Want to Boost Machine Reliability? Start With Choosing the Right Sensor Cables
Colin Cartwright
Supercharging machine builders with simplified control systems that short-circuit their competition!
When I first started out as a controls engineer in the early 1990s, choosing a sensor cable was relatively easy because there weren’t many options to choose from. If your sensor had a connector at all (many still used screw terminals), it was either a 7/8" Mini connector or an M12 connector. At that time, the M8 version was only just emerging, and most sensor manufacturers weren’t paying much attention to it yet.
Selecting a sensor cable back then really boiled down to five simple questions:
Although double-ended cables did exist, we typically wired everything back to the main control panel in those days, so we rarely used them. As for the cable colour—well, any shade you liked, so long as it was bright yellow.
Things have changed a bit since then. The beloved 7/8" connector is rarely seen on sensors anymore, whereas M12 and M8 connectors reign supreme, with no signs of that changing any time soon.
The Real Revolution: Cable Jacket Materials
But connectors themselves aren’t where the real revolution has happened. These days, you can choose from a startling range of cable jacket materials, from low-cost PVC to wear-resistant and flexible PUR to heat-resistant and chemical-resistant variants of TPE.
"That humble cable jacket could well be the difference between a smoothly operating machine and your phone lighting up at 2 a.m. with an urgent breakdown call."
Connector Size, Type, and Style
Choosing the connector size, type, and style is still straightforward. Most modern sensors use either M12 or M8 connectors with three or four poles, and you basically only need to decide whether you want a straight or right-angle version based on how you want your cable orientated from the sensor.
If you only need a single-ended cable, that’s half the job done, but if you plan on connecting the other end of your cable to an IP67 I/O module, for example, you’ll need a double-ended cable with a male connector that matches the ports on the I/O module. Be especially mindful of I/O modules with M8 ports because 3-pin and 4-pin M8 connectors/ports are not compatible and won’t connect together.
Shielding Considerations
Shielding is another consideration that should be taken into account. Most digital sensors with NPN or PNP outputs will be fine without shielded cables, but analog sensors (4–20mA or 0–10V) typically need that extra shielding to protect the signals from electrical noise.
You might also want to consider shielded cables for IO-Link devices if they are located in or close to noisy environments—think welders, VFDs, servo motors. If you’re unsure, ask someone who’s had to troubleshoot unexplained signal spikes at 5 p.m. on a Friday—believe me, it’s worth the peace of mind.
Cable Length and Routing
Cable length might seem like the most straightforward question in the whole equation until you start tracking how that cable must snake its way around your machine. If you're connecting your sensors directly to the PLC in the control cabinet, you’ll likely need a longer cable. And don't forget the vertical direction runs that go up and over parts of the machine in addition to horizontal runs when determining the length you'll need.
If you've moved over to decentralized automation concepts, your cable pulls will be much easier and faster with a short double-ended cable connecting to a nearby IP67 I/O module that's been strategically mounted on the machine close to your sensors.
If you have a really long cable run back to your main panel, say 20m or 60ft, you may also need a heavier wire gauge to save yourself from issues caused by voltage drop.
"There’s nothing quite like realizing you’re two feet short of cable when you’ve spent 30 minutes routing a cable from one end of the machine to the other!"
Environmental Factors
Environmental factors are also crucial. Selecting a cable that shrugs off weld spatter doesn’t necessarily mean it can survive high-pressure water jets and chemical cleaners in a food-processing area. The chemical exposure, temperature swings, and even the certifications required (think FDA/Ecolab compliance if you’re in food or pharma) must be weighed carefully. The more extreme the environment, the more you’ll want to confirm temperature ratings, chemical resistance, and IP ratings for both cable and connector.
