John wrote:
"The Wanderer" wrote in message

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.

Brilliant explanation Wanderer, thanks very much.

Yup, very useful. ;-)

I would add to that, IT equipment in particular will respond to these
things in different (and sometimes surprising) ways.

Most PC power supplies have enough internal capacity to ride out short
interruptions of several hundred ms. So a short glitch that affects some
equipment may go unnoticed by the PC. However as the duration increases
more and more will reset (hence the "some computers in the office"
problem). The reverse can also be true, the PSU in the HDD recorder may
be able to cope with longer interruptions than the (typically more
heavily loaded) computer PSU can.

Brownouts (sustained low volts) pose a different sort of problem - how
it copes with this will depend on the PSU technology. Most IT kit will
have switch mode PSUs, and some have very wide ranging input
capabilities (laptop PSUs for example are often happy with anything from
90V up), and may carry on with half mains voltage and not bat an eyelid.
Others may treat it as an interruption and shut the machine down. What
happens on power recovery is controllable in the BIOS on many PCs now.
So you have have them stay off, or start, or return to whatever state
there were in at the time of interruption. Most non IT kit will just run
dimmer / slower or not work at all on low volts. Although some things
like fridges are actually at risk of being damaged by persistent low volts.

For IT kit a UPS is your friend!

(and if you power is anything like ours, gets well used sometimes!)

--
Cheers,

John.

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