Reasons to use shielded cabling
You asked about using shielded cabling to protect ethernet from interference from heavy power currents. We mentioned above that shielded cabling isn't required in this case, but there are a few valid use cases for it:
- If you run cabling where there truly is a high potential for interference, such as ethernet cabling that runs near aircraft radar transmitters.
- If you find that you're at the 100m distance limit for standard ethernet runs, it's not unheard of to squeeze a few more feet beyond the 100m limit of a Cat5e run by using shielded cabling. That said, the 100m distance limit is there for more than just cross-talk and noise limits, but some people still get away with doing this. It helps if you're in bind, but it's much better to lay out your IDFs correctly; you shouldn't do this intentionally.
It obviously won't hurt to use shielded cabling in your case, but it doesn't sound like you really need it.
Grounding Shielded Cabling - one side, or both sides?
As our 'porta cabin' only has 2 PC's we are not planning to put a patch cabinet in there, so my question is - if I terminate the STP cable to a socket and run a standard patch cable that side to a small switch (and then cables to the PC's) would I need some way to earth it?
Terminating one side of shielded cabling is sufficient and industry practice; I am assuming the shielded cabling is terminated correctly. Grounding shielded cabling is also discussed in BICSI Mythbusting: Shielded cabling, which I thought was a great presentation. If you only ground one side, you're eliminating the possibility of ground loops on the cabling itself. Ground loops tend to form at power-cycle frequencies, which don't interfere with ethernet; however, it's best to avoid ground loops altogether.
Terminating shielded cabling to a patch panel
The patch cabinet side would also be terminated to a socket (with patch cable going to main switch (with fibre link to our main office)). I have no STP RJ45 terminations (only standard UTP).
If I was to run an earth wire from the STP shield/screen to the patch cabinet would that be enough (cabinet grounded through equipment connected to a UPS)? I would also look to use ferrite chokes on the cable (to negate any outside RF interference it would pick up) if we found problems.
This is a little unclear without a diagram, but let's cut to the essentials:
- Ground one side of your shielded cabling
- If you terminate into a patch panel, use patch panels / modular inserts designed for shielded cabling because the mod plug (shown below) terminates to the shield inside the cable, and that all should be grounded.
Shielded mod plug:
As the same power source will be powering both ends of the installation I understand we won't have problems with earth differential.
This isn't in the critical path of your main questions, but it's worthy of discussion. Everything depends on whether the buildings are grounded individually, or whether the power cabling to your 'porta cabin' carries the ground conductor with the hot and neutral. If the buildings indeed have different ground rods, you could see a difference in ground potential between the buildings. Please see BICSI Mythbusting: Shielded cabling, which discusses why this shouldn't be a problem for your shielded twisted pair termination, but it's a point worth mentioning in case you run into other equipment grounding concerns; you should not intentionally form ground loops, if possible.