Shielded Ethernet & ground loops

Thread Starter

strantor

Joined Oct 3, 2010
5,908
I've been looking into powering some 24VDC industrial Ethernet/IP devices over their Ethernet cables. Low power devices only (<500mA), using passive POE injector/splitters such as this. I did the math for voltage drop along the intended distance and concluded that there would be no problem, so I ordered the devices I linked to, connected everything, and indeed it works fine. Voltage drop is not the problem.

The problem (up for debate) is that the injector/splitter pair has shielded female jacks but unshielded male plugs. All signal & data cables for this application are shielded, and all the comms are 100% Ethernet/IP (physical Ethernet spec, industrial protocol). All my Ethernet/IP devices (15+) have dual (for daisy-chaining) metal clad ports and are intended for shielded Ethernet cables to be used.

I ordered the previously linked injector/splitter pair specifically because of the metal clad female ports but I missed the fact that the plugs are unshielded. So when I utilize this pair, it causes my shielded Ethernet cable to now have a floating/unterminated shield. So I ordered this 4-port device which has metal-clad female ports for input & output, which (I assumed) would allow the shield termination to pass through the POE injector. It does not. The metal cladding of the input ports is not connected to the metal cladding of the output ports. None of the metal cladding in the device is connected at all, to anything. My initial thought was that this "iCreatin" company has no clue what shielded Ethernet is, or how it is supposed to work.

Then I questioned, if maybe I'm the clueless one. So I did some Googling about how shielded ethernet is supposed to work, if it's supposed to be grounded on both ends, if it's supposed to be grounded at all, Etc. And I did not find any concrete answers. I found a lot of people talking about shielded cables in general, single point grounding, and a lot of disagreement and contradictions. So then I decided to investigate the devices that I have, and see if I can resolve the intent behind their design. My industrial ethernet switches have their port cladding internally terminated to ground. One of my field devices has its dual ports connected and tied to ground. One of them has the ports connected and floating. One of them has the ports isolated from one another and floating. And the last type of device has 1Mohm to ground on each port (2Mohm between them - presumably some kind of filter?).

Now I resign that I don't know WTF is going on. I can't find any credible reference that definitively states what (if anything) the metal cladding inside a shielded Ethernet port is supposed to be terminated to inside a device, if/where the shielding of a shielded Ethernet cable is supposed to be grounded (one end? Both ends? At each device? Only at the switch?), or how shielded Ethernet ports are supposed to be designed.

(I think) I understand the concept of a ground loop so it would make sense that the switch has grounded ports and none of the other devices do (except for the ones that do) - but what if you're going from one switch to another switch? Half of the voices on the internet are telling me that ground loops are fiction except for when devices are powered from separate supplies and have different ground potential. Some are even so bold as to state that ground loops are fiction, period. Or that they don't apply to shielded Ethernet because it uses differential signals.

So please shed some light. How is this supposed to work? I opened up the 4-port device and soldered a jumper between the metal cladding of the input and output ports, and it's working just fine like that. But have I potentially caused a problem down the road if I use this method with a different kind of device that has a differently grounded port?
 

Thread Starter

strantor

Joined Oct 3, 2010
5,908
After reading through that reference, I am still of the opinion that the passive injectors/splitters, if marketed as shielded Ethernet devices, should have...
For the single injector/splitter pair: shielded plugs with the shields connected between jack & plug.
For the 4-port device: input port cladding terminated to output port cladding.
 
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