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Darryl R

Shielded vs. unshielded Ethernet and Grounding

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My dCS Network Bridge is connected to my DAC with dual AES interconnect, and connected to my music server via shielded Ethernet cable.  The music server is in turn connected to my LAN with the same shielded cable.  A reputable manufacturer advised me my system may grounded to the LAN because of the shielding on the Ethernet cable.  I have some occasional light humming in the speakers.

 

Has anyone experienced any grounding issues with shielded Ethernet?  I see it is a known issue in various on-line sources.

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On 7/12/2018 at 6:13 PM, Darryl R said:

My dCS Network Bridge is connected to my DAC with dual AES interconnect, and connected to my music server via shielded Ethernet cable.  The music server is in turn connected to my LAN with the same shielded cable.  A reputable manufacturer advised me my system may grounded to the LAN because of the shielding on the Ethernet cable.  I have some occasional light humming in the speakers.

 

Has anyone experienced any grounding issues with shielded Ethernet?  I see it is a known issue in various on-line sources.

 

In addition to dCS's official reply, I did experiment on the subject and found the best results with simple CAT 6 UTP to prevent from ground loops via shielding and a Emo Systems EN-70HD (hospital grade) network isolator to block any possible common mode noise. The network isolator is placed just before the Network Bridge.


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Thanks for this.  There seems to be a consensus among cable manufacturers with a sense of responsibility for their products that if shielding is used it should be grounded only at the source end.

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The main reason for having the shield on the cable is to avoid HF being emitted from inside the cable and avoiding RF to get in the inside conductors.

 

The problem arises when the shield is a different potential at the source and the receiver in a classic ground loop. A second conductor run in parallel with shielded cable’s shield will achieve the same potential at both ends. It may not be attractive, but it works. 


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On 7/16/2018 at 2:35 PM, skatbelt said:

In addition to dCS's official reply, I did experiment on the subject and found the best results with simple CAT 6 UTP to prevent from ground loops via shielding and a Emo Systems EN-70HD (hospital grade) network isolator to block any possible common mode noise. The network isolator is placed just before the Network Bridge.

 

Ethernet is transformer isolated at both ends, you don't need those network isolators if you use UTP cable. Common mode noise is cancelled by the transformers and receiver circuit. That network isolator probably just adds extra transformers on the path and may degrade signal quality leading to higher transmit powers through 802.3az standard which all good new network equipment supports. It is usually better to let go with lower transmit powers on the ethernet than trying to add extra isolation.

 


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1 hour ago, Miska said:

Ethernet is transformer isolated at both ends, you don't need those network isolators if you use UTP cable. Common mode noise is cancelled by the transformers and receiver circuit. That network isolator probably just adds extra transformers on the path and may degrade signal quality leading to higher transmit powers through 802.3az standard which all good new network equipment supports. It is usually better to let go with lower transmit powers on the ethernet than trying to add extra isolation.

Those isolators intended for hospitals and such are meant to protect equipment against much higher voltages than a typical Ethernet jack can withstand. Low-level noise isn't a consideration, and they may or may not block it any better than the regular transformers.

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6 hours ago, mansr said:

Those isolators intended for hospitals and such are meant to protect equipment against much higher voltages than a typical Ethernet jack can withstand. Low-level noise isn't a consideration, and they may or may not block it any better than the regular transformers.

 

And they don’t do anything to block AC leakage currents—which John has measured traveling over Ethernet lines.  That’s an important factor to address for audio—especially when one purpose of using small end-point Ethernet renderers is to provide a truly isolated music source.

There are a few models of Ethernet switches which, if grounded, will shunt incoming (from all ports) high-impedance leakage.  Such is never seen as a specification and it takes a special test setup to check if a switch does or does not block leakage.

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9 hours ago, Em2016 said:

 

Is this stuff unshielded? There's no mention. But can you tell by the specs mentioned?

 

 

There are different types of shielding "S" refers to a screen shield, "F" refers to a foil shield and "U" refers to no shield.

The shield can be around the entire cable, or around each twisted pair (TP) hence the nomenclature:

 

S/STP -- the first "S" is the outer screen, the second "S" are screen shields around each twisted pair (TP).

U/STP -- no outer shield, each twisted pair has a screen shield

S/FTP -- there is a screen around the cable, and foil around each twisted pair.

etc...

 

https://www.belden.com/blog/digital-building/stp-utp-ftp-cable-more-7-types-when-to-use-them


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9 hours ago, Miska said:

Do NOT use shielded ethernet cables with audio equipment! Only use standard inexpensive CAT6 UTP patch cables - those are the best.

 

Cat 6 U/UTP is best for audio (10Mb -> 1GbE)

The shields around each twisted pair reduce crosstalk between the twisted pairs and are used with 10 Gbe... e.g.

U/FTP

For the vast majority of home applications there is no need for outer shielding -- particularly with bonded twisted pair there is not significant EMI emission ... for example your Ghz Wifi works great right next to a Gb Ethernet cable -- nor is there significant RF pickup.

 


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11 minutes ago, jabbr said:

Medical/hospital Ethernet isolators absolutely are exactly designed to block leakage currents. In fact that's their entire purpose.

 

The standard Ethernet PHY transformers are fine to block high level voltages/currents.

Ethernet jacks/transformers are not generally rated according to IEC 60601, the standard specifying safety requirements for medical electrical equipment. A random TE Connectivity datasheet I checked doesn't even mention the relevant parameters.

 

11 minutes ago, jabbr said:

Imagine an ICU setting where there is a catheter in the heart which is measuring (electrical) cardiac signals -- imagine what a very low level (AC) -- perhaps microamp --  leakage current might do! Leakage currents are literally a life and death issue.

