>What I'm getting at is - these homes who have 300 meters of CAT5 strung >around the place - under these abnormal fault conditions you describe, >can you generally say that the cat5 cable will not present a risk >_during_ the fault conditions ? Generally the cat5 cable will not present a risk under abnormal fault conditions IF it is plugged into a NIC at each end AND by fault conditions we mean 400/230 volt supply. For high tension faults you can figure out the volts per meter back to where the HT is earthed (where it is supplied from, which is normally lots of kms away) and get a rough idea it it is safe or not. EG, if this 11kv line comes down, it has 6.6kv to ground, it is suppled from the substation 4km away, so that's 1.65kv per km, so 300 meters of cat5 will have 495 volts difference between each end IF each end were connected to ground (which it isn't). You have to take into consideration the breakdown voltage of the insulation on the cat5 also. When thinking about lightning, well if lighting wants to go somewhere, it goes somewhere. >This is the clincher. Sure it will be safe 99.999% of the time - almost >anything is. What about one day when a fault or a strike pushes the >grounds' PD gradient above a safe level for a 300 metre cable ? How >regularly is *that* going to happen, IYO ? Personally, I've seen 11kv get through onto the 400/230v once. 11kv to ground is more common. Lightning strike onto HT is common. Lightning strike onto 400/230 I've seen once. Lightning definitely hits things like wires and trees and the ground. >> >How many of the abnormal conditions could not have been foreseen ? >> >> I would say 99% of abnormal conditions can be foreseen, and the way they can >> be foreseen would be to figure out all the things that can go wrong: cars >> running into poles, trees falling through power lines, snow snapping lines, >> high wind banging lines together* etc, etc. The problem is that such >> conditions are statistically unlikely and so are not really taken into >> consideration. And then they happen. > >okay, the risk for the supply authority, and for consumers is not >economically worth lowering any further. perfectly reasonable. > >But what happens when someone strings a 300m wire and takes it into >their living room? These faults that spot-crank the earth potential >around could (and will) present quite a large voltage in someones' >lounge - enough to break down a tiny 2kv isolator that was not designed >to do that job. comments ? Personally, I would happily string a bit of cat5 10, 20 or 30 metres between houses. I'd be much more wary of running longer distances, like 100 or 300m, instead I'd build a Ronja. http://ronja.twibright.com/ Wayne ------------------------------------------------------------------- To remove yourself from this list, email nzlug-request@linux.net.nz with "unsubscribe" in the body of the message.
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