|
Vintage Telephony and Telecomms Vintage Telephones, Telephony and Telecomms Equipment |
|
Thread Tools |
23rd May 2017, 1:14 pm | #1 |
Nonode
Join Date: Jan 2010
Location: North Somerset, UK.
Posts: 2,129
|
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 ? |
23rd May 2017, 1:21 pm | #2 | |
Moderator
Join Date: Jan 2003
Location: Ipswich, Suffolk, IP4, UK.
Posts: 21,192
|
Re: Did very large phone cables get warm
Quote:
In these days of broadband things may be different.
__________________
Graham. Forum Moderator Reach for your meter before you reach for your soldering iron. |
|
23rd May 2017, 2:18 pm | #3 |
Guest
Posts: n/a
|
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.
|
23rd May 2017, 2:30 pm | #4 |
Tetrode
Join Date: May 2013
Location: Enniscorthy, County Wexford, Ireland.
Posts: 74
|
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
|
23rd May 2017, 2:33 pm | #5 |
Dekatron
Join Date: Feb 2005
Location: West Cumbria (CA13), UK
Posts: 6,118
|
Re: Did very large phone cables get warm
...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) |
23rd May 2017, 4:14 pm | #6 |
Nonode
Join Date: Jan 2010
Location: North Somerset, UK.
Posts: 2,129
|
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. |
23rd May 2017, 5:24 pm | #7 |
Rest in Peace
Join Date: Feb 2004
Location: Hexham, Northumberland, UK.
Posts: 2,234
|
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. |
23rd May 2017, 7:35 pm | #8 |
Dekatron
Join Date: Feb 2005
Location: West Cumbria (CA13), UK
Posts: 6,118
|
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) |
23rd May 2017, 9:04 pm | #9 | |
Triode
Join Date: Aug 2016
Location: Lampeter, Ceredigion, Wales, UK.
Posts: 35
|
Re: Did very large phone cables get warm
Quote:
|
|
23rd May 2017, 9:05 pm | #10 |
Moderator
Join Date: Jan 2003
Location: Ipswich, Suffolk, IP4, UK.
Posts: 21,192
|
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. |