[Gllug] ADSL problem

[Christopher Hunter]

On Sunday 27 Feb 2005 22:26, Chris Bell wrote:

>    There should be (or should have been) a spark gap between the incoming
> wires, hidden inside the BT master box, and much computer comms equipment
> now has an opto-isolator on the input. 

All "approved" equipment is opto-coupled and transformer coupled to the line 
side - it's mandatory (I used to work in the telecomms industry and used to 
design approved equipment).

> If you are in an area prone to 
> lightning strikes you can feed the incoming pair through a ferrite core a
> few times, (an old TV line output transformer core would be ideal), or get
> a rather more beefy metal cased protector with a good earth connector (more
> expensive, but claimed to work well).

NOTHING will prevent destruction of the equipment on a line by a direct hit to 
the 'phone lines, but you'll often get way with an induced spike.  The 
"protection" inside a BT master socket is utterly useless, as there's no path 
to earth (except through the connected equipment).  many times I've scraped 
the crunchy remnants out of computer cases after lightning strikes.....

The only protection that works at all are a pair of tritium-filled gas 
discharge tubes connected from each of the line wires to a good, solid earth 
where the line enters the building.  These used to be available from Farnell 
for pennies, but I haven't seen them for a while.

>    I have been told that telephone boxes often sustained lightning strikes,
> and there was often very little damage visible, just a pair of holes in the
> cable outer where the spark jumped a bend in the cable.

The 'phone box was well earthed, and provided a Faraday cage for the contents!

Chris

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