UK Telecoms (uk.telecom) Discussion of UK telecommunications.

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Old August 4th 04, 10:02 PM posted to uk.telecom
Currie, R Currie, R is offline
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Default Telecommunications Lines - Earthing

Hello All,

As I am sure most persons present are aware, BT direct exchange lines have
the 'A' wire of the pair earthed at the exchange, giving the old

A Earth = 0V
B Earth = -48V

What I was wondering is whether the earthing of one side of the pair is
essential in modern telecommunications, and whether anyone is aware of any
large networks which are unearthed?

Does the earth on A play any particular part in modern networks, or is it a
hangover from the past?

All comments welcomed........

Cheers,

Ross



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Old August 6th 04, 06:45 AM posted to uk.telecom
Steve Maudsley Steve Maudsley is offline
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Default Telecommunications Lines - Earthing


"Currie, R online.co.uk" [email protected] wrote in message
. ..
Hello All,

As I am sure most persons present are aware, BT direct exchange lines have
the 'A' wire of the pair earthed at the exchange, giving the old

A Earth = 0V
B Earth = -48V

What I was wondering is whether the earthing of one side of the pair is
essential in modern telecommunications, and whether anyone is aware of any
large networks which are unearthed?

Does the earth on A play any particular part in modern networks, or is it

a
hangover from the past?

All comments welcomed........


Several things come to mind:

1. Having one side of the cables at earth means that so long as a linesman
earths himself to the same potential he knows what the voltage is of any
line with respect to himself (I recall when ISDN started to be installed a
linesman getting sparks to the end of his nose because the potential was
higher and his normal working position for analogue was too close for ISDN
(ISTR he did have a large nose as well).

2. Having a common earth means that physical spacing can be standardised in
line equipment.

3. Makes it easier to use random phases from the 3 phase mains for
terminating equipment.

4. Reduces the chance of sparks in hazardous environments (and any enclosed
underground space can get build-ups of gases like methane or petroleum
vapours.

5. Allows the use of cheaper line interfaces in cheap terminals.

I'm sure that there are some more...

Stephen




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Old August 6th 04, 08:22 AM posted to uk.telecom
Alan J. Flavell Alan J. Flavell is offline
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Default Telecommunications Lines - Earthing

On Fri, 6 Aug 2004, Steve Maudsley wrote:

A Earth = 0V
B Earth = -48V


Several things come to mind:


All of which seem to me to point to benefits in having the A and B
at roughly equal polarity above and below earth (+/- 24V). Odd. Is
this something to do with electrolytic corrosion effects?

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Old August 6th 04, 11:53 AM posted to uk.telecom
Steve Maudsley Steve Maudsley is offline
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Default Telecommunications Lines - Earthing


"Alan J. Flavell" wrote in message
la.ac.uk...
On Fri, 6 Aug 2004, Steve Maudsley wrote:

A Earth = 0V
B Earth = -48V


Several things come to mind:


All of which seem to me to point to benefits in having the A and B
at roughly equal polarity above and below earth (+/- 24V). Odd. Is
this something to do with electrolytic corrosion effects?


The linesmen would still need an earth reference for test gear - there's a
lot to be said for being able to hang stuff off the frame.


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Old August 6th 04, 05:58 PM posted to uk.telecom
Currie, R Currie, R is offline
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Default Telecommunications Lines - Earthing

Is this something to do with electrolytic corrosion effects?

Interesting you should mention that - I have read in many published works on
electrical engineering and telecommunications that in DC systems, earthing
the positive conductor produces less corrosion than earthing the negative
conductor.

However, whilst this perhaps explains the place of connection of the earth
on a telecoms pair, it does not explain its existance.

Many thanks,

Cheers,

Ross




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Old August 6th 04, 06:32 PM posted to uk.telecom
Ivor Jones Ivor Jones is offline
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Default Telecommunications Lines - Earthing


"Currie, R online.co.uk" [email protected] wrote in message
...
Is this something to do with electrolytic corrosion effects?


Interesting you should mention that - I have read in many published

works on
electrical engineering and telecommunications that in DC systems,

earthing
the positive conductor produces less corrosion than earthing the

negative
conductor.


I remember many years ago that cars could be either positive or negative
earth; the Austin 1100 my dad used to have was positive earth and caused
no end of problems when trying to install anything designed for negative
earth. Not long after, -ve earth became the norm and +ve was phased out.

