Outer Foil Shield/Unshielded Twisted Pair (F/UTP Cable) GROUNDING - Page 1
Forum section: electronics / projects

author: mcinque
author_rank: Unknown
timestamp: March 21, 2023, 10:17:55 am
content: Hello all,at work we are having a discussion on the grounding of F/UTP (Outer Foil Shield/Unshielded Twisted Pair) cable, basically 4 twisted pairs wrapped in thin aluminium tape.Some say that this type of cable absolutely must be grounded at both ends, with shielded plugs and sockets.Others say that grounding only one end is ideal, otherwise ground loops can be inserted, which would make things worse.Others say that shielding with aluminium is not "true" shielding but something in between: it reduces interference and emissions but cannot be considered a "real" shielding, so it should not be treated as such (what's more, it is also fragile and difficult to crimp into connectors).I always referred to "shielded" when talking of cables with proper copper/alluminium braid AND eventually alluminium foil (not only foil).All this doubts remains to some, even though a Fluke CableAnalyzer has certified that the cable's performance is perfect, even though it has only been grounded at one end.Where is the truth? Some manufacturers speak of grounding at both ends, others say "depending on the case". Do you have a paper that can clarify this doubt? is there a specific standard?Attached pictures of the cable inside the patch panel and the alluminium foil grounded (one side, at the rack).Many thanks in advance

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 21, 2023, 11:31:08 am
content: What is being shielded?The analyzer isn't going to tell you anything, it's just going to see UTP like anything else. The question is, what was the purpose of purchasing shielded cable in the first place if you're not going to use it?Tim

author: TERRA Operative
author_rank: Super Contributor
timestamp: March 21, 2023, 11:49:08 am
content: For those clamps in the image, I would be folding the shield back over the outer insulation and clamping over the whole lot.How it's shown in the image will give a tenuous ground connection at best.Also, shielded Ethernet cable is mostly unnessecary in all but high noise environments, industrial etc. But if you have it already, may as well use it.I used it at home anyway just for fun, and have the shield connected from the network switch, through the grounded jacks in the rack all the way to the metallic Panduit keystone jacks on the wall.For those connections where the shield connection continued to the grounded end device, I haven't noticed anything but perfect connection.I'm sure there's a standard out there somewhere for grounding Ethernet cables, but my system works well enough that I haven't looked too hard to find it.

author: mcinque
author_rank: Unknown
timestamp: March 21, 2023, 12:06:17 pm
content: Quote from: T3sl4co1l on March 21, 2023, 11:31:08 amnetwork cables for intranetQuote from: T3sl4co1l on March 21, 2023, 11:31:08 amThe analyzer certifies network speed, and the result is signed by the company that installed the cables.Question are:1. both sides should be grounded?2. just one side should be grounde to avoid ground loops?3. aluminium foil IS a proper shielding in a cable?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 21, 2023, 12:21:47 pm
content: The other side, what's being shieldedout?Both sides must be grounded to get full value of the shield, but ground loops may occur. This is the tradeoff. An AC ground (lots of bypass capacitors) is all that's necessary, which may alleviate some ground loop issues, but I don't know if this is a common item; more of an electronic design option.If one or neither side is grounded, it's about equivalent to UTP. You get no value from the shield, but it's no worse than UTP, which does pretty well in most cases.Even wholly unconnected foil still serves as a shield to very local / intense / pointlike fields, which could otherwise couple into individual twists of the pair(s), corrupting the differential signal directly. This is pretty minor, but it's still a little advantage.The other common case, I think, is when cable is simply sourced with foil, whether that foil is desired/needed or not. In that case, it's no worse to leave it floating.Foil is inferior versus braid, or multiple layers of either kind, but it's all a matter of degree. Foil might afford 20-40dB of shielding (under certain specified conditions), while braid does 60-80; with proper dress, bulkhead connectors and multiple layers, over 100dB of shielding is achievable.20dB would be enough to extend, for example, a marginal link passing through ~30V of noise (typically enough to corrupt 100BASE-T Ethernet, I think), but not 100 or 300V. The former might arise in an industrial setting, say in the general environment around unfiltered VFDs and switchgear, but not in direct contact with their wiring (especially the output (motor) cables); the latter might arise in contact with such motor cables.If very low bit error rate is required, quite a bit of additional shielding may be necessary to afford transient immunity. For example, relays/contactors switching inductive loads typically generate fast transients (10s of ns pulse width, lots of ringing) with amplitudes of several kV. That environment might be reason to use foil + braid cable and improved connector grounding practice (like coaxial bulkhead connectors).Tim

author: mcinque
author_rank: Unknown
timestamp: March 21, 2023, 03:52:31 pm
content: Thanks Tim!Quote from: T3sl4co1l on March 21, 2023, 12:21:47 pmthe cables start from a patch panel and ends in single ethernet wall sockets across various rooms.QuoteI have installed several wired burglar alarm systems and all the manufacturer's installation manuals state that the shielded cable must have the shield and gnd connected to the gnd of the control unit board, leaving the shielding floatingon the sensor side.Think that alarm cables often run together with the 220v wires so they can catch much noise.Of course I understand there are HUGE differences in signals between simple IR sensors that goes NC or NO and high speed differential signals...But I guess that generally speaking there must be a difference from neither side or one side grounded. I saw that kind of grounding also in some cables inside instruments: grounded only one side.Having a manual, a reference would be very usfeful. Can you suggest me a couple of lectures if you have them by hand?

