Understand that the holiest of holies, dial tone, was
a sacred thing to the telcos for many years. I'm not
sure where this came from, but working with a lot of
independent telcos years ago, I had it explained to me
that the worst, absolute worst thing that could ever
happen to you as a service provider was to not have a
dial tone. The level of reliability was 100%.
Period. Nothing less was acceptable.
As a result of this thinking, the telcos were set up
years ago to run on 48 volts DC, the origin of this
being unknown to me personally. However, that's why
when you go into a CO (central office) or switch,
you'll find a large room, in some in a major exchange
the whole floor of a building, filled with nicad wet
cells set up in series-parallel on huge racks.
Utility power is used to run the inverters to keep the
batteries charged, and "runs over the top" of the
system during normal operation, that is, the inverters
provide the power while charging or maintaining the
charge on the batteries. Should utility power fail,
the load simply shifts to the batteries and there is
no interruption to the customer, or (G-d forbid) a
loss of dial tone.
If there are generators on site, they are set up to
provide power to the inverters, and will power them up
during an outage to keep the batteries from
discharging. In a lot of cases they will set up a
time delay on the generator start up so that it's not
coming on until the outage is sustained for a short
time, maybe 10-30 minutes. regularly discharging the
batteries is a good thing, and in some cases they'll
even have resistive load banks that can be applied to
the battery banks on a scheduled basis to load test
them. You certainly don't want to find a dead cell
when there's a power outage...
The telcos I have worked at typically had 18-24 hours
of run time on batteries alone, possibly more
depending on the load on the system. This gives them
lots of time to come up with alternative sources of
power in the event of a catastrophic event
(sometimes.) Look closely at the outside of the CO
buildings and in many cases you'll see a big
Crouse-Hinds connector for connecting a trailer
mounted genset. This is very common amongst the Baby
Bells, most of whom maintain a large fleet of
generators spread out across their service area, ready
to be dispatched in the event of an emergency.
Dan
--- Tom Hargrave <[EMAIL PROTECTED]> wrote:
> I used to work on the Telco equipment at Browns
> Ferry Nuclear Power Plant.
> Our main switch, a Dimension 2000 (we called it the
> Demented 2000) supported
> 1700 extensions. The system ran on a stack of Edison
> batteries that were
> housed & maintained in the next bay.
>
> The phone system was actually powered by the
> batteries 24/7. The batteries
> were kept charged by a huge charger that was backed
> up by a generator set.
> This configuration guaranteed 100% uptime since a
> loss of line power would
> just stop the batteries from charging.
>
> The batteries were huge clear glass cells that were
> wired in series to make
> 48 volts. There were two banks of batteries so that
> one could be taken off
> line while the other was serviced.
>
> Our second "modern" system was a System 75 that
> supported an additional 750
> phone lines. This system had a more conventional UPS
> that measured 4' square
> by 4' tall.
>
> Thanks,
> Tom Hargrave
> www.kegkits.com
> 256-656-1924
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