I'm in the process of ducting the aforementioned cable through a conduit that I've "trenched" between my house and my office which is in an outbuilding 30m away.

Since buying the cable and starting this project I've learnt about the worries of lightning strikes and ground loops, but I don't fully understand what I should be doing to mitigate these issues.

I'm in the UK (where we have three-prong mains sockets that TEND to have an earth wire connected), and plan to put an rj45 faceplate on each end of the cable. In the house I will connect my router (or maybe a switch if I'm running short of router ethernet ports) and in the office I'll connect a second router (with DHCP off, running just as an access point).

Firstly I don't even know how you ground a cat 7 cable. Do you have to connect all of the shieldings together somehow, or is that what the drain wire does?

Am I right in thinking that I should be earthing one end of the cable, and very definitely NOT the other? How do I go about doing this? And does it make more sense to ground the house end or the outbuilding end?

Any help appreciated... I've spent more time than I care to admit digging a trench and drilling through stone walls, and have spent £200+ on the access point, 40m of cable, various tools, etc etc.

Should I be aborting the ethernet and running fibre instead? If so what does that look like, cost wise, for a 40m run with converters at both end? Is fibre "easy" to set up, just plug and play, or is it a steep learning curve?

Thank you, all help appreciated!

  • How are you providing primary protection for this cable? (i.e lightning/surge protection) Commented Feb 25, 2020 at 1:55
  • 1
    No idea, you tell me!
    – Codemonkey
    Commented Feb 25, 2020 at 7:50
  • 2
    If you actually need Cat7, and on the presumption that you do not have much experience terminating ethernet cables, for 30 meters my strong suggestion would be to buy a factory made patch cable, and I hope your conduit is big enough for the connectors to pass through. Learning to properly terminate Cat7 on your first and only run seems like an unnecessary risk. Fiber is a better idea, but the same caveat would apply ... I hope your conduit is big enough for pre-terminated fiber.
    – jay613
    Commented Apr 11, 2022 at 12:14

2 Answers 2


Fibre/fiber is by far the preferred/best solution to the problem when running external to buildings.

It can be inexpensive or expensive, and that does not always correlate to quality. Typically the most afforable "fiber converter" is just a switch with SFP (Small Form Pluggable) [ or XFP slots if you have application for 10Gigabit. ]

The market has moved a great deal - at this point there are very affordable single-mode SFPs out there, and that makes the mutli-mode versions that have less range and need more expensive fiber far less of an attractive option. Likewise, the price premium for "reduced bend radius" singlemode fiber has come way down, as it's become the default single-mode fiber in many applications.

  • Yeah, direct burial type (water resistant/outdoor rated) fiber is how I'd prefer to do it, although I'd drop in a Cat 5 or 5e with primary protectors at each end as well to provide copper pairs if need be (mostly for POTS or PoE reasons) Commented Feb 25, 2020 at 2:02
  • Could you recommend a fiber to me @ThreePhaseEel? fs.com recommended me fs.com/uk/products/71448.html which at 40m and a fibre count of 2 runs to £180... and I need two of them (I'm actually doing two separate outbuildings). That is a LOT more than I want to spend... also it wouldn't arrive until March 23rd!
    – Codemonkey
    Commented Feb 26, 2020 at 16:25
  • Thanks @Ecnerwal - I can't find any cheap outdoor rated cables though, see my comment above...
    – Codemonkey
    Commented Feb 26, 2020 at 16:26
  • I don't know what the market in the UK is like - I've found excellent deals on ebay, of all places, but it's not a consistently reliable source. I hunted around quite a bit before finding a "reasonable" supplier of fiber cable semi-local to me. There are many unreasonable ones. "drop" cable is a particularly low-cost option for exterior rated, usually.
    – Ecnerwal
    Commented Feb 26, 2020 at 18:41
  • For example (just a mostly-happy customer, no other connection) fiberinstrumentsales.com/… is about 27 UKP if I'm doing the math correctly for 80 meters, but then you need to get it terminated/connectorized, shipped across the Atlantic, etc...
    – Ecnerwal
    Commented Feb 26, 2020 at 18:57

I would comment, but i am lacking rep here.

Essentially, you can not do it safely.

The problem is when the lighting strikes near one of the building. Even if you ground both sides, your "grounds" will be at a different level and there will be current flowing through the shield.

That is the smaller problem.

The bigger issue is that the same voltage difference will be present at the data lines (across the input coupler, because each building will be on a different potentia), and the input coupler is only safe for some 100V, while the voltage difference incurred by a lighting strike can be much more, even if it does not directly strike one of the buildings. What i did in a similar Situation (although only ~10m apart) was to connect the ground levels right at the foundation earth of the two buildings with a large diameter wire. That way, the path of least resistance will be through there and not through the ethernet cable, and both sides will have the same reference potential. Disclaimer: Don't do that if you don't know exactly what you do and understand the implications. In your case, fiber is the way to go. If you absolutely can not go with fiber, a sluggish workaround would be to have a cheap switch on either side and pray that in case of a surge, the dying input couplers will still protect the rest of the network. Another option would be to have only a short line of fiber for seperation, but then each coupler would need to get power from it's "own" end.

  • I went with a Ubiquiti NanoStation 5AC Loco on the house and another on the outbuilding. Nearly 3 years on and it's worked as flawlessly as a wired connection.
    – Codemonkey
    Commented Dec 21, 2022 at 20:09

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