housing for small to medium machines and control cabinets.
4. Audible signal
Light alone fails when the operator is looking the other way. Add a buzzer or siren when a fault must be noticed immediately or when the machine is not in the operator's line of sight.
- Choose a sound level clearly above the ambient noise. The RST 56 buzzer reaches a maximum of 90 dB, and compact towers such as the Werma CST55 offer an optional 85 dB pulsed buzzer.
- In very loud areas such as press shops, the KombiSIGN 71 offers a self-adjusting siren element that adapts to the ambient noise level, and a vocal element that plays your own mp3 or wav messages.
- Use different tones for different states, and give operators an acknowledge button so a sounding alarm does not get disconnected permanently.
5. Mounting and connection
- Base mounting on top of a machine or cabinet is the most common.
- Tube mounting raises the tower above guards and nearby equipment so it can be seen across the floor.
- Angle or wall mounting puts the tower on the side of a machine or a wall; KombiSIGN 72 is suitable for floor and angle mounting.
- Built-in mounting suits panels and operator desks, for example the M30 installation mounting of the RST 56.
For wiring, a pre-assembled M12 connector speeds installation and replacement compared with a cable and terminal block. Check the supply voltage (most towers run on 24 V; some, like KombiSIGN 40, accept 24 V AC/DC) and the logic of the control outputs.
6. Environment
Look at the IP rating against where the tower will live. Indoor dry areas such as electronics assembly can use IP40 towers. Food, dairy and pharma lines with washdown need IP66 or IP69K. The Werma CleanSIGN is designed for clean rooms and food applications with IP67/IP69K protection and an 85 dB buzzer.
7. Connected signal towers
When the PLC already uses IO-Link, a connected tower reduces wiring to one standard sensor cable and makes the tower configurable from the program. The Werma eSIGN is available in 24 V and IO-Link versions and can show classic tiers, full-surface signalling or a fill-level indication from 0 to 100%, with IP66/IP69K protection and a UV-resistant UL Type 4X housing.
Quick selection checklist
- Machine states to show, mapped to colours.
- Number of tiers, and whether flashing or steady light is needed.
- Audible signal and required sound level.
- Mounting position and viewing distance.
- Supply voltage, connection (cable or M12) and control type (discrete or IO-Link).
- IP rating and cleaning conditions.
Signal towers from Rajdeep
Rajdeep Automation supplies Werma signal towers, beacons and integrated lights across India, from ready-configured three-colour towers to IO-Link connected units. Browse the range under stack lights.
Tell us the machine, the states you want to signal, the mounting position and the environment through the enquiry form, and our team will recommend a configuration with the datasheet and price.