Cat. 7 network from main house to annex underground

Exactly, if there are different supplies, one must be careful.

What

Exactly, if there are different supplies, you have to be careful.

What do you have to be careful about then?

If the supply of the two buildings is done via L1/L2/L3/PEN, different qualities of grounding in various buildings can lead to potential differences between the buildings. These can then be equalized through the shielding of the network cable, which can be annoying when touching the devices or can instantly destroy the entire device. I am not an electrician, just a button presser and thinker, but for these reasons, either a potential equalization is created (thick cable between the buildings) or it is made galvanically isolated, i.e., via a short fiber optic segment. However, if the second building is simply connected to the main building via a five-core cable, for example, there are fewer problems in this regard.

But if the second building, for example, is simply connected to the main house via a five-core cable, there are fewer problems in that regard.

Great, thanks for the clarification. But it really is a smaller kind of extension and is therefore supplied by the main house.

What kind of five-core cable would that be and what would need to be considered? Just for planning, I won't be doing the connection myself.

Best regards

I once saw a video from the USA on Youtube where the creator originally had two adjacent houses (his parents' and his own with family) networked together. At some point, lightning struck the tree standing between them and caused major damage to the network components in both houses. After that, he switched to fiber optics.

Long story short: I would at least consider networking the two buildings with fiber optics and then using a switch for distribution within each building. Yes, it is more expensive, especially with 40 GBit or more on the trunk, but it is also safer.

How can one possibly advise a layperson to connect switches via [LWL Kabel]? In the unlikely event that lightning strikes, there is surely a household insurance.