[Techtalk] OT but I'm desperate.... Windows....!

Maria Blackmore mariab at cats.meow.at
Sat Oct 19 22:23:34 EST 2002


On Sat, 19 Oct 2002, L J Laubenheimer wrote:

> The problem isn't voltage, it's current.  The voltage is regulated by
> the utility.

but is vulnerable to sagging

> If you have a 15 amp circuit powering 2 outlets, and those outlets are trying 
> to suck 20 amps of draw combined, something loses, and a brownout condition 
> can occur before circuit breakers trip.  Computers, for some reason, are 
> sensitive to brownout conditions, in spite of their converting (AC to DC) 
> power supplies.  That is why line conditioners are used in most datacenters.

If you try to draw 20 amps from a 15 amp circuit, you will get 20 amps,
and the circuit will get warm.

This is why fuses exist :) There is nothing limiting the current on a 15
amp circuit to 15 amps, there are merely breakers or fuses to cut the
supply if the current goes past 15 amps.

Moreover, despite it being called a 15 amp fuse or breaker, this is a
somewhat nominal value, there is actually a current/time curve that it
obeys, this is especially important in circuits with a high inrush
current, such as motors, large power amplifiers, etc.  eg, a 15 amp class
B breaker will allow say, 40 amps to pass, but only for a short time

A good demonstration of this is a theatre I used to work in, the power
amplifiers for the auditorium sound had been installed using Class C
domestic breakers, when you turned on an amplifier, about 80% of the time
the inrush current would trip the breakers.  Which was annoying, but a
good demonstration, since the breakers were rated correctly for the in
service current of each amp, but would trip on the inrush current.

Another good demonstration is the UPS where I currently work, which is
protected by three 63A Class B three phase breakers.  One day one of the
modules in it let go in a *really*big* way, and all three breakers tripped
at the same time, the current was so great that it passed the current/time
curve on all three breakers at the same time.  That smelt bad, lots of
magic smoke escaped then :)

> > Mains voltage is very well regulated, and has a lot of oompf behind it to
> > back it up.  The voltage drop across properly installed cables when
> > addition load is applied should be pretty much negligible, and the voltage
> > droop caused by the additional load should be easily corrected by
> > regulating transformers after a short delay.
> 
> But that doesn't say anything about current, which is what is restricted in 
> household circuits.

Nothing restricts current in any circuit, unless you have a little widgit
which increases the circuit resistance in response to a higher current
flow, for example.  Household circuits don't have this, it's just copper
wire, there's no current limiting other than the resistance of the wire
itself.  eg, when I turn my monitor on it makes the lights dim briefly,
but it doesn't even make my UPS beep (though the UPS does make offended
noises when you plug the monitor into it and turn it on :).  Another good
instance of this was at another theatre I had worked in, where someone had
decided it would be a really good idea to connect four 1 kW lights to a
circuit rated for 13 Amps in the auditorium, it worked just fine ... until
a fuse blew, I was asked to investigate and had to replace a plug that had
apparently turned into something rather like brown glass.  The lights had
been drawing 16 Amps from a 13 Amp circuit and nothing had stopped them at
all, but things had got a little warmish.

If the voltage sags a lot on incrush current, the wiring needs to be
replaced.  I believe in the USA the fashion is to supply each wall socket
from its own single wire from the distrubution panel, so this could be a
significant problem.  In the UK we usually use ring mains, which are rated
at between 32 and 40 A on the distribution board, and a ring of wire goes
around all the sockets, the whole wire around the ring being rated for the
whole current of the circuit.  This gives two paths for the current, and
thus the wire resistance has a lesser effect.  Also in the UK, in a lot of
places you use pryotenax for circuits that are likely to see abnormally
high surge currents.

I don't know about any other countries, because I've only worked in the UK
and USA

> If your power supply isn't getting the amperage it needs to convert,
> it starts flaking out.  Talking about voltage alone tells nothing.[1]

This assumes that there is something limiting the current, which there is
not for most cases.

eg, one of my friends, at a very young age, decided that she wanted to try
to make a light bulb out of a 3 pin mains socket.  She looked at a light
bulb and figured that if there was one wire in a light bulb, and it made a
lot of light, then she could make a better light bulb by using three
wires.  So she took apart a three pin plug, and found some 60 Amp fuse
wire (which she deemed suitable because it's nice and bendy :) she wound
the fuse wire around all three pins of the plug nice and tight, quite a
few times, probably half a dozen (she wasn't clear on this). She put her
"lightbulb" into a wall socket, turned it on, and found that it did indeed
produce light.

It also threw her across the room and blacked out her entire town.

The next day someone from the electricity board was there replacing the
100 Amp "service fuse"[1] for the house, he noticed that one of the fuses
on the distribution board had a certain kind of bubbley melty effect to
it, and that some of the wiring was done in a kind of charred and
blackened motif. They sent someone around the next day to have a little
chat with her about electricity, and how dangerous it is, and how it's not
really acceptable to black out a town for an hour. :)

Just for interest, this wouldn't black out so large an area nowadays, and
it would probably only go off for about 5-10 seconds, since all
substations are remotely controlled nowadays, and the breakers are
automatically reset if they trip.  Investigation is only carried out if
they trip again when reset.  It makes electrical storms much easier to
deal with, but the beeping from the UPS every time the power goes off does
get annoying. :)

have fun

Maria

[1] A service fuse is what the UK calls the main big chunky fuse that sits
between the electricity company wiring, and the wiring in your house. It
usually sits between the armoured cable coming out of the ground, or the
cables from outside and the meter, it is owned by the electricity company.  
Usually found inside under the stairs or in a cabinet on the outside wall
of the house.  I don't know what other countries call these.




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