On 3 May, Michelle Sullivan wrote: > > > Michelle Sullivan > http://www.mhix.org/ > Sent from my iPad > >> On 03 May 2019, at 03:18, N.J. Mann <n...@njm.me.uk> wrote: >> >> Hi, >> >> >> On Friday, May 03, 2019 03:00:05 +1000 Michelle Sullivan >> <miche...@sorbs.net> wrote: >>>>> I am sorry to hear about your loss of data, but where does the >>>>> 11kV come from? I can understand 415V, i.e. two phases in contact, >>>>> but the type of overhead lines in the pictures you reference are >>>>> three phase each typically 240V to neutral and 415V between two >>>>> phases. >>>>> >>>> Bottom lines on the power pole are normal 240/415 .. top lines are >>>> the 11KV distribution network. >>> >>> Oh and just so you know, it’s sorta impossible to get 415 down a >>> 240v connection >> >> No it is not. As I said, if two phases come into contact you can >> have 415v between live and neutral. >> >> > > You’re not an electrician then.. the connection point on my house has > the earth connected to the return on the pole and that also connected > to the ground stake (using 16mm copper). You’d have to cut that link > before dropping a phase on the return to get 415 past the distribution > board... sorta impossible... cut the ground link first then it’s > possible... but as every connection has the same, that’s a lot of > ground links to cut to make it happen... unless you drop the return on > both sizes of your pole and your ground stake and then drop a phase on > that floating terminal ...
A friend had a similar catastrophic UPS failure several years ago. In her case utility power was 120V single-phase, or 240V hot to hot. Neutral was bonded to ground at the meter box. Under normal circumstances, any current imbalance between the two hot legs returns to the utility distribution transformer center tap over the neutral wire. In her case, the neutral connection failed at the pole end of her power line. In that case, the imbalance current was forced to return via the ground rod outside her house and then through some combination of the ground rods at neighboring houses and the transformer ground connection at the base of the pole. Any resistance in this path will reduce the hot to neutral voltage of the heavily loaded side and increase the voltage by the same amount on the lightly loaded side. Fire code specifies a maximum 25 ohm ground resistance, but it seems this is seldom actually measured. In addition her house was old, so there is no telling what the ground resistance actually was. If we assume a 25 ohm resistance, it only takes 1 amp of imbalance current to increase the voltage on the lightly loaded side by 25V. At that rate, it doesn't require much to exceed the continuous maximum voltage rating of the protective MOVs in the UPS. Once you get past that point, the magic smoke escapes. The UPS was actually a spare that I had lent her. I thought about repairing it by replacing the MOVs after I got it back from her, but I abandoned that plan after I opened the UPS and found this insides were heavily coated with a layer of conductive-looking soot. Two of the MOVs were pretty much obliterated. The third was intact, but charred a bit by its neighbors. _______________________________________________ freebsd-stable@freebsd.org mailing list https://lists.freebsd.org/mailman/listinfo/freebsd-stable To unsubscribe, send any mail to "freebsd-stable-unsubscr...@freebsd.org"
