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If I run S/FTP Ethernet cable ran next to 120V conductors (inside 3/4" conduit) for about 70 ft. is there chance that I'll have a usable data connection? I'm fully aware that this is a terrible way to layout a network but this is more of a retrofit and running cable outside the conduit would be huge undertaking. Can anyone share some practical experience with what you can get away with when using shielded Ethernet cable?

The Ethernet would plug into a PLC that controls a couple of pumps and some valves, and a switch or Ethernet adapter at the other end. The bandwidth requirements are minuscule - just basic telemetry from the PLC and monitoring for faults - not looking for stable gigabit speeds here.

The 120V pumps are off most of the time and the control circuit with the PLC (always on) doesn't pull much current. I'm a bit worried that when the pumps kick on the inrush current might induce enough current in the Ethernet conductors and/or the shielding that it would damage the network equipment - again looking to distinguish theoretical from practical concerns here.

Other alternatives I've considered:

  • Wi-Fi bridge - the enclosure that houses the PLC doesn't have a lot of space available for additional equipment - namely the wireless AP and power supply - and I hesitate to tap into available circuits for power. Also the Wi-Fi signal will face its own challenges (distance, concrete, etc.).
  • Fiberoptic bridge - same concerns as the Wi-Fi in terms of space and equipment seems much more expensive.
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    There are some questions and answers here that are related. For example, this one and this one. This is not something you should mess with; leave it up to professionals. The laws and penalties for improper cabling can really hurt you if you do it incorrectly. – Ron Maupin Dec 12 '18 at 0:35
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    I did find something: "There is an Informational Note to 300.3(C)(1) that refers us to 725.136(A) for Class 2 and Class 3 circuit conductors. So if the 0 to 10 volt circuit is a Class 2 or 3 circuit then you are correct they are not permitted in the same raceway with the 277 volt lighting circuit." You should really read the sections on low-voltage cabling, too. – Ron Maupin Dec 12 '18 at 1:32
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    Do NOT mix high voltage and low voltage in the same conduit. I've seen this seriously hurt people, and cause fires and explosions. – Ricky Beam Dec 12 '18 at 17:19
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    You have three different professionals with probably half a century (or more) of experience between us saying it isn't a good idea. I doubt any of us are going to flip and say, "sure, go ahead, why not?" just because you are trying to leverage exceptions within the code to fit your perceived use case. At this point, get your local knowledgeable electrician and building inspector to sign off and if they both give you the nod you can go ahead. However doing so is a bad idea, even if done "properly" per code as it can lead to confusion and human error in the future. – YLearn Dec 12 '18 at 20:24
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    You can certainly argue here, with us, and look for loopholes, but you cannot do that with the building inspectors, fire marshals, etc. People try that and fail all the time. If your building gets red-tagged for a violation, then it cannot be occupied until the situation is corrected, and there will probably be fines every day until the situation is corrected. If there was a fire, then you can usually be held personally liable for civil lawsuits, and face criminal charges if anyone is hurt or killed. It simply isn't worth the risk; leave it to a professional. – Ron Maupin Dec 16 '18 at 18:13
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As everybody has said, including the original question, this is a terrible idea and don't do it. I recommend some kind of fibre solution, which can be cheaper than you might think. I recently did an economy addition for a charity with the cheapest ethernet switches I could find that had SFP sockets, with ready-made LC-LC multimode 50 m fibres; this used no more equipment, space, or power than copper ethernet, and surprisingly little money. (If it's just the PLC plugging in, then certainly you'd have to accommodate the media converter and its power; conceivably it might be possible to find a module for the PLC directly.) I don't recommend anything radio-based. If you really, really, can't get fibre down the conduit, perhaps even consider powerline communications, where the data goes down the power line instead of a cable alongside, although general-purpose non-domestic ones can be hard to find, and I have never seen one in professional networking and have no experience to say how well it would work.

But your question is about the practical matter of what happens if you do it nonetheless.

If it's not illegal where you are and somehow you convince yourself that it's safe and you don't get fined for building regulations and you can stand the legal exposure and you find yourself doing it against your better judgement or someone else did already did it ...

You don't get failure, you get unreliability.

