UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio and TV Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > Specific Vintage Equipment > Vintage Telephony and Telecomms

Notices

Vintage Telephony and Telecomms Vintage Telephones, Telephony and Telecomms Equipment

Closed Thread
 
Thread Tools
Old 23rd May 2017, 1:14 pm   #1
broadgage
Nonode
 
Join Date: Jan 2010
Location: North Somerset, UK.
Posts: 2,129
Default Did very large phone cables get warm

Most telephone circuits involve a small current circulating continually even when the phone is not being used.

The current involved is of course minute if compared to say domestic power or lighting circuits.
However the conductors used are very small, and in the largest multi pair cables huge numbers of circuits are involved.

IIRC 2,000 pair cables were the largest in general use, and several such cables might be in close proximity.

With say 50ma per circuit, a 2000 pair cable would be carrying about 100 amps in total. And probably with a total cross sectional area of copper not much greater the 25mm or 35mm that would be commonly used for a 100 amp power cable.

So, did they get perceptibly warm to the touch ?
broadgage is offline  
Old 23rd May 2017, 1:21 pm   #2
Station X
Moderator
 
Station X's Avatar
 
Join Date: Jan 2003
Location: Ipswich, Suffolk, IP4, UK.
Posts: 21,192
Default Re: Did very large phone cables get warm

Quote:
Originally Posted by broadgage View Post
Most telephone circuits involve a small current circulating continually even when the phone is not being used.
Are you sure about that? In the local network current would only be flowing if the phone was off hook, or ringing current was being applied to the line.

In these days of broadband things may be different.
__________________
Graham. Forum Moderator

Reach for your meter before you reach for your soldering iron.
Station X is offline  
Old 23rd May 2017, 2:18 pm   #3
Guest
Guest
 
Posts: n/a
Default Re: Did very large phone cables get warm

A telephone on hook should be an open circuit so there would be no appreciable current. Having fun years ago I tried to suck some power from an on hook line, I think all I could get was a lot less than a mA without tripping the exchange. Broadband is a fairly low current thing and (usually) only local so no large cables would be carrying it anyway.
 
Old 23rd May 2017, 2:30 pm   #4
space charge
Tetrode
 
space charge's Avatar
 
Join Date: May 2013
Location: Enniscorthy, County Wexford, Ireland.
Posts: 74
Default Re: Did very large phone cables get warm

As previously mentioned no current flows in the on hook situation.however in a catastrophic situation ie a mechancal digger causing a failure of the insulation .there would be some current flowing to earth or to a short circuit. Never felt any significant heat due to these conditions. Regards ei7ka
space charge is offline  
Old 23rd May 2017, 2:33 pm   #5
Dave Moll
Dekatron
 
Dave Moll's Avatar
 
Join Date: Feb 2005
Location: West Cumbria (CA13), UK
Posts: 6,118
Default Re: Did very large phone cables get warm

Quote:
Originally Posted by merlinmaxwell View Post
A telephone on hook should be an open circuit...
...at least as far as the always-present DC line voltage is concerned. It will draw current when the AC ringing voltage is applied, but there should be no current flowing otherwise.
__________________
Mending is better than Ending (cf Brave New World by Aldous Huxley)
Dave Moll is online now  
Old 23rd May 2017, 4:14 pm   #6
broadgage
Nonode
 
Join Date: Jan 2010
Location: North Somerset, UK.
Posts: 2,129
Default Re: Did very large phone cables get warm

I was under the impression that traditional phone lines had a small circulating current at all times, hence the need for vast storage batteries at exchanges.
It would seem that I was mistaken in this, and that no significant current flows when the line lies unused.
Thanks to all who replied.
broadgage is offline  
Old 23rd May 2017, 5:24 pm   #7
Biggles
Rest in Peace
 
Join Date: Feb 2004
Location: Hexham, Northumberland, UK.
Posts: 2,234
Default Re: Did very large phone cables get warm

Our exchanges at work will supply about twenty milliamps max to line, but line loop resistance will eventually limit this, which is why there is a maximum working distance from the exchange to the subscriber. A domestic line/exchange works on a current loop principal. A good pair should read infinite resistance. You are lucky if you get this. There are normally small leakage currents across a pair through connectors etc, but not enough to cause the exchange to seize the line (thinking someone has picked up the phone). I found that for our telephone lines to work properly you need in the order of 10mA plus flowing, up to a max of 20mA. We have problems on some of our lines with insulation and this will cause the exchange line card to sense a fault, and time out giving the classic wailing alarm call as if you had left the phone off the hook. An idle telephone line should read open circuit and not draw current. Saying that I used to work on DC signalling equipment and these often had a current, (positive or negative wrte) flowing, to drive the interface at the remote end. These went out of fashion a long time ago.
Alan.
Biggles is offline  
Old 23rd May 2017, 7:35 pm   #8
Dave Moll
Dekatron
 
Dave Moll's Avatar
 
Join Date: Feb 2005
Location: West Cumbria (CA13), UK
Posts: 6,118
Default Re: Did very large phone cables get warm

I assume that "wrte" is "with respect to earth".
__________________
Mending is better than Ending (cf Brave New World by Aldous Huxley)
Dave Moll is online now  
Old 23rd May 2017, 9:04 pm   #9
Euros1951
Triode
 
Join Date: Aug 2016
Location: Lampeter, Ceredigion, Wales, UK.
Posts: 35
Default Re: Did very large phone cables get warm

Quote:
Originally Posted by broadgage View Post
I was under the impression that traditional phone lines had a small circulating current at all times, hence the need for vast storage batteries at exchanges.
It would seem that I was mistaken in this, and that no significant current flows when the line lies unused.
Thanks to all who replied.
In the small exchanges batteries in the exchange were used for back-up in case of a mains power fail. In larger exchanges the batteries were used in between power fail and the stand-by generator starting up to provide the power. The batteries were also used to smooth out the 50v when in normal use with the power on, and were therefore charged and ready.
Euros1951 is offline  
Old 23rd May 2017, 9:05 pm   #10
Station X
Moderator
 
Station X's Avatar
 
Join Date: Jan 2003
Location: Ipswich, Suffolk, IP4, UK.
Posts: 21,192
Default Re: Did very large phone cables get warm

Time to draw a line under this thread.
__________________
Graham. Forum Moderator

Reach for your meter before you reach for your soldering iron.
Station X is offline  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 9:24 am.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2024, vBulletin Solutions, Inc.
Copyright ©2002 - 2023, Paul Stenning.