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I have knowledge terminating CAT5 UTP.

But, for an installation in a remote office, the cable will be routed near a power source which we already know causes a substantial amount of 'noise' on standard telecom cables.

We have purchased some CAT5e STP cable (screened) for this particular run. It will be running from the patch cabinet, past the power source (industrial equipment) and out of the building (a run of a few feet) to a 'porta cabin' (where the power feed also goes to).

As the same power source will be powering both ends of the installation, I understand we won't have problems with earth differential.

As our 'porta cabin' only has 2 PC's, we are not planning to put a patch cabinet in there.

My question is:

If I terminate the STP cable to a socket and run a standard patch cable that side to a small switch (and then cables to the PC's) would I need some way to earth it?

The patch cabinet side would also be terminated to a socket (with patch cable going to main switch (with fibre link to our main office)). I have no STP RJ45 terminations (only standard UTP).

If I was to run an earth wire from the STP shield/screen to the patch cabinet would that be enough (cabinet grounded through equipment connected to a UPS)?

I would also look to use ferrite chokes on the cable (to negate any outside RF interference it would pick up) if we found problems.

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    I'm curious about this noise source; is it 50/60Hz noise, or a higher frequency? How did you find this noise source?
    – Mike Pennington
    Mar 12, 2014 at 8:55
  • I'm unsure of the frequency. The porta cabin in question has a telephone extension cable running alongside the power feed, when speaking to anyone on this line there is a constant overpowering 'hum'. Dialup modems are used on the line with occasional line drops, but with no packet loss or any other symptoms.
    – HaydnWVN
    Mar 12, 2014 at 9:05
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    Ok, but be aware that telephone signals are analog, and that's why you get the hum (because the phone line acts like an antenna, which picks up the 60Hz noise). Ethernet is very different, it uses digital signals, differential signalling, much higher frequencies, and specially conditioned cabling (via twists); all this makes it quite immune to power line noise. I work at a factory, where we have machines operating all the time. We have never used shielded in our environment. Perhaps it's worth testing with a simple hub / laptop and unshielded cabling.
    – Mike Pennington
    Mar 12, 2014 at 9:21
  • Ok thanks Mike, very useful information. As we have the STP now we'll use it (with chokes) and then any problems I'll try grounding the 1 end of it. I'll leave question open as would like experience from someone using STP.
    – HaydnWVN
    Mar 12, 2014 at 11:00
  • I think you're going to introduce more variables and potential issues with the ferrite chokes on the Cat5; I'd just run vanilla UTP and test it for a day or so...
    – Mike Pennington
    Mar 12, 2014 at 11:14

2 Answers 2

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Reasons to use shielded cabling

You asked about using shielded cabling to protect ethernet from interference from heavy power currents. We mentioned above that shielded cabling isn't required in this case, but there are a few valid use cases for it:

  • If you run cabling where there truly is a high potential for interference, such as ethernet cabling that runs near aircraft radar transmitters.
  • If you find that you're at the 100m distance limit for standard ethernet runs, it's not unheard of to squeeze a few more feet beyond the 100m limit of a Cat5e run by using shielded cabling. That said, the 100m distance limit is there for more than just cross-talk and noise limits, but some people still get away with doing this. It helps if you're in bind, but it's much better to lay out your IDFs correctly; you shouldn't do this intentionally.

It obviously won't hurt to use shielded cabling in your case, but it doesn't sound like you really need it.

Grounding Shielded Cabling - one side, or both sides?

As our 'porta cabin' only has 2 PC's we are not planning to put a patch cabinet in there, so my question is - if I terminate the STP cable to a socket and run a standard patch cable that side to a small switch (and then cables to the PC's) would I need some way to earth it?

Terminating one side of shielded cabling is sufficient and industry practice; I am assuming the shielded cabling is terminated correctly. Grounding shielded cabling is also discussed in BICSI Mythbusting: Shielded cabling, which I thought was a great presentation. If you only ground one side, you're eliminating the possibility of ground loops on the cabling itself. Ground loops tend to form at power-cycle frequencies, which don't interfere with ethernet; however, it's best to avoid ground loops altogether.

Terminating shielded cabling to a patch panel

The patch cabinet side would also be terminated to a socket (with patch cable going to main switch (with fibre link to our main office)). I have no STP RJ45 terminations (only standard UTP).

If I was to run an earth wire from the STP shield/screen to the patch cabinet would that be enough (cabinet grounded through equipment connected to a UPS)? I would also look to use ferrite chokes on the cable (to negate any outside RF interference it would pick up) if we found problems.

This is a little unclear without a diagram, but let's cut to the essentials:

  • Ground one side of your shielded cabling
  • If you terminate into a patch panel, use patch panels / modular inserts designed for shielded cabling because the mod plug (shown below) terminates to the shield inside the cable, and that all should be grounded.

Shielded mod plug:

Shielded mod plug

Avoiding ground loops

As the same power source will be powering both ends of the installation I understand we won't have problems with earth differential.

This isn't in the critical path of your main questions, but it's worthy of discussion. Everything depends on whether the buildings are grounded individually, or whether the power cabling to your 'porta cabin' carries the ground conductor with the hot and neutral. If the buildings indeed have different ground rods, you could see a difference in ground potential between the buildings. Please see BICSI Mythbusting: Shielded cabling, which discusses why this shouldn't be a problem for your shielded twisted pair termination, but it's a point worth mentioning in case you run into other equipment grounding concerns; you should not intentionally form ground loops, if possible.

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  • Thanks Mike for an excellent answer, I'll be going through processing everything contained before doing the actual run. Your information has certainly helped and I'm not quite sure why I didn't find that BICSI article before!
    – HaydnWVN
    Mar 17, 2014 at 17:22
  • Hi Mike, so after reading through the BICSI article I'm not too worried about the grounding of the cable for interference purposes, but the fact it's running outside makes me think I should for safety purposes. The cable will not be at height (as I first thought), as I'm not using a patch panel (just wall sockets) do I simply run a 'grounding wire' from the shield to the cabinet?
    – HaydnWVN
    Mar 19, 2014 at 9:50
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    Thanks for following up with the results :-). I'm afraid I don't quite understand the distinction you're making between a shielded mod plug and shielded RJ45 end... those are the same thing to me
    – Mike Pennington
    Mar 25, 2014 at 9:21
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    Its modular in the sense that you can cut off that Rj45 and replace it with another one. On this side of the Atlantic we call it a modular plug
    – Mike Pennington
    Mar 25, 2014 at 12:29
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    I've had ground-loop problems in the same building. If two PCs are grounded differently or one is not grounded at all, you'll see sparks when connecting the Ethernet cable because of the potential difference, not to mention the serious shock you'll get. So it's always better to ground just one side, preferably the side that plugs into the better grounded equipment, this way shielding the Ethernet wires from electromagnetic interference. This could make the difference between a 1Gbps link and a 100 Mbps link, even in smaller runs than the OPs, depending on how close to electrical wires you get.
    – Paul-Sebastian Manole
    Apr 8, 2016 at 13:09
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A cable shield grounded at one end only is a low pass filter for magnetic fields and a high pass filter for electric fields. A shield it is not. Chassis grounds are all supposed to interconnected anyway for safety reasons. Ground loops are a circuit design problem.

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  • A similar question about grounding shielded twisted pair was raised over on Electrical Engineering. The conclusion endored by Kortuk (a diamond mod) was only grounding one side to avoid ground loops. Sorry, but you haven't presented enough evidence to override the clear guidance from an expert in the field.
    – Mike Pennington
    Jun 19, 2018 at 15:25

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