It is quite likely you'll get an apparently fully working 100baseT link -- ethernet is surprisingly robust, and you have 20 m and 120 VAC with occasional switching -- and might not notice anything immediately or directly. You might even get away with it with perhaps just a few lost packets. In my experience, however, the problems tend to be indirect and often manifest later. Here are two sorry tales from my experience, one current, one old.

  • I have a client site which is very similar (25 m three-phase armoured cable adjacent to shielded ethernet running 100baseT, PLC running Modbus/TCP for telemetry, occasional SSH for config.) The site is extremely inaccessible. Everything worked, they thought, and left site. It turns out when a once-per-day event happens, half the time a particular device crashes and stays down. It took 20 days to find the pattern. The event is only about a 1 A inductive load going on, and somehow that's disrupting one specific unit through high-quality German DC supplies. It has cost about four months and 30-person days to debug the situation and find an absolutely ridiculous and unsustainable temporary fix by hideous trial and error (replacing crashing unit with an out-of-production one from a different manufacturer). My client very nearly bust its deadline over this, and still doesn't know how to address the problem without a very, very, expensive trip. My client receives daily phone calls from its client on this matter. I suspect they won't get paid until this is resolved.

  • A subcontractor cut some corners at a company I used to work for and didn't put in enough cable tray in a machine room. A staff member who did know better but just-this-once-can't-hurt laid a lot of ordinary Cat-5 100baseT patch cables next to single-phase 230 VAC lines (just switches and similar current), 5 m here, maybe 10 m there. Everything worked fine, he thought, and finished installation of the machine room. Unwisely, I didn't push the point though we had some very heated exchanges, and allowed it to continue. Two months later, they found the main UPS was kicking in and draining the batteries and losing main server, switches, and routers at random times of day until they went and reset things. Debug loop was about a week on this, took about a month to finally understand what was happening: it was some very unpredictable interaction between noisy lines and UPS control software. As the time was so long after their work, we never got the contractor to make good, and ended up having to pay to put in all the extra traywork, which was then all awkward and had to be done expensively out of hours, with said staff member working nights for about a week recabling. Disastrous for SLAs and credibility and budget.

Don't do it.

  • Powerline is a crap shoot. In most commercial/industrial applications I have seen it attempted to be used, it was unreliable at best. Have personally worked on several projects with startup companies who had products (stats collection on solar power collection, building automation, etc) that tried to use powerline comms and failed, often due to "noisy" equipment drawing high amounts of power like fans, electric motors, or pumps (like the OP's situation). – YLearn Dec 17 '18 at 16:36
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Definitely a case for fiber. While there are some things to optimize twisted pair:

  • use high-quality cable
  • use shielded cable with excellent grounding on both sides
  • bundle data lines and power lines separately, keep bundles as much apart as possible
  • reduce data rate down to 10 Mbit/s if you don't require more

there's only so much you can do. If you need to rely on the connection there's little alternative to fiber. Compare the additional cost to that debugging and recovering from an unreliable connection and you've got your answer.

If cost really is a problem stick to preterminated fiber and proven third-party transceivers which usually are a fraction of the original vendor's.

And forget WiFi if you need a highly reliable connection - especially in a noisy environment.

  • And given this particular case, perhaps consider Modbus/RS-485 at 9600 bit/sec to the PLC. – jonathanjo Dec 16 '18 at 16:13
  • Doesn't really answer the OP's question, unless he hasn't actually stated his question correctly. He mentions alternatives he has considered, but isn't asking about the viability of those. He is asking about the "Practical limits of shielded Ethernet cable" when run in the same conduit as his high voltage lines. – YLearn Dec 16 '18 at 16:19
  • @YLearn ... which I've tried to point out. Without hard figures it's impossible to answer Serguei's question. I don't think it's practical to do some worst-case measurements and without those it's more a general "don't do it". – Zac67 Dec 16 '18 at 16:34
  • @Zac67, which makes your answer theoretical and not practical, which is exactly what the OP wrote they weren't looking for in an answer. You don't provide personal experience with problems you had nor do you don't provide practical data showing why it shouldn't be done. You provide reasoning for using/not using alternatives the OP say they considered and you provide theory on how to get a better copper connection, but neither address the OP's question. The answer from jonathanjo contains two practical experiences, which does address the OP's question. – YLearn Dec 16 '18 at 16:41

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