Why most telecommunication equipment use -48V supply voltage

This question was asked while I attending an interview for a job of
telecommunication technician.

A big question mark was appearing in my brain at the time, although i
am working in this area for more than 10 years.

1. Why chose negative voltage?
2. Why chose 48V?

Any comment is welcome, just for remind our knowledge that we learnt
before.

t.m.fan@green-part-hk.servehttp.com wrote:
> This question was asked while I attending an interview for a job of
> telecommunication technician.
>
> A big question mark was appearing in my brain at the time, although i
> am working in this area for more than 10 years.
>
> 1. Why chose negative voltage?
> 2. Why chose 48V?
>
> Any comment is welcome, just for remind our knowledge that we learnt
> before.
>

Spoiler:

http://answers.yahoo.com/question/in...1095739AANQXjv

<t.m.fan@green-part-hk.servehttp.com> wrote:

> This question was asked while I attending an interview for a job of
> telecommunication technician.
>
> A big question mark was appearing in my brain at the time, although i
> am working in this area for more than 10 years.
>
> 1. Why chose negative voltage?
> 2. Why chose 48V?
>
> Any comment is welcome, just for remind our knowledge that we learnt
> before.

48v is 4 x 12v batteries. (google)

It comes in under the 50v requirement for low voltage circuits. (google
again)