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Movement and Dynamic Applications
While we’re at it, consider whether your cable will be put through its paces in a dynamic application. A standard cable might hold up for a million bend cycles, but a cable specifically designed for continuous flex applications (like a C-Track) can go ten million or more. If torsional twisting is on the menu—as is typical on 6-axis robots—you’ll need a cable designed and rated to handle 360-degree torsional rotation. Otherwise, you’re just setting yourself up for frequent failures and more time wondering why on earth you went with a cheaper cable.
M12 Connectors: Not All Equal
Of course, not all M12 connectors are created equal, either. Beyond the number of pins, there’s also the coding to consider—A-coded is standard for sensors, D-coded for Ethernet, L-coded for power, and so on. While most industrial sensors use A-coded connectors, it’s good to be aware of these other variations so you don’t accidentally buy a cable meant for networking or power and wonder why the connector doesn’t fit or the keyway doesn’t line up.
Colour Choices
Colour is one of the more minor decisions, but it can help keep your machine looking organized and straightforward to troubleshoot. Some people pick black for photo or proximity sensors, grey for cylinder reed switches, yellow for safety, and so on. Bear in mind that some colours might be less commonly stocked, so factor in lead times before you commit to a rainbow of cable colours.
"A word to the wise: fancy cable colours are great unless they take sixteen weeks to arrive."
Maintenance and Spares
Speaking of inventory, maintenance and spares are worth mentioning. Standardizing on a few types of cables can simplify everything from initial installation to later expansions or repairs. If a particular machine frequently operates in high humidity or washdown areas, you might designate a specific high-resistance TPE-E cable for it and keep spares on hand. Trust me, there’s no better time-saver than having the right cable available when you need one rather than scrambling to find one at the eleventh hour.
Wire Gauge and Proper Torque
Finally, there’s the matter of wire gauge and extended runs. Most applications will be fine with the standard 0.25 mm2, but if you’re pushing out a 24V signal halfway across the factory, voltage drop can become a real nuisance. Upgrading to a thicker gauge wire (0.34 mm2) or splitting the run can spare you a lot of grief (and a few exasperated phone calls). And don’t forget to torque your connectors properly—especially if you need to maintain an IP67 or IP69K seal. A slightly loose connection can lead to moisture ingress and sporadic signals.
A Real-World Example
I once worked on a packaging line that kept experiencing intermittent signals from a sensor. After a few hours of troubleshooting, we discovered that the sensor cable wasn’t rated for the constant bending in the overhead track system and was the culprit causing the problems. Swapping it out for a continuous-flex version with the correct bend radius solved the problem and kept on going for many years afterward. It was a textbook case of how a seemingly minor detail—sensor cable type—can have a massive impact on production uptime and reliability.
Conclusion
Sensor cables may not be as flashy as a gleaming new servo motor or the latest PLC, but they can make or break a machine's OEE. Too often they’re chosen on price rather than their suitability for the job, which leads to breakdowns, downtime, and electrical designers hiding under their desks. On the other hand, putting in a bit of research—or consulting an industrial connectivity specialist at Murrelektronik, for example—can pay off enormously in terms of machine uptime, reliability, and your reputation.
So the next time you think of buying sensor cables, remember they’re not just commodities. They’re literally the lifelines of your machine.
Which outcome would you prefer? A cable that was cheap and inevitably leads to a late-night breakdown call, or one that’s designed to keep your machines humming and your stress levels in check? I know which one I prefer.
Final Thought: Spending a bit of time selecting the right sensor cable from the start doesn’t just save money—it saves you from those dreaded 2 a.m. calls when everything seems to go wrong. When in doubt, consult the experts, invest in quality, and give yourself the peace of mind that comes with robust, reliable connections.
About the Author: Hey there! I'm Colin, and I've worked with industrial automation and control systems for over thirty years. I enjoy collaborating with machine builders and OEMs to increase their competitiveness. By fine-tuning your control system installations and commissioning processes, we can reduce skilled labour challenges, boost your machine’s OEE, and deliver better ROI.?
Want to see how we can work together? Connect with me!