On Fri, 27 Jul 2007 14:24:17 +0100, John wrote:

OK, not DIY but interesting anyway.

Yesterday, I was sat at my computer at about 3.00pm and suddenly the
computer and the wireless router went off, I was listening to the radio
tuner of the hi-fi and that went off, but at the same time I was using the
DVD/HDD recorder to dub some stuff from the hard drive to DVD (with the
telly on so that I could see what was going on, but the sound turned down)
and that *remained on* and OK - all of these being fed on the same ring
main.

Meanwhile, my wife was in the kitchen (which is on a seperate ring main) and
she came in to say that her telly had gone off but the kettle was still on.
I looked at the consumer unit, not really knowing what to expect - MCBs
couldn't have tripped or everything on those circuits would be off, but they
weren't. As expected, nothing was untoward. Then a neighbour came knocking
to say that something similar had happened at their house and was wondering
if we'd noticed anything.

Just been talking to a mate of mine on the phone and he said, "Did your
lektrickery go off at about 3 o'clock yesterday?" Apparently, he was in town
shopping and various shop lights and stuff went off. Half an hour later, he
rings his wife at work and she says that she'll not be home on time as
*some* of the computers (not all) had thrown a wobbly - her office is in
town.

When he got home, he found that his kitchen fridge was off and that a
clock/radio in the bedroom had gone off (because the display was flashing).
His beer fridge in the garage was OK though (PHEW!!! ))

Now, my house, his house, and town are roughly in a triangle geographically,
with each side of the triangle being about 3, maybe even 4 miles. How the
hell could this have happened and more to the point, just exactly *what*
happened??

John (this is in Preston, Lancashire, BTW)

Probably a fault at a higher voltage (33kv or even higher) some long way
from you pulled the volts down enough for a 'brown out'.

Fault protection for predominantly overhead systems have both an
instantaneous element and a time-delayed element. The instantaneous element
trips the circuit in a matter of a very few milliseconds and is intended
for transient faults, like wind-borne material, birds flying into and
clashing the wires, tree branches, which can cause a very short duration
fault but which then clears itself, so the circuit can be re-energised
after a few seconds delay. In the event of a sustained fault the
time-delayed protection operates. At 11kv, that time delay is still very
small, typically 350-400 milliseconds.

On underground systems, there isn't usually any need for instantaneous
protection, as a fault will be sustained anyway. The higher the voltage the
longer the (relative) time delay, to get discrimation between voltages -
you don't want the protection at, say, 33kv or 132kv recognising and
operating with a fault at 11kv, so the time delay to operate could be two
or three seconds.

So, a fault at a much higher voltage, and quite possibly some distance from
you will be pulling down the voltage for perhaps two or three seconds where
you are until the fault is cleared. Some equipment at home will be more
sensitive to (severe) voltage fluctuations than others, hence some items
affected, some not.

--
the dot wanderer at tesco dot net