Shielded Cat 5 cables

Alan Case

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Join Date
Apr 2002
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Wagga Wagga
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Hi.
Does anyone here know the advantages of using shielded cat 5/6 cables in lieu of unshielded when joining together Rockwell Powerflex 753 VFDs in a Device Level Ring. (DLR)
The VFDs are fitted with the EN2TR Ethernet cards that have 3 Ethernet ports.
The 2 front ports are for a 753 VFD and the back port is reserved for a 755 VFD.
My DLR utilises the 2 front ports.
Now for the strange thing. The 2 front ports are metal shielded but the metal on the port has no connection to any earth point nor to the other port.
The rear port which also has metal shielding does have a connection to the frame of the drive. (test with an ohm meter)

I am trying to see a reason for using shielded cable if the shielding is not grounded anywhere or continuous through all the cat 6 cables.

Regards Alan Case
 
I am trying to see a reason for using shielded cable if the shielding is not grounded anywhere or continuous through all the cat 6 cables.

Because if you don't, and you have problems with it, Rockwell will say "well, you're not using screened cables" regardless of whether it makes any sense or would make any difference 🤾

Personally, I'd use them purely for the reason above, but you're right - if there's no connection to ground anywhere on the screen, the screen is not actually doing anything.
 
Dont rely on the connection between connector and port, regardles if there is a connection to ground in the port or not.

The screens in screened cables should grounded by a dedicated shielding clamp with large surface to a copper bus bar or the galvanized mounting plate.
Something like this:

Shield_clamp.png
 
That you have not seen it done before just means that malpractice is rampant.
I am betting that such practice is a spill-over from the dear IT department. Que the proud IT person with his most cherished tool, the crimper for plastic RJ45 connectors.
Office or home --> plastic RJ45 and no shielding.
Industrial installation + shielded cables --> industrial grade shielded connectors and shield clamps.
We allways ground all shielded cables in this manner.
 
Interestingly, ODVA (Ethernet/IP standardisation committee) and the Profinet Standardisation committee see it differently, despite that the topic of noise and shielding should behave exactly the same irrespective of the protocol.

ODVA seems to emphasize avoiding ground loops:
https://www.odva.org/Portals/0/Libr...IP_Media_Planning_and_Installation_Manual.pdf
page 3.50:
Eliminating ground loops is extremely important [..]Consequently, to eliminate ground loops the Ethernet/IP cable should be grounded at the switch end only.
Whereas Profinet seems to emphasize the quality of the ground connection and makes no distinction of which "end" the cable is.
http://www.sci-eng.mmu.ac.uk/ascent...Guideline_Cabling_Assembly_8072_V10_Jan09.pdf
page 9:
At the cabinet entry, connect the
shields of all cables run into the
cabinet to the equipotential
bonding conductor. For this, the
shields should be connected to
operational ground across an area
of appropriate size.
 
Hi.
I assume that the port of a Stratix switch is grounded.
When you then series through a run of EN2TRs then there is no connection that I know of via the card.
Interesting reading from the ODVA spec.
Regards Alan
 
I assume (he, he) that the statement of grounding at the switch is based on a 'star' architecture, which would make sense. But, when you use a 'ring' or 'daisy-chain', there doesn't appear to be any way to accommodate the ground. Interesting!
 
Interestingly, ODVA (Ethernet/IP standardisation committee) and the Profinet Standardisation committee see it differently, despite that the topic of noise and shielding should behave exactly the same irrespective of the protocol.

ODVA seems to emphasize avoiding ground loops:
https://www.odva.org/Portals/0/Libr...IP_Media_Planning_and_Installation_Manual.pdf
page 3.50:

Whereas Profinet seems to emphasize the quality of the ground connection and makes no distinction of which "end" the cable is.
http://www.sci-eng.mmu.ac.uk/ascent...Guideline_Cabling_Assembly_8072_V10_Jan09.pdf
page 9:


Jesper,

I've heard that the difference in recommendations is at least partially related to grounding expectations between US vs Europe. I'm told that in the US, it is common to have many independent grounds with different potentials, and so ground loops are a big problem. I'm also told that in Europe, it is common for the grounds to all be tied together. Variation in potential is typically less, and ground loops are less of an issue.

Take it with a grain of salt, though. I'm mostly a software guy reporting hearsay, none of this is personal experience.
 
in the hydro electric power stations I work on we are careful to ground things only in one place, or sometimes not at all and isolated on purpose as we use floating DC systems. The ethernet cables are armoured so the armour acts as a shield between panels but within each panel it is unshielded. No issues with the ethernet in the stations yet.

In one instance the owner did use shielded ethernet cable but it wasn't armoured, and I brought all the shield drains out to a copper bus, which was pretty time consuming.

I would shy away from using metal connectors to connect the ethernet cable shield to the ethernet switch as I would prefer not to ground all the cables through the ethernet switch, and as others have pointed out this is an invitation for ground loops. I have a hard enough time getting the electricians to terminate the ethernet at all, let alone to specify plastic connectors in some panels and metal ones in others.

If we did use shielded cable I would look for the setup Jesper posted.
 

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