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From: (John Higdon)
Newsgroups: alt.dcom.telecom
Subject: Re: How do I become a telco?
Date: Tue, 30 Apr 1996 00:39:23 -0700

In article <4m2t4r$>, Bruce A. Pennypacker
<> wrote:

> really blew his mind was the rather large interior room that was
> filled from floor to cieling with rows and rows of lead acid
> batteries.  Just like most other telco systems, a backup in case
> the first backup fails.

It is not exactly like that. The central office equipment runs on those
batteries which float on a charger. The charger can either be powered from
commercial AC (which it normally is), or from the backup generator.

When the power fails, there is absolutely no service interruption since
the batteries can easily power the connected equipment during the start
and runup time of the backup generator.

The batteries are there to provide smooth DC power (sort of like filter
caps), and to keep the operation alive during transition to backup power.
There are not there as backup in case of failure of another backup.

John Higdon  |    P.O. Box 7648   | +1 408 264 4115  |      FAX: | San Jose, CA 95150 | +1 500 FOR-A-MOO |+1 408 264 4407
             |         |

From: (Floyd Davidson)
Subject: Re: How do CO -48V plants work?
Date: Sun, 26 Oct 1997 11:56:50 GMT

<wdg@[]> wrote:
>In article <62sp1t$dej$>
>(Aaron Nabil) writes:
>>In a typical CO, how are the batteries charged?  Are they simply
>>paralled across the office -48v rectifier without any sort of
>>charge management/limiting?
>The switching machine runs off batteries, not a -48 rectifier.

During normal operation the office runs off the rectifier plant,
not the battery.  (Opening the battery string by removing straps
between cells will not even cause a minor glitch in operation!)

The battery is in parallel, but the voltage is adjusted such
that the string of cells is "floating", which means there is no
discharge, and actually there is a slight charge current at all
times.  Under that circumstance the primary effect of the
battery string is an exceedingly low impedance, making it an
excellent smoothing filter for the -48 volt bus.

Typically the battery plant would measure -48vdc with no load.
The float voltage is about 53.5 volts.  Occasionally (e.g.,
anytime water is added to the cells) an "equalize" charge is
used, and that might be as high as 56vdc.

Typically the low voltage cutoff will be set to 42vdc.

The "typically" description is used because in some odd given
case things are done differently!  Also, despite all of above
voltages, the actual -48 Volt Bus voltage is normally 52vdc.
When the string is floating at 53.5 volts that is accomplish
via any of various magic tricks known as CEMF cells, End Cells,
or other contraptions that I've gladly forgotten (they are all
a pain).

One other point worth noting is that the portion of a telephone
office which is part of the Public Switched Telephone System is
required to have 8 hours of batter backup.  Private line
circuits are not required to have that, and sometimes it happens
that they don't.


Floyd L. Davidson in named for the siltiest
                           most mud filled glacial river in the world. So  here I sit, both literally and figuratively,
Salcha, Alaska             on the banks of the Tanana, muddying the waters.

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