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Old August 7th 04, 03:19 PM posted to uk.telecom
Jock Mackirdy Jock Mackirdy is offline
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Default Telecommunications Lines - Earthing

In article , R Currie wrote:
Not that elderly - finials weren't being fitted to poles when I started

with
the PO in the early 60s.


I think finials may go further back than that even - I'll check the date on
one of our local poles fitted with both earth wire and finial when I next
pass it.


Other way round chronologically - finials stopped being fitted ? c. 1960; tops
of poles chamfered instead.

And the payback there was mainly the reduced cost from doing away
with the need to keep pairs "straight" all the way from MDF to subs.


Straight? How is a "straight" pair defined? Would be interested to learn the
meaning of this term.


Guaranteed "A" wire to earth, "B" wire to battery at all times, which meant
that cable jointers had to ensure they didn't introduce any polarity reversals
either in cable joints or in cross-connection cabinets and frames. Doing away
with that restriction speeded up jointing enormously and eliminated
maintenance visits to reverse the line at the subs. premises.

--

Jock Mackirdy
Bedford



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Old August 7th 04, 03:19 PM posted to uk.telecom
Jock Mackirdy Jock Mackirdy is offline
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Default Telecommunications Lines - Earthing

In article , Stephen Radford wrote:
The telephone line pair is 'balanced about earth' to ensure common mode
balance. This is necessary to avoid noise pickup in the line pair, e.g.
mains hum. A line pair is also twisted for the same purpose. In simple terms
most interference will also be reference to earth, the common mode balance
ensures cancellation on each leg. The twist endeavours to keep the
interfering signal equal on each leg - imagine a telephone cable adjacent to
a mains cable! On lines with a an earth fault (leakage from one leg to
earth) the line will be noisy with lots of mains hum. The balance is
achieved at the central exchange, in Strowger days by means of a feeder
relay/bridge with two coils one fed from Batt +ve, the other from Batt -ve.
On System X and other modern exchanges/PABXs by means of a solid state SLIC
(Subscriber Line Interface Circuit).


There's not a lot of twist in a local cable pair. Whilst balance is a factor,
avoiding split pairs is also a major consideration. Long overhead open wire
lines had a reversal every so often to maintain balance.


--

Jock Mackirdy
Bedford


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Old August 7th 04, 03:26 PM posted to uk.telecom
Ian Bartholomew Ian Bartholomew is offline
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Default Telecommunications Lines - Earthing

Jock Mackirdy wrote:

Must have been a bit later than 1972. Bacup (the first production
TXK1) was about 1970, and the GSC replacement version took some time
to develop after that.


Not much later. There were quite a few GEC contractors who were transferred
to Hastings after the work at Dover was completed (I used to have breakfast
with a number of them in the local cafe). Hastings was RFS at the end of
1973 so Dover must have been 1972 or 1973..

--
Ian

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Old August 7th 04, 03:57 PM posted to uk.telecom
Jock Mackirdy Jock Mackirdy is offline
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Default Telecommunications Lines - Earthing

In article , Ian
Bartholomew wrote:
Jock Mackirdy wrote:

In your dreams! There's about 90 years of history to plough through
(the starting point being central battery manual exchanges, the last
of which ?Dover? only closed about 1980.


Dover's TXK opened around 1972 but I'm not sure if it was manual before
that.


Must have been a bit later than 1972. Bacup (the first production TXK1) was
about 1970, and the GSC replacement version took some time to develop after
that.

Hastings manual board(s - there were 3 at the end) closed in 1974 and
ISTR we were told at the time that it was the last _large_ manual to close.


In fact the last few big manual exchanges were among the last to close because
Strowger GSC installation had ceased before TXK1 GSCs got going properly.

I would guess that there were a few small rural ones left but I don't think
they would have been around much longer than the mid 70s


They were very few and far between. Funding for Scotland and Wales meant that
UXD5s were quickly mopping up the last of the CBS's as well as many tiny UAXs.


--

Jock Mackirdy
Bedford


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Old August 9th 04, 02:09 PM posted to uk.telecom
Phil Partridge Phil Partridge is offline
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Default Telecommunications Lines - Earthing

In article , Jock Mackirdy
writes
In article , Stephen Radford wrote:
The telephone line pair is 'balanced about earth' to ensure common mode
balance. This is necessary to avoid noise pickup in the line pair, e.g.
mains hum. A line pair is also twisted for the same purpose. In simple terms
most interference will also be reference to earth, the common mode balance
ensures cancellation on each leg. The twist endeavours to keep the
interfering signal equal on each leg - imagine a telephone cable adjacent to
a mains cable! On lines with a an earth fault (leakage from one leg to
earth) the line will be noisy with lots of mains hum. The balance is
achieved at the central exchange, in Strowger days by means of a feeder
relay/bridge with two coils one fed from Batt +ve, the other from Batt -ve.
On System X and other modern exchanges/PABXs by means of a solid state SLIC
(Subscriber Line Interface Circuit).


There's not a lot of twist in a local cable pair. Whilst balance is a factor,
avoiding split pairs is also a major consideration. Long overhead open wire
lines had a reversal every so often to maintain balance.


So that must have been to prevent one conductor migrating to the other,
and all the poles leaning one-way due to the weight imbalance! ;-)
Phil Partridge

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Old August 9th 04, 08:40 PM posted to uk.telecom
Jock Mackirdy Jock Mackirdy is offline
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Default Telecommunications Lines - Earthing

In article , Phil Partridge wrote:
In article , Jock Mackirdy
writes


There's not a lot of twist in a local cable pair. Whilst balance is a factor,
avoiding split pairs is also a major consideration. Long overhead open wire
lines had a reversal every so often to maintain balance.


So that must have been to prevent one conductor migrating to the other,
and all the poles leaning one-way due to the weight imbalance! ;-)


OK then - please define the unit of force "Pull on Pole". A genuine parameter,
though you may need to consult Herbert and Procter for the answer. I presume the
electricity companies have a similar unit.

--

Jock Mackirdy
Bedford


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Old August 9th 04, 10:02 PM posted to uk.telecom
Phil Partridge Phil Partridge is offline
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Default Telecommunications Lines - Earthing

In article , Jock Mackirdy
writes
In article , Phil Partridge wrote:
In article , Jock Mackirdy
writes


There's not a lot of twist in a local cable pair. Whilst balance is a factor,
avoiding split pairs is also a major consideration. Long overhead open wire
lines had a reversal every so often to maintain balance.


So that must have been to prevent one conductor migrating to the other,
and all the poles leaning one-way due to the weight imbalance! ;-)


OK then - please define the unit of force "Pull on Pole". A genuine parameter,
though you may need to consult Herbert and Procter for the answer. I presume the
electricity companies have a similar unit.

Jock,

It was meant light-heartedly, hence the smiley, and not as a slight on
any Persons posting to the thread..
Normally, I would 'just consult the tables', so without going into the
loft, here goes; cue creaking in the brain department..

Two messenger wires, or one with change of direction, on a pole. Project
one line of messenger to other side of pole, measure angle between
projected line and second messenger. - Angle = A (degrees).
Assume same tension in both messenger wires. T (pounds).
Also, assume messengers are horizontal.

This will give a pull on pole of 2 x T x sin(A/2)

Always supposing I can remember my maths right, and I have the right
term from the guy-wire strength formula.

Sorry it's not a 'wordy' definition.
Phil Partridge

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Old August 10th 04, 10:11 AM posted to uk.telecom
Jock Mackirdy Jock Mackirdy is offline
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Default Telecommunications Lines - Earthing

In article , Phil Partridge wrote:
In article , Jock Mackirdy
writes
In article , Phil Partridge wrote:
In article , Jock Mackirdy
writes


There's not a lot of twist in a local cable pair. Whilst balance is a factor,
avoiding split pairs is also a major consideration. Long overhead open wire
lines had a reversal every so often to maintain balance.


So that must have been to prevent one conductor migrating to the other,
and all the poles leaning one-way due to the weight imbalance! ;-)


OK then - please define the unit of force "Pull on Pole". A genuine parameter,
though you may need to consult Herbert and Procter for the answer. I presume the
electricity companies have a similar unit.

Jock,

It was meant light-heartedly, hence the smiley, and not as a slight on
any Persons posting to the thread.


No offence taken. I do have a sense of humour. Well done for remembering the
definition; after 40+ years I could only remember the name.

Normally, I would 'just consult the tables', so without going into the
loft, here goes; cue creaking in the brain department..

Two messenger wires, or one with change of direction, on a pole. Project
one line of messenger to other side of pole, measure angle between
projected line and second messenger. - Angle = A (degrees).
Assume same tension in both messenger wires. T (pounds).
Also, assume messengers are horizontal.

This will give a pull on pole of 2 x T x sin(A/2)

Always supposing I can remember my maths right, and I have the right
term from the guy-wire strength formula.


From the curvy state of some present-day DPs I wonder if the art has been lost.

--

Jock Mackirdy
Bedford


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Old January 15th 12, 06:14 PM
Erlang Erlang is offline
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Default

There is a great advantage in earthing one 'leg', mainly for wiring as in older relay sets, just earth one side no need to individually wire back to a polarity point. Event though we say the A leg Earth = OV is was through a 200ohm coil. If both legs were independent of earth a mains voltage contact could be fatal for a linesman!

BTW any good source as to how the famous Detector No. 4 was 'supposed' to be used in fault finding?

Thanks,
Fiachra
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Old August 5th 13, 08:06 PM
Dr.Who Dr.Who is offline
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Hi,

It's all in Atkinsons Telephony, and even Herbert & Proctor. The confusion here is that the line in the idle state is different that when a call is in progress.

Idle state

This goes back to Strowger days and also manual days. The B wire is at -50v (-40v for manual exch) and the voltage is via a line relay of some 1000 ohms. The A wire is at full earth potential. Hence the use of a detector No. 4 et al.

In use state.

Here either the siezed 1st selector will operate the K relay via the P wire, or the answer cord on the manual system will operate the CO relay via the sleeve circuit. The end result is that the full earth is removed from the A wire and the line relay is removed from the B wire.

Battery and earth to the line is now via the cord circuit, or the A relay in the 1st selector. Both battery and earth are supplied to the line via 200 ohm relay coils which now gives a balanced circuit. Depending upon the line resistance the A wire will be about - 21v and the B wire at about -29v in the case of the auto system.

In the case of digital exchanges the line card mimics this condition for compatability purposes but the A relay is replaced with a suitable transformer which feeds a codec chip, so the audio signal is digitised.

This arrangement balances out noise and hum. Also in PBX working this allows earth loop recall as this unbalances the line current in each leg of the line. This condition can be detected by a differential relay in the exchange line card.

I hope this helps.


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