However, whilst this perhaps explains the place of connection of the

earth
on a telecoms pair, it does not explain its existance.


Signalling on old party lines springs to mind, but not sure if there are
any still in existence.


Ivor


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Old August 6th 04, 07:01 PM posted to uk.telecom
Alan J. Flavell Alan J. Flavell is offline
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Default Telecommunications Lines - Earthing

On Fri, 6 Aug 2004, Currie, R wrote:

Is this something to do with electrolytic corrosion effects?


Interesting you should mention that - I have read in many published works on
electrical engineering and telecommunications that in DC systems, earthing
the positive conductor produces less corrosion than earthing the negative
conductor.


Indeed. When I was a youngster, this was explained to me as the
reason why cars had positive earths (negative supply). Then,
suddenly, new cars all had negative earths (positive battery), without
any clear explanation being given, and I still don't know why.

Probably the same reason that motorcycle gearchanges suddenly changed
to the other side and upside down - "Not Invented Here". Ho hum.
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Old August 6th 04, 07:29 PM posted to uk.telecom
Russell Hafter Russell Hafter is offline
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Default Telecommunications Lines - Earthing

In article , Ivor Jones
wrote:

Interesting you should mention that - I have read in
many published works on electrical engineering and
telecommunications that in DC systems, earthing the
positive conductor produces less corrosion than
earthing the negative conductor.


It is the other way round. Corrosion of metals involves
electron loss, ie becoming +ve.

Making the earth -ve forces electrons onto the metal and
prevents corrosion.

Used to be used for seaside piers etc. Connect the -ve to
the pier and the +ve to a pile of scrap elsewhere in the
sea, but nearby. The scrap corrodes faster than it would
normally,because it is doing the corroding for the pier too.
Process is "Cathodic protection".

I remember many years ago that cars could be either
positive or negative earth; the Austin 1100 my dad used
to have was positive earth and caused no end of problems
when trying to install anything designed for negative
earth. Not long after, -ve earth became the norm and +ve
was phased out.


Once the car manufacturers realised that their wiring +ve
earth was actually encouraging their cars to rot...

--
Russell

http://www.russell-hafter-holidays.co.uk
Russell Hafter Holidays E-mail to enquiries at our domain
Holiday specialists for Germany, Czech Republic, Belgium, bits of France...
Tel 0870 740 4963 Fax 0870 870 5515
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Old August 6th 04, 07:55 PM posted to uk.telecom
Jock Mackirdy Jock Mackirdy is offline
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Default Telecommunications Lines - Earthing

In article , R Currie wrote:
Hello All,

As I am sure most persons present are aware, BT direct exchange lines have
the 'A' wire of the pair earthed at the exchange, giving the old

A Earth = 0V
B Earth = -48V

What I was wondering is whether the earthing of one side of the pair is
essential in modern telecommunications, and whether anyone is aware of any
large networks which are unearthed?

Does the earth on A play any particular part in modern networks, or is it a
hangover from the past?


It's simple to have "battery" connected to one leg and thus "earth" to the
other. No subs. apparatus has been polarity sensitive for yonks (various
external extension plans with ringing leg-to-earth were probably the last).
There's still a need for a d.c. loop (pulse-dialling telephones are still
catered for, even they no longer operate an A relay). Ringing current is a.c.
superimposed on d.c. to detect ring trip.

--

Jock Mackirdy
Bedford


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Old August 6th 04, 07:55 PM posted to uk.telecom
Jock Mackirdy Jock Mackirdy is offline
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Posts: 169
Default Telecommunications Lines - Earthing

In article , Steve Maudsley wrote:
"Alan J. Flavell" wrote in message
la.ac.uk...
On Fri, 6 Aug 2004, Steve Maudsley wrote:

A Earth = 0V
B Earth = -48V

Several things come to mind:


All of which seem to me to point to benefits in having the A and B
at roughly equal polarity above and below earth (+/- 24V). Odd. Is
this something to do with electrolytic corrosion effects?


The linesmen would still need an earth reference for test gear - there's a
lot to be said for being able to hang stuff off the frame.


That's what the earth wire on the pole is for. Try fault-finding on an
ADSL-enabled line with a Detector No. 4.

--

Jock Mackirdy
Bedford




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