Cable pit LV+ELV terminating??

Humm I suppose it depends on whether you consider a the inside of a draw-pit to be "underground" - I suppose the worst case would be to assume so, in which case from a BS 7671 point of view you just need to maintain a 100mm clearance between them (or added protection - 528.2). BS 6701 and/or BS EN 50174 might have other ideas though (I'm afraid I don't have either of them to hand).

That said, running copper data cables between building isn't always a good idea - especially if the buildings concerned have less than especially well interconnected earthing systems (various events can reasonably result in one or other earthing system going up to a reasonably high voltage, and that voltage difference can then be imposed on the electronic data equipment - which often doesn't end well). (Although I must confess I do have CAT5 running between a the house and a couple of outbuildings at home.) Fibre optics are usually preferred.

   - Andy.

The Ethernet cards, usually as standard have tranformer isolation.

The google AI reports:

Ethernet isolation is the capability of a network to separate electrical circuits to protect against voltage surges and electrical noise. By default, Ethernet uses isolation transformers to provide a minimum of 1500 VRMS of transient isolation, which complies with the IEEE 802.3 standard and protects against high voltage spikes like lightning strikes. This capability can be enhanced with specialized network isolators for higher protection levels, often used in sensitive applications like medical or audio systems.  

The google AI reports:

Undoubtedly at least partly true! I'd add thought that not everything that runs over structured cabling is necessarily Ethernet (the structured cabling concept allows for all sorts, from analogue phone lines to RS 232, although admittedly 'earth return' types are getting quite rate these days); I suspect not all Ethernet ports are fully protected by transformers either - e.g. PoE pushes DC down the Ethernet wires, I understand, so that can't go through a transformer; and finally protection against short duration surges isn't quite the same as protecting against longer duration voltage differences (most SPDs would fail if subject to an overvoltage for several seconds (time to clear a L-PE fault) never mind minutes or hours (broken PEN)).

   - Andy.

Useful points. I can see that the PoE stuff (of various implementations) could be a problem. I did think that the Ethernet signalling side did expect to have isolation transformers, which then expects independent power at each end.

Never a dull day ;-)

Useful points. I can see that the PoE stuff (of various implementations) could be a problem. I did think that the Ethernet signalling side did expect to have isolation transformers, which then expects independent power at each end.

Never a dull day ;-)

Well in a sense a pit is a very small room, that occasionally floods.

As such, when there is no makers guidance to the contrary  the same standard separation rules really should apply as would be applied indoors. 

Actually combined power and data cables are entirely possible, (example )but they have to be made correctly, and the insulation of the signalling parts insulated to a level that is not affected by the voltages on the power section.  So much for safety, the next issue is EMC, and the modern equivalent to the coupling of mains hum onto the phone lines.

This can be addressed with appropriately shielded cables and the use of transmission schemes like Ethernet that is transformer coupled and quite immune to induced low frequency voltages - but as noted a lot of cheap kit does not actually implement the transformer isolation properly, and cheaper capacitor coupling is used instead, which is functional in a clean environment, but far less immune than a system with magjacks at both ends. And POE blows a hole in that anyway and gets you back to where you came in with phone lines and RS232 links etc . 

In general separated ducts and cables properly spaced within the manholes & any larger service tunnels  is the proper way, but the rules can be broken when there is knowledge of the cable types and exactly what will be at either end of both systems involved.. If you should is another matter, as it can later restrict what can be done with the installation when kit needs replacement and so on.
Mike.

Mike.