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Author Topic: Power Over Ethernet (POE) - Grounding (bonding?) Shielded Twisted Pair?  (Read 7789 times)

frank kayser

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As a new owner of an EnGenius ENH500 (outdoor) Access Point, new questions pop up not covered in the manual - or if there are pieces, the why is missing. 


Straight to it, the POE injector has a terminal on the side labeled "ground".  It is connected internally to the metal-clad RJ45 sockets on the input and the output end of the injector.  There are also metal Rj45 sockets on the AP itself.  No ethernet cables were supplied.


The manual is silent on the connection. Let's talk about hookup first - bare wire to a piece of grounded equipment (mixer, rack)?  For what?


If instead, I were to use shielded twisted pair ethernet cable with metal shells on the RJ45, connected between the mixer (QU16) and the POE, that would complete the POE "ground" circuit at the mixer.
Extending the shielded cable with metal shells between the POE and the AP would then completely interconnect the ground through all three devices through the shielding.


My reading of the use of Shielded Twisted Pair leads me to the conclusion that that would lessen interference to/from that cable. But given my run will never exceed 25' (stick the AP on a pole next to the mixer location), I'm not sure of the benefits in my situation.  I can see it if it were to be used for its intended purpose - permanent install outdoors at the end of a longish run most likely. 


Which leads me to the real question: what is this "ground" terminal is supposed to be?

- EGC?
- Lightening protection?
- Shield connection/drain?


One thing I've learned here is how much I do not know (the list is increasing), and if in doubt (possible safety issues), I should ask.  Grounding and bonding are two of these things about which the more I learn, the less I know.  Any help on the whys and wherefores of connecting/ignoring the "ground" is always appreciated.


frank



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John Rutirasiri

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Re: Power Over Ethernet (POE) - Grounding (bonding?) Shielded Twisted Pair?
« Reply #1 on: November 04, 2014, 04:24:42 pm »

You don't need or want to use shielded cat5/cat6 (STP) for PoE.  PoE does not use the drain wire or the shell of a metal RJ45 connetor at all.  It was designed to use cheap UTP cabling. 

PoE injector has to be galvanically isolated from the device, and (depending on the design of either), using a shielded cable can cause problems such as blowing out the device's transceiver.

I use standard cat5e patch cables between my PoE injectors and devices.  Works just fine.  Supposed to be good for 300ft, but I've only done 100ft.

My injectors have the standard 3 prong IEC power socket, which of course has the ground connection.  If yours doesn't, then the ground pin is for ESD protection and yes, lightning.  DO NOT connect that ground pin to the device getting the power -- you'd be defeating the design.

-JR

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Re: Power Over Ethernet (POE) - Grounding (bonding?) Shielded Twisted Pair?
« Reply #1 on: November 04, 2014, 04:24:42 pm »


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