How to ground shielded cable

[Deac99]

Does anyone have practical experience in grounding cat 6 and 7 cables?  I have instances where I've got runs of cat7 close to 100 meters.  My area has a lot of lightning and I understand that even though I don't have a direct hit, the long cable can act as an antenna and the shield can create a high voltage spike which is obviously not a good idea.  In reading more, I understand that I do not want to ground both ends because this could make things worse due to a ground loop.

I've seen the shielded cat6&7 connectors that presumably would plug into a grounded patch panel, but I'm not using the ends as such I'm just using the twisted pairs to connect sensors etc.

In a situation like that, how best should I ground all that shielding (neatly) and does anyone have an opinion / rationale of which side is best to ground (the far sensor or camera side or the close side near the Loxone?

Thanks

[Rob_in]

If you Google 'cable shield clamp' you will find a few. Eg. Wago 791 series. These are mostly designed to clamp cable to an earthing bar in your cabinet. However, they are quite expensive.

What I decided to do was use Fix & Fasten KTCC clamps (bought from TME where they are about 10c each!). I drilled a load of holes in the DIN rail holding the cable termination points and just bolted the incoming cables, with shields exposed of course, to the back of the DIN rail which is itself grounded. I then just loop the cable up in front of the rail to connect the signal cores.

I did think about bolting these clamps onto a threaded steel rod fixed in the side or top of the cabinet but thought behind the DIN rail was better. Could have just bolted directly the the grounded steel cabinet too. I'm sure there are any number of places you can clamp with these.

HTH,

Robin

[Deac99]

Thanks Robin,  That looks like a good solution, looked up the connectors on TME and I'll get those ordered.

[Rob_in]

No worries.

I'd hold off for ordering until you absolutely need therm because maybe you'll discover you need some more small parts during planning, and TME is a great source. Eg. I bought loads of cable ties and fixing points, Wago connectors for other stuff, etc. from them.

Cheers,

Robin

[Seb]

That is a great solution! I have been wondering how to keep my wires tidy before they enter the cabinet, I can surface mount a din rail on the wall, attach these to it and then run the wires into the cabinet. When the rail is full I can mount one slightly further out/down the wall.

What I was doing before this was an interconnected terminal block, crimping the shielding in a ferrule to link them all together and then one linked to the cabinet earth bar.

One question. If I have a cat 7 going from a switch to the cabinet, should I only attach the shielding in the cabinet to earth and not in the switch to the metal box?

[Kevp]

You can connect the shielding at both ends, it's still essentially only earthed at one end.

[Andy Wain]

Network cables should only be grounded at one end to prevent an ground loop.

In the context of Loxone grounding both ends shouldn't cause a problem. However good practice is to ground only one end just in case

[Seb]

Grounding in the cabinet will take it to the earth bar, that is connected to the earth bar in the consumer unit (that is then connect to gas and water pipes). However, connecting the other end of the wire's shielding to the metal box in the wall, do you see that as grounding as well? Ie what you would do if it were a 240v switch.

[Kevp]

That wouldn't be classed as grounding unless the back box is connected in some other way to earth.

[Seb]

Thanks, that is what I thought, but wanted to be 100%

[Seb]

For anyone that is interested, I did a little research on this and found the products [1] & [2]. The UK supplier is www.buttkereit.co.uk and you can contact them on sa...@buttkereit.co.uk. Both of the below can be mounted on 35mm DIN rails.

[1] SFS|SKL 6-8, Part No. 36820 is £2.38/each

[2] SFZ|SKL 6-8, Part No. 36860 is £2.72/each. This Item provides strain relief.

NOTE: The above are the 6-8mm, but after removing the jacket to expose the shielding the wire is not clamped very well (with jacket on, its clamped nice and tight - but obviously defeats the point), you may therefore prefer the 3-6mm. They kindly sent me a sample of each and I have asked for the next size down. Will let you know what works best.

My thoughts on this was to have a DIN rail above the cabinet on the wall and use the clips to both tidy up the wires and ground them. Will send a photo when I have something that is neat and tidy.

Seb

[1] http://www.icotek.com/en/product-catalogue/emc-cable-clamps/sfsskl/  

[2] http://www.icotek.com/en/product-catalogue/emc-cable-clamps/sfzskl/ 

[Rob_in]

Interestingly, one of those is already in my browser history ;)

But yes, they seem a good option. Will be interested to see the result.

Robin

[Seb]

Hi, so here are some photos (all Cat 7, NOT Cat 6).

Image 1 shows both clip types all with wires in. For two wires to sit side by side, it take up ~14mm for the SFSSKL [1] and ~28mm  for SFZSKL [2]. The 6-8mm are good for clamping the wire, but if you remove the sheath the wire is loose in the grip (image 2), so the 3-6mm are best for that, but the 3-6mm are too small to clamp the with with the sheath still on.

[RSin]

Question my spark just raised. The cabinet, DIN rail etc is grounded back to the meter box where the earth is connected to the neutral (at least with our connection). If you have an earth fault  it comes up the neutral and you've grounded the shielding to the cabinet/DIN rail then you could get 230v going down your shielding. Any thoughts? 

He says he's being pedantic but it could happen.

FYI i've got draka cat6a that has a separate drain wire that I can connect to the earth bar rather than faffing with clips. Bought from canford audio.