author: Terry Bites
author_rank: Super Contributor
timestamp: March 21, 2023, 04:38:43 pm
content: You don't need to worry about groundloops in an a network cable. The signals run on (150 ohm) twisted pairs with diff drivers and receivers.CM currents injected from the mains power system are unlikely to create an issue.Its important to note that a separate ground needs to be run between switching systems to prevent a fault current finding its way down the shield.I live next door to a twisted pair- I wish they'd keep it in, or a least save it for the car.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 21, 2023, 05:01:18 pm
content: That's correct practice, if the sensors are floating and the shield is still terminated to them. That is, you don't want some dingus adding an extra (galvanic) ground at the far end making a huge loop susceptible to mains and other induction sources. All that matters for signals is the local ground environment. Signals in the cable are referenced to the cable (and its shield in turn). Signals on board are referenced to the board. Tie the references together, everything's clean and happy.It may also be that the sensors are well enough filtered that shielding is irrelevant, and plain twisted pair would do. Perhaps they don't care if you use shielded, and just want it grounded somewhere so it isn't floating; or for safety purposes, so if someone cuts into it or cross wiring happens and there's a ground return to blow the fuse properly.The most common source of such advice, for actual signal purposes, dates back to tube amplifier days, where shielding might be needed to separate signals, but the impedances are very high (often 100s kohms) so electric field coupling dominates. It doesn't matter where the shield is connected, just that it's connected to ground somehow, so it can short out the electric field lines that would otherwise couple between wires and introduce crosstalk or oscillation.At the same time, the transmission line stubs thus formed are prone to oscillation near the tubes' cutoff frequency (often 100s of MHz), necessitating resistors ("grid stoppers") to either dampen the resonant modes, or kill bandwidth of the circuit (more or less, RC filtering against electrode capacitance; the full effect is a bit more complex than that, but that's an adequate explanation).Such effects were generally passed off as low-frequency or high-frequency parasitic oscillation, as if poorly understood and nebulous in nature, and usually dealt with in equally unsubtle means -- like grid stoppers (or other RC or RL networks; some turns of wire wound over a carbon composition resistor (thus making an R || L element), in series with the plate terminal, was another common method), or just tossing in enough supply bypass to quash motorboating or the like. This applies to audio and radio amplifiers, of course for the audio case the LF oscillations will be quite slow indeed (i.e. under 20Hz), hence the name "motorboating", but might be fractional to many MHz for transmitters, while still a fraction of the intended passband.But really, there's nothing that can't be understood. The bigger problem is having good boundaries between signal paths, so that loops can be separated, and understoodeasily. This might've been harder in the tube days, with everything tossed inside a metal chassis and wired point-to-point on terminal strips. We have the luxury of ground planes within every PCB we make, if we choose to, which grants excellent isolation between signals, as well as controlled impedance (I mean, maybe it's within 30 or 50% of target impedance without specifying traces as "impedance controlled" per se, but to say it's still within some modest range I mean, not wildly out say by a factor of 2 or more).Which, going back to the case for wiring, when units are isolated from ground, and only interconnected in a chain or tree topology (or most generally, an acyclic graph), it's no problem, tie the shields together and you have a solid shell around everything keeping outside fields out and inside fields in.When there are cycles in the network, is when you get problems, like ground loop. Mainly the problems are low frequency, because the induced currents at 50/60Hz can be large, or there are DC or low frequency offsets for other reasons (say the voltage drops between battery, alternator and etc. in an automobile), and because high frequencies can be adequately shielded by brute force of the shielding itself (assuming it can be made thicker: compare foil to braid to multilayer braid, case in point!), whereas low frequencies drop across the resistance of the shield itself thus offsetting the voltage sensed at the connector.Hence why, if you can bond the shield with an array of capacitors, you can get the best of both worlds (AC ground and DC isolation), given still other considerations of course (like, if the signal bandwidth overlaps with the isolation bandwidth, you still have a problem; and the capacitors need to be numerous enough that their ESL doesn't dominate at very high frequencies; and being capacitors, they can resonate with cable inductance so some damping resistance may be necessary; etc.).Ethernet being AC coupled already, shouldn't care about ground loop, so the shield can be grounded at both ends. The worst that can happen is ground loop currents cause the cable to heat up and fail, and that would be indicative of a much more pernicious and urgent fault in the facility grounding itself.The cables in this case, dead-ending in wall sockets (themselves unshielded, I assume), means the shield has nowhere to go at the far end anyway (nowhere still related to the signal, which is the only important part), so should be left open. Grounding at the patch panel probably doesn't matter.Where it would matter, is if the sockets are isolated (there's a transformer inside), in which case shield should be terminated to whatever ground/shield is associated with the same side of the transformer (if any). The transformer then isolates noise (as well as ground loop, solving that concern too) from whatever's plugged in, which might just be some laptops, maybe someone plugs in an obnoxiously long cable and it's picking up nearby radio stations, who knows, but it's all isolated by the transformer. This is possible because Ethernet is AC coupled (indeed with transformers), and a few transformers can be added to the line without degrading performance (and this would only add one, which is fine).I have no idea if isolated wall jacks are normally a thing.Also keep in mind, this is just my opinion with respect to EMC/signal quality. Always look up and respect local laws when it comes to wiring. Networking cables are not at all exempt from electrical codes, AFAIK. They may or may not be doing things "properly" with respect to signal quality, but that's just something we have to deal with (and Ethernet itself deals with very well indeed).Tim

author: Marco
author_rank: Super Contributor
timestamp: March 21, 2023, 06:38:50 pm
content: Quote from: mcinque on March 21, 2023, 10:17:55 amLets just add a third option for completeness, grounded at one end, capacitively grounded at the other.It should ideally be grounded at the receiver. A floating shield can couple high frequency EMI to the twisted pair as a common mode signal, common mode rejection deteriorates at high frequency for differential receivers. If the shield isn't grounded at the connector, it's defacto floating regardless of the other end.