Conduits in a trench

That should be fine. I've done the same thing with a 16" trench and didn't have any problems. Cat 5/6 uses balanced pairs which are pretty immune to interference. RG6 is, of course, shielded. If what you've hooked the 240vac up to produces too much interference, you can always use shielded cat 5/6 but I doubt you'll have to.

conductors,

Should be ok for interefrrence that's directly coupled but bear in mind the RG6 may form a big loop.

Any high power machinery about?

If the RG6 does form a big ground loop, then make sure the shield is only grounded at one end not both ends, (IOW "lift" or break the ground at one end). You'll still have a ground, but there will be no potential for AC to flow in the shield due to differing ground impedances at each end. Same if you go with shielded cat5.

This is true. However, it would be wise to either provide a bonded ground between all three buildings (something which is probably not practical) or use fiber to completely isolate all current carrying conductors. Even data lines inside a shield can carry destructive currents during a lightning hit.

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Yes, a shield ground lift only works if the equipment being deprived of the shield ground also has a ground reference of its own via the power ground. The incoming a/v signal is then referenced to that local ground instead of the interconnect ground which is undependable because it is floating at some level above 0 volts, forming the hum loop. He did say he was running a ground wire to the other building.

We're talking about two different concerns. I was referring to ground strike lightning rather than AC issues. When multiple structures are wired together and there's no bonded ground there is an added danger of lightning entering through one structure's ground, passing through your hardware and out through the other structure's ground. Even in a single structure this is enough of an issue that the recommended procedure is to connect the grounds of all incoming services to a single point or to a bonded ground system. With multiple buildings the danger can be worse than a single structure due to the increased potential between points.

I assume you already know all this but I point it out for the benefit of others who might not. I don't disagree with what you've said above.

There have been some interesting discussions concerning this on Usenet. Some want only one point grounded, while others want both points. See

FWIW, the cited pages were not Usenet threads.

There are several purposes for grounding and they are not all served in the same manner. Shielded cable is often grounded at one end only. Electrical safety grounds are typically grounded from the exposed metal components all the way back to the breaker panel and from there to the ground rod. Multiple services within a single structure should be connected to a single grounding conductor or to a bonded ground system. When multiple buildings near to each other are interconnected, use a bonded ground or isolate them.