For such purposes, you'll need a barrier much closer to the probe, or else it will be disturbed by noise generated within the equipment itself.

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3 minutes ago, mansr said:

Ethernet jacks/transformers are not generally rated according to IEC 60601, the standard specifying safety requirements for medical electrical equipment. A random TE Connectivity datasheet I checked doesn't even mention the relevant parameters.

 

For such purposes, you'll need a barrier much closer to the probe, or else it will be disturbed by noise generated within the equipment itself.

 

Yeah in fact, the standard specifies allowable leakage currents as well as methods of testing: https://www.mddionline.com/leakage-current-standards-simplified

 

In the USA, the equipment itself is regulated by the FDA.

 

Those old heavy "ultra isolator" low leakage current transformers were not made with audio in mind ;)

 


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5 hours ago, jabbr said:

Medical/hospital Ethernet isolators absolutely are exactly designed to block leakage currents. In fact that's their entire purpose.

 

Sorry, I misspoke. Of course the medical EN isolators block low-impedance (touch current) leakage.

John tested a popular one that someone sent him and found that it did not block high-impedance leakage.  Can't seem to find his post right now, but he did use his leakage test setup on it--during the time he was comparing switches--and reported that the medical isolators were only effective with the low-impedance leakage.  Not that it is difficult to design a switch that shunts the high-impedance leakage.

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29 minutes ago, Superdad said:

 

Sorry, I misspoke. Of course the medical EN isolators block low-impedance (touch current) leakage.

John tested a popular one that someone sent him and found that it did not block high-impedance leakage.  Can't seem to find his post right now, but he did use his leakage test setup on it--during the time he was comparing switches--and reported that the medical isolators were only effective with the low-impedance leakage.  Not that it is difficult to design a switch that shunts the high-impedance leakage.

 

As I said before, there is more than one issue with leakage current. "Touch current" is the easiest to understand, and most widely applicable, and ultimately you don't want your 2 year old kid to get shocked either by a low impedance path, but there are more stringent situations as well, i.e. the cardiac compromised ICU patient with an electrode actually inside the heart. In the most sensitive situations e.g. brain monitors, actual fiberoptic sensors are preferred due to their blockage of even higher impedance leakages!

 

What are you defining as "high impedance"? IEC 60601 defines the test mechanism which requires a minimum >1 M ohm input impedance on the meter. If you look at the literature which if course exists and is fairly extensive, issues with even higher input impedance e.g. Gigaohm are discussed, as well as issues arising from insulator breakdown (can be in the Teraohm range). That can indeed be hard to measure!

 

I can't speak for the Ethernet isolators that you tested, but Topaz ultra-isolators have certainly been used in healthcare environments and are known to have low parasitic capacitance e.g. < 0.0005pF etc.

 

In defense of the IEC 60601 it does really make sense to measure the net leakage at the end of the day rather than merely the individual components because the circuits can get complicated.


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Let me expand a bit:

 

Every current path is governed by the equation V = IZ (Z=R at DC). Nothing magic about leakage current, they just travel along a certain path, typically one that involves parasitic capacitance/inductance -- if the current didn't travel along the parasitics then it would be a "normal" part of the circuit.

 

So the higher the path impedance, the lower the current at a specific voltage.

Now consider the measurement equipment itself -- the probe etc have their own parastics. Youve discussed providing a low impedance alternate path to shunt away parasitic leakage currents -- when the measuring device can be this!!!

 

So ... in order to be able to measure low level currents (high impedance leakage circuits), the input resistance of the measuring equipment needs to be substantially higher than that of the circuit impedance. That's why (duh!) IEC 60601 requires a high impedance measurement -- and if you want to get even more sensitive then you need even higher impedance.

 

So, to flip this around: what is the minimum current that you want to measure? 1 nA, 1pA? --- really now what is your background noise folks??? 


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@jabbr:

Thanks for your technical observations! John is super-busy this week, but I sent him a link to the above and encouraged him to engage with you and to clarify how he has measured and what he has seen.  

[And of course leakage is but one of the issues we will be addressing effectively in our EtherREGEN switch.]

 

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16 minutes ago, octaviars said:

Look here @jabbr I think this is the one @Superdad was looking for were John measured the Baaske isolator.

 

Oh wow, thank you! That’s the exact post I was searching for.  You just saved John a whiole bunch of time, so double-thanks.  

That post shows both what he measured and how.

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3 hours ago, octaviars said:

Ah good. FWIW there are “medical grade”/rated SMPS that have lowered leakage. I tend not to use SMPS when possible although some of the old HP test equipment have internal SMPS which are quite good. Similarly a SMPS is used in the LPS1 and also mitigated so there are indeed different strategies!. I think it’s the brand/quality that’s a big issue and John has shown that they can be modified to greatly reduce. Also as we all know noise from cheap SMPS can travel into other devices so tackling the source of the problem is indeed better than mitigating.

 

BTW: what are the levels (Y axis) here? Are we talking microAmps? nano? pico?

 

I dont use Baaske isolator myself — I have a fiberoptic network which isn’t an issue. 


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While I'm with the unshielded ethernet group, I was looking through an Icron user manual and noticed this:

 

1006401164_ScreenShot2018-08-01at10_41_21pm.thumb.png.387f97bd14e4bb267a18aed5f8ad4802.png

 

http://www.icron.com/pdf/el5363-hdmi-usb-2-0-manual.pdf

 

They have quite a bit of experience in both networked audio AND video, so there must be something to their recommendation, especially their notes on interference?

 

They're obviously not fans/believers of the balanced and isolated nature of unshielded ethernet.

 

It doesn't change the fact that shielding may open the door to a ground / leakage loop but they must have seen/measured this interference they mention.

 

I'll tag the respected experts @JohnSwenson @Miska @mansr @jabbr

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