Jock Mackirdy wrote:
> In article <df70ar$ifp$(E-Mail Removed)>, Mike H
> wrote:
>
>> Having said that, surely ground stakes are not part of a standard
>> line installation?? Perhaps a BT eng. can shed some light on this!
>
> Shared service, earth-calling PABXs and external extensions were the
> main types of line which used an earth connection. They all had to be
> abolished prior to the introduction of "modern" exchanges such as
> crossbar (TXK1 and TXK3) and electronic (TXE2 and TXE4). That also had
> the benefit that lines were no longer polarity-sensitive. The
> reduction of line and apparatus maintenance time/cost was
> significant, offsetting the initial costs of changing apparatus and
> increasing the number of lines.
>
> Earth spikes had been standard provision from time immemorial on all
> lines to provide lightning surge protection. The carbon "protectors"
> were a common source of low insulation faults and were abolished for
> new lines when the 700-series telephone was introduced (c. 1960).
> Special provision (gas-discharge tubes) was then made for
> lightning-sensitive lines.
Only problem is that those where very seldom connected to earth, so they
took the earth terminal away so the gas disharge tube was only connected
from A to B (even though there was still an earth terminal in the NTE) &
now even the discharge tube has been removed in favour of a diode, once
again connected between A & B legs (the earth terminal having been
removed ). AFAIR the only equipment accesable to the CAL man,
containing a earth connection point, is the in the block 80b or the RF3
connection blocks (or the larger internal DP type blocks, 301 etc of
course)...
Earth discharge protection/over voltage protection is something which is
sadly lacking in the BT local loop nowadays, I've even known a 240v
flash over (during a house fire) to be able to get back to the exchange
card & damage that.
The only voltage protection devices still used are in hot sites (power
stations) where all circuits have to go thru line protectors (unless the
lev 2 says other wise, go figure) so things like ADSL & Redcare are
effictively blocked & specialised modules have to be used for BTHighway
or ISDN but even with those they can have problems.
Now I'm sure that someone is going to tell me that what I'm saying is
incorrect practice, or not correct by the regulations (i.e long multi
span lines are supposed to have earth discharge prtotection etc) but
what's in the book & what happens outside in the big bad world is & can
be completely different (even the 'fuses' used in the exchanges are
being replaced by metal bridging pieces if not already done so)...
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