Key Takeaways
A flame detector in an industrial plant needed only four conductors for its alarm and fault signals, even though the cable carried eight cores in four colour pairs.
The colours printed on the engineering drawing did not match the real cable colours, because drawing colours were only visual aids and not a strict wiring code.
The relay terminals marked NO and COM on the detector are simple switch contacts: they close only during an alarm or a fault.
The field cable was an unshielded twisted-pair type, while the metal cable gland with a circular spring was an EMC gland designed for shielded cable, so in this case the spring had no electrical role.
Clear local colour rules on site—red for alarm, yellow for fault, green for power, blue for data—made the final installation easier to understand for anyone opening the enclosure in the future.
Story & Details
The scene is a production plant in the Netherlands in late November 2025. High above the floor, a technician stands on a steel walkway, holding a red control cable in one gloved hand and a phone with a wiring diagram in the other. The task is simple on paper: connect a new flame detector so that the control system can see its alarm and fault signals.
The cable in the hand tells a different story from the lines on the screen. Under the outer red sheath sit eight slim conductors. They form four twisted pairs: green with white, yellow with white, red with white, blue with white. The drawing, however, shows only thin orange, blue and other bright lines snaking across the page. The line colours in the diagram do not match the insulation colours in the real cable, and that mismatch raises a natural question: is there a hidden standard that ties the two sets of colours together?
In many control and telecom cables, manufacturers still follow the old German DIN 47100 colour chart, which lists core colours and combinations such as white, brown, green and yellow, even though the standard was officially withdrawn years ago [1][2]. These charts describe the colours inside the cable, not the colours used by drafting software. On most engineering drawings, line colours are simply there to keep different circuits apart. They can hint at functions, but they are not a legal wiring code.
The flame detector in this story is like many industrial models. Inside its housing sit relay outputs for alarm and fault, with terminals marked in a small printed row: Vin plus and minus for the power supply, three fault terminals (NO, COM, NC), three alarm terminals (NO, COM, NC), optional analogue outputs such as 4–20 milliamp, and sometimes a communication pair called RS-485 [3][7][8]. Only some of these are used in the plant. The diagram for this installation shows wires on terminals 4 and 5 for the fault relay, and 7 and 8 for the alarm relay. The RS-485 pair on terminals 13 and 14 is not used at all.
Another small part ties the inside of the detector to the outside world: a grey and green plug with four factory-fitted leads. Two of the leads are yellow, two are red. The plug snaps onto a resistor module inside the detector. From left to right, the four internal wires carry fault normally open, fault common, alarm common and alarm normally open. The yellow pair belongs to the fault relay. The red pair belongs to the alarm relay. The plug does not know anything about the colour pairs in the field cable; it only exposes four switch contacts.
In the control room, the meaning of the relay markings is standard. A contact marked NO, for “normally open”, sits open when the detector is healthy. It closes only when the detector wants to signal an event. For the alarm relay, that event is a fire. For the fault relay, that event is an internal problem, such as a blocked lens or an electronics error [3][7][8]. The terminal marked COM, for “common”, is simply the other side of the switch. When the control system feeds a voltage through COM and uses NO as a return, the contact acts like any light switch: open in normal times, closed in a fire or fault.
The technician decides that four conductors from the red field cable are all that is needed. One twisted pair will carry the alarm relay. Another pair will carry the fault relay. The power supply and the unused RS-485 pair travel on other cables, so the green and blue pairs inside this cable can stay parked as spares. To keep things clear, the team on site agrees on a local rule: red cores stand for alarm, yellow cores stand for fault, green is reserved for power, and blue is reserved for data lines. The white partners in each pair become the “common” side of each relay.
The final mapping is simple. The red core in one pair connects to alarm normally open, and its white partner connects to alarm common. The yellow core in another pair connects to fault normally open, and its white partner connects to fault common. The four unused conductors are neatly cut short, insulated and folded back so they cannot touch anything. A quick test with a multimeter confirms that each relay contact appears on the correct pair at the far end of the cable.
While the wiring is taking shape, another small mystery appears on the workbench. The cable entry kit supplied with the detector includes a bright metal cable gland and, oddly, a delicate ring made from fine spring wire. The gland body is nickel-plated brass with a rubber sealing insert. The spring ring is shaped like a tiny crown. When assembled, the ring sits inside the gland and presses inward.
Manufacturers of electromagnetic compatibility, or EMC, cable glands explain that this spring is meant to press firmly all around the metal shield of a cable [5][6][10][18][26][30]. In shielded cable, a braided or foil layer encloses the inner conductors. When the installer strips the outer jacket and folds the shield back over the gland entry, the spring ring bites gently into the metal braid. The shield then bonds to the metal housing of the detector with a full 360-degree connection, helping stray interference currents flow safely to earth instead of into sensitive electronics.
In this installation, the red control cable has no shield at all. Under the jacket there is only plastic insulation and copper cores, with no braid, foil or bare drain wire. Without a shield, the spring has nothing conductive to grip. It simply presses against plastic. The gland still seals the cable and provides strain relief, but its EMC function sleeps unused. For RS-485 communication links over long distances, shielded twisted pair is often recommended to reduce noise [3][4][5][9][13][17][21][25][29]. Here, though, RS-485 is not connected, and the short run of unshielded cable for simple relay contacts is more than adequate.
The name printed near the detector symbol on the drawing hints at its origin. The word is a compound Dutch label for flame detector, formed by joining the Dutch words for “flames” and “detector” into one. That small detail is a quiet reminder that engineering language on paper can shift from country to country, even when the hardware and the core electrical ideas remain the same.
A few hours later, the work is done. The four live conductors are tight in their terminals. The unused ones are safely parked. The gland grips the cable and holds the enclosure closed against dust and moisture. Back in the control system, the alarm and fault inputs change cleanly as the detector is tested. The colours on the screen are still not the same as the colours on the cable, and that no longer matters. Function, not ink, is what counts.
Conclusions
The story of this flame detector shows how easily colour can mislead in industrial wiring. Diagram lines exist mainly to help the eye, while the real logic lives in labels such as NO and COM and in the relay functions behind them. A simple agreement on local colour use—red for alarm, yellow for fault, green for power, blue for data—turned a confusing cable into a clear and reusable pattern.
The unused features of the hardware, such as RS-485 terminals and an EMC cable gland designed for shielded cable, highlight a common reality in plants: equipment is built for many scenarios, yet each site uses only what it needs. When a cable has no shield, the elegant little spring inside an EMC gland has nothing to do, but the gland still offers good sealing and strain relief.
Careful reading of terminal names, a modest multimeter check and tidy treatment of spare conductors are enough to turn a worry about mismatched colours into a clean, maintainable installation. The flame detector is ready to watch for real problems, not wiring puzzles.
Selected References
[1] Wikipedia. “DIN 47100.”
https://en.wikipedia.org/wiki/DIN_47100
[2] Eland Cables. “What is the DIN 47100 Core Colour Chart?”
https://www.elandcables.com/the-cable-lab/faqs/din47100-colour-chart
[3] Texas Instruments. “RS-485 Design Guide (Application Report SLLA272).”
https://www.ti.com/lit/pdf/slla272
[4] ABB. “RS-485 Design and install best practices – Guidelines for system planning and field installation.”
https://library.e.abb.com/public/19382ad529ef49f0803e1ec89fbbf6b3/LVD-EOTKN121U-EN_RS-485designandinstallbestpractices_REVA.pdf
[5] PFLITSCH. “EMC cable glands.”
https://www.pflitsch.de/en/cable-gland/emc-cable-gland/
[6] Amphenol LTW via CODICO. “EMC-Shielded Cable Glands by AMPHENOL LTW.”
https://www.codico.com/en/current/news/emc-shielded-cable-glands-by-amphenol-ltw
[7] Det-Tronics. “IR Multispectrum IR Flame Detector Model X3302 – Product Guide.”
https://www.det-tronics.com/wp-content/uploads/sites/8/2025/04/90-1232-1.1-Enhanced-X3302.pdf
[8] Spectrex. “40/40 Series UV/IR Flame Detector Models – Application Guide.”
https://www.spectrex.net/documents/guide-40-40-series-uv-ir-flame-detector-models-40-40l-lb-40-40l4-l4b-spectrex-en-us-1459808.pdf
[9] Renesas. “RS-485 Design Guide Application Note.”
https://www.renesas.com/en/document/apn/rs-485-design-guide-application-note
[10] HX Cable Gland. “The Block Type EMC IP68 Brass Plated Nickel Metric Cable Glands.”
https://www.hxcablegland.com/the-block-type-emc-ip68-brass-plated-nickel-metric-cable-glands/
[11] YouTube – RST. “Euro-Top EMC (4th generation).”
https://www.youtube.com/watch?v=025XtU7qWKQ
Appendix
Cable gland
A cable gland is a mechanical fitting that lets a cable enter an enclosure while gripping the jacket and sealing against dust and moisture. Some cable glands, especially metal types, also help connect cable shields to the enclosure.
Common (COM)
Common is the shared terminal of a relay contact set. It forms a simple switch with a matching normally open or normally closed terminal and carries the current that the relay is meant to control.
Electromagnetic compatibility (EMC)
Electromagnetic compatibility is the ability of electrical equipment to work correctly in the presence of electromagnetic noise and to avoid emitting excessive interference. EMC cable glands help by bonding cable shields to metal housings so unwanted currents can flow safely to earth.
Flame detector
A flame detector is a safety device that senses the presence of fire, often through ultraviolet or infrared light, and reports its status through relay contacts and sometimes analogue or digital outputs to a control system.
Multicore cable
A multicore cable is a cable that contains several separate insulated conductors under a single outer jacket. In control work these conductors are often arranged as twisted pairs to reduce electrical noise.
Normally open contact (NO)
A normally open contact is a relay switch contact that stays open when the device is in its normal state. It closes only when the relay coil is energised, such as during an alarm or a fault.
Relay output
A relay output is an electrically isolated switch inside a device. It uses a small internal signal to move mechanical contacts that can switch a separate circuit carrying higher voltage or current.
RS-485
RS-485 is an electrical standard for balanced digital data lines. It uses differential signalling over twisted pairs and supports long cable runs and many devices on the same bus when installed with suitable cabling and termination.
Twisted pair cable
Twisted pair cable is a type of wiring where two insulated conductors are twisted around each other at regular intervals. The twist helps cancel out electromagnetic noise and is widely used for data and control signals.
My Diary - Leonardo Cardillo 