[plug] Telstra CLI/CID Question

[David Campbell]

To:             	plug at linux.org.au
Subject:        	Re: [plug] Telstra CLI/CID Question 
Date sent:      	Sun, 15 Aug 1999 08:30:14 +0800
From:           	John Summerfield <summer at os2.ami.com.au>
Send reply to:  	plug at linux.org.au

> But not for one to fiddle around on the computer's side of the modem. 
> Shall we stick with external modems? Internals seem likely to muddy the 
> waters a little

Most modems (especially the older ones) have an isolation transformer 
that is meant to be rated at some obscene isolation voltage (2kV?). 
The connection to the phone lines was directly to one side of the 
transformer with the other side to the circuitry.

The characteristics of the transformer is such that despite the fact 
it is a nominal 1:1 winding ratio that even if 240VAC was to find its 
way to across the secondary then the worst that could happen to the 
phone lines is something like 80VAC (old pulse dial phones used 
voltages in this vicinity) due to transformer core losses and 
internal transformer resistances (deliberate design flaw/feature).

Some of the newer modems have probably done away with the old 
isolation transformer and are probably using optocoupled isolation 
with an internally isolated power supply (there are available potted 
power isolators, about 10mm x 40mm x 10mm which tansfer 5VDC power 
across an electrical isolation of 1kV. Don't ask me how but they 
can...).

Originally Telecom Australia (the bit that is now Austel) set the 
standards to prevent 240VAC appearing on their equipment and zapping 
their technicians (duty of care).

As far as frying their equipment is concerned, much of the equipment 
should be able to handle 500V surge peaks without blinking an eyelid 
(it has to do with line inductance and the hideous back EMF you can 
achieve). But still electronics tend to loose the "magic smoke" when 
exposed to 240V for any significant period of time.

David Campbell

PS: "magic smoke" refers to the critical part of all electronics, 
typically released by electronic components when exposed to high 
voltages hence they stop working. Once relased it is very hard to put 
back into the components again.
=======================================================
campbell at torque.net
"This is not an office, rather Hell with fluorescent lighting"