Author Topic: Preventing ground loops on shielded twisted pair (STP) cabling  (Read 769 times)

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Offline Zeyneb

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Dear people,

I think I have to try my luck here to find an answer. As I have asked about the following on Network Engineering Stack Exchange but my question was discarded because they don't allow topics on home networks.

Maybe for my home network I don't really need STP. But still I would like to know how to properly build such a network. And I'm actually implementing this with STP cable.

Regarding grounding of STP cables I was facing with contrasting recommendations. I believe this topic is much more general than Ethernet networking. One camp claims you should only ground on one end of the cable to prevent a ground loop. The other camp claims to ground both ends in order to have an effective way to prevent EMI entering the cable. To be sure with a ground loop for my case I mean a loop in the ground/earth wire of my residential mains electricity installation. And I think it is useful for this case to state that I live in the Netherlands.

After searching around the internet I found the following question on Network Engineering Stack Exchange valuable:
https://networkengineering.stackexchange.com/questions/6773/shielded-twisted-pair-stp-termination-and-grounding?newreg=87f74fa1868f434eb44dd79f275e5602

And especially the answer from Mike Pennington there. I've also read his linked powerpoint: http://www.pennington.net/archives/cabling/BICSI_Mythbusting_Shielded_Cabling.pdf

Based on this I was drawn to the conclusion to ground the STP shield only on one end. Which means for my case that only the patch panel side will have connections from the STP shields to the breaker box ground which in turn is connected to the ground electrode of the apartment building I live in. I hope the majority of the Eevblog audience agrees with this plan.

Now leading to my question. Regarding the wall plate side where you should interrupt the ground connection. What products should you use for that? Obviously you do buy an STP cable and practically this is F/UTP or SF/UTP. But I'm worried that I will make an accidental ground loop with the shielded keystone jacks I currently have. You know, you can connect a shielded patch cable from the wall plate to a computer and for the computers I have they all make electrical continuity with the network port shield and the power plug ground. For the assembly of the cable to the shielded keystone I did cut away the foil, braided shield wires and the drain wire. Now the shielded cover is only attached to cable jacket. But I believe I'm better off buying the more regular plastic UTP keystones (for the wall plate side) even for this STP network. However a proper strain relief is still a good thing to have in my opinion.

I'm curious what other people think of this.
« Last Edit: March 02, 2020, 04:13:04 pm by Zeyneb »
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Offline David Hess

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Re: Preventing ground loops on shielded twisted pair (STP) cabling
« Reply #1 on: March 02, 2020, 03:28:47 pm »
You pretty much covered it.  Analog Devices makes the same recommendation not to connect the shield at both ends.

If you did connect the shield at both ends, then the high frequency common mode current from various switching power supplies which is coupled through their input RFI filters into the ground connection would be applied across every cable shield.

I would use an unshielded plug on the side where the shield is not connected to indicate how it is actually wired.
 

Offline Zeyneb

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Re: Preventing ground loops on shielded twisted pair (STP) cabling
« Reply #2 on: March 02, 2020, 04:24:11 pm »
Hi David,

Good to know reputable sources share the approach to only ground on one end.

In your last sentence you talk about an unshielded plug. I think you actually mean an unshielded keystone jack. The one that is put in a wallplate. As it is common for my nanny state that on the living space side you can deal with the most naive people. Who don't care to plug in an shielded or unshielded patch cable.
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Offline David Hess

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Re: Preventing ground loops on shielded twisted pair (STP) cabling
« Reply #3 on: March 02, 2020, 06:14:26 pm »
In your last sentence you talk about an unshielded plug. I think you actually mean an unshielded keystone jack. The one that is put in a wallplate. As it is common for my nanny state that on the living space side you can deal with the most naive people. Who don't care to plug in an shielded or unshielded patch cable.

I am not used to thinking in terms of keystone jacks but that seems right.  I usually terminate the cable in a plug and use a wall plate with a socket on each side.
 
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Offline Zeyneb

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Re: Preventing ground loops on shielded twisted pair (STP) cabling
« Reply #4 on: March 03, 2020, 05:27:30 pm »
Hi there!

So I was looking around at Digikey for a suitable unshielded keystone jack and found the following:

[attach=1]

It's from Assmann WSW Components, part A-KEY-8-EG-GIB-WIW2.
Digikey product page:
https://www.digikey.com/product-detail/en/assmann-wsw-components/A-KEY-8-EG-GIB-WIW2/123-A-KEY-8-EG-GIB-WIW2-ND/10392871

The thing is specified for Power over Ethernet PoE. However I want to use it for normal Ethernet, either 100BASE-TX or 1000BASE-T.

So what is this PoE thing doing to my Keystone? Is it just that voltage rating and/or dielectric strength is higher than usual?
« Last Edit: March 03, 2020, 05:29:11 pm by Zeyneb »
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Offline reboots

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Re: Preventing ground loops on shielded twisted pair (STP) cabling
« Reply #5 on: March 03, 2020, 07:29:43 pm »
So I was looking around at Digikey for a suitable unshielded keystone jack and found the following:

The thing is specified for Power over Ethernet PoE. However I want to use it for normal Ethernet, either 100BASE-TX or 1000BASE-T.

So what is this PoE thing doing to my Keystone? Is it just that voltage rating and/or dielectric strength is higher than usual?

I believe this is a standard CAT 6 jack which happens to be designed and rated suitable for PoE use, but is not intended exclusively for that application.
 

Online T3sl4co1l

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Re: Preventing ground loops on shielded twisted pair (STP) cabling
« Reply #6 on: March 03, 2020, 07:51:31 pm »
Regarding grounding of STP cables I was facing with contrasting recommendations. I believe this topic is much more general than Ethernet networking. One camp claims you should only ground on one end of the cable to prevent a ground loop. The other camp claims to ground both ends in order to have an effective way to prevent EMI entering the cable. To be sure with a ground loop for my case I mean a loop in the ground/earth wire of my residential mains electricity installation. And I think it is useful for this case to state that I live in the Netherlands.

Note that neither camp is wrong.  But their actions are exclusive.

If you are leaving one end open, you are letting in 100% of the EMI on the cable.

So why even bother getting the shield in the first place?  It's literally not doing anything.

The only thing a single-grounded shield can protect against is magnetic induction between the signal lines, which the twisted pair already deals with reasonably well, and electric induction, which the differential lines don't care about.  It cannot deal with electromagnetic (wave) induction, i.e. where the offending frequency is higher than the electrical length of the cable.

To avoid ground loop, you might compromise by using bypass capacitors around the ground.  This retains galvanic isolation (low frequency -- mains and DC -- ground loop voltages are allowed) while shunting EMI.  But this isn't a connector-level thing you can do, and I'm not aware of anything other than board-level design, that handles this.


Points against the ground loop camp are:
1. Who cares if the EMI is doing ground loops?  If the shield is complete, EMI isn't getting into your signals!  It's fine, end of story.  (Resonances are common -- the low impedance connections allow cables to act as resonators -- but this is easily tamed with a ferrite bead or two.)
2. If you have ground loops that are so bad they're actually causing problems, you have bigger concerns than mere network wiring -- namely, your building wiring is fucked and needs to be repaired immediately before the defective grounding starts a fire or something!

Note that #1 is predicated on what is "fine".  Ground loops are offensive in audio applications because the standard signaling method is single-ended, and the signal is low frequency where shields are ineffective.  That is, the shield's resistance dominates, and ground loop and signal currents are allowed to mix.  The resulting voltage drop is additive to the signal.

This goes away with a differential signaling method, like XLR audio jacks, or Ethernet which is differential and high frequency (AC, transformer coupled).

Tim
« Last Edit: March 03, 2020, 07:54:36 pm by T3sl4co1l »
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Offline ve7xen

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Re: Preventing ground loops on shielded twisted pair (STP) cabling
« Reply #7 on: March 03, 2020, 09:09:48 pm »
Quote
If you are leaving one end open, you are letting in 100% of the EMI on the cable.

For an explanation:

https://www.emcstandards.co.uk/cable-shield-grounded-at-one-end-only

It's not really doing nothing, but it's not doing much for the interference you care about, as T3sl4co1l points out.

Quote
Maybe for my home network I don't really need STP.
Almost no networks need STP. Ethernet over UTP is very robust, and STP would only be necessary in a truly horrific EMI environment. And it creates a lot of complications, you need special patch panels, special keystones, special cables, all of which are much more expensive... Then you need to actually think about your ground system and make sure everything is bonded correctly. Really not worth it. The discussion here just proves this point. If you are working in an environment where STP is actually used/required, you should get domain specific training.
73 de VE7XEN
 
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Online Ranayna

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Re: Preventing ground loops on shielded twisted pair (STP) cabling
« Reply #8 on: March 07, 2020, 10:34:43 pm »
Hi there!

So I was looking around at Digikey for a suitable unshielded keystone jack and found the following:

[attach=1]

It's from Assmann WSW Components, part A-KEY-8-EG-GIB-WIW2.
Digikey product page:
https://www.digikey.com/product-detail/en/assmann-wsw-components/A-KEY-8-EG-GIB-WIW2/123-A-KEY-8-EG-GIB-WIW2-ND/10392871

The thing is specified for Power over Ethernet PoE. However I want to use it for normal Ethernet, either 100BASE-TX or 1000BASE-T.

So what is this PoE thing doing to my Keystone? Is it just that voltage rating and/or dielectric strength is higher than usual?
If that thing is explicitly rated for PoE, it might (or rather should) have a couple of "flaps" on the pins. Those are supposed to help reduce arcing when you unplug, which can actually be an issue, especially with the high powered new standards where you can supply up to 90 watts.
In any case, it will work for normal, non PoE network.

Regarding STP: In my experience (and i am a network admin), connecting the shield on both ends should not give you any issues. In our network, we exclusively use CAT 6a rated cabling (actually CAT7 S/FTP cables, with CAT6a keystones), which have the shield connected on both ends. In our server network, we also use exclusively CAT6a S/FTP patch cables. So our shields are connected, and can utilize 10GBaseT without issues.
For the clients we use a mixture of STP und UTP patch cables.
 

Offline Qmavam

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Re: Preventing ground loops on shielded twisted pair (STP) cabling
« Reply #9 on: April 18, 2020, 02:50:36 pm »
Since this seems to have run it's course, I'll add my question.
 I'm trying to decide whether to use UTP or STP Cat6.
But, I have an off label use, I'm using it as a feedline for
a directional MW/HF antenna, specifically because of the
twisted pair rejection of external RF.
My feedline is 230 ft long. And my question comes down to
If I use STP am I possibly adding a source of noise with the shield
by grounding one end, both ends?
                           Mikek

 


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