Telephone CO supply

PeterT

On Fri, 16 Nov 2007 17:46:43 GMT, "Peter Andrews"
<p.andrews@blueblueyonder.co.uk> wrote:

>
><abhi.15sep@gmail.com> wrote in message
>news:8b8a3b8a-e02d-404a-b16a-d231440406df@e25g2000prg.googlegroups.com...
>> why telephone offices have -48V sypply ?
>
>I suspect that the answer lies back many, many years - my copy of Telephony
>Vol 1, Herbert and Proctor, first published in 1934 says that speech quality
>using carbon granule microphones is better with 40v than 24v. Why -50v I
>really don't know, but the UK has used -50v since time immemorial and since
>strowger type exchanges had a nice big stable power supply of -50v
>everything to follow was designed to use it. Even with relatively modern
>equipment the -50v central supply, already in place, was convenient, very
>secure, and was often used to power rack convertors to provide 12v, 5v, etc.
>supplies to specific equipment.
>
>
>Peter
>

And of course repeater stations were poerered at 24 Volts, certainly
until the 1980's (ISTR) until TEPIE kit was btought into service - and
the telex exchanges seemed to like 80+80-ve
--
Cheers

Peter

John Riggs

"steveybar" <steveybar@NOSPAMyahoo.co.uk> wrote in message
news:5q6dfhFu7pnbU1@mid.individual.net...
>
>>>
>>> and why not have used +50v and earthed the -ve ?
>
>
>> To prevent electrolytic corrosion of the cables.
>>
>> Steve H
>
> Are you saying that:
>
> -50v and earthing the +ve prevents electrolytic corrosion of the cables
> but
> +50v and earthing the -ve doesn't prevent it?
>
> If so, can you qualify that?
>

I know your query has already been answered, but I thought I would add my
minor contribution by recalling back in the mid 1960's, as an apprentice, I
helped take readings for the cathodic protection of HM ships which involved
placing "sacrificial" anodes in the water and connecting them to the deck of
the ship. Every day we disconnected the cable and measured the voltage
between the anode and the deck. The anodes corroded and the ship didn't.

I've no idea if they still do it.

Regards

